Technical Manual
THERMOFORMING
INDEX
Thermoforming principles
-History of thermoforming industry
-Products manufactured by thermoforming
Suitable polymers for thermoforming
-Thermal properties -Temperature -Heat measurement -Specific heat -Thermal conductivity Heating plastics
-Heat transfer: conductivity, convection and radiation
-Thermal properties of plastics -Heat transmission media
-Temperatures and forming cycles -Establishing the right temperature
for the material
Thermoforming equipments
-Gas furnaces with pressured air circulation -Infrared heating furnace
-Lineal heating electric resistors
Complementary equipment: vacuum, pressured air and mechanical forces
-Vacuum forming -Pressured air forming -Mechanical forming -Combined techniques -Mechanical support design
Thermoforming molds
-Choosing thermoforming technique -Criteria to design thermoformed products -Criteria to design thermoforming molds -Considerations in designing thermoforming
molds
-Materials used to manufacture tthermoform - ing molds 4 7 11 17 25 31
Thermoforming techniques
-Bi-dimensional thermoforming
-Tri-dimensional thermoforming (with molds) -Molding techniques in infrared heating
furnace
Cooling thermoformed products
-Conventional cooling methods -Non-conventional cooling methods
Cutting thermoformed products
-Cutting equipment -Cutting techniques Thermoforming variables -Material variables -Mold variables -Pre-stretching variables -Mechanical support variables
Problem and solution guide Appendix
-Glossary
-Glass fiber reinforced plastic -Unit conversion table
46 51 53 58 62 68
Thermoforming principles
Since the beginning of the XX century some techniques to form sheets, with materials such as metal, glass and natural fibers, have been known. The true thermoforming principles emerged as thermoplastic materials were developed, which happened dur-ing the second world war. The post-war period brought about mass commercialization and rapid development of equipment and machinery capable to adapt to the manu-facturing modern methods, to make more useful and income yielding products. In the 50s, the volume of thermoplastic material production and the products made with it reached impressive figures. In the 60s, by developing the thermoforming indus-try, the foundations for the future were established. Then huge consumers and prod-uct competitiveness, in the 70s, required high speed prodprod-uctive machinery. Equipment manufacturers met those needs by making machinery capable to produce about one hundred thousand thermoformed individual containers per hour. Sophisticated controls were also required.
Since the 80s up to the present, thermoformers have so much relied on their process that they have gone beyond their expectations and have established production lines that can produce finished thermoformed products, not only from sheets but also from resin pellets; besides, they recycle the scrap with minimum control. Equipments have been computerized and at present, they can perform auto-monitoring and diagnostic functions. Nowadays, very complex equipment does not require more than one work-er to handle and control it thanks to electronic advances. Thus, it is believed that the thermoforming industrial labor market will undergo a shortage of technically trained and experienced personnel, since traditional knowledge will no longer be enough. Therefore, lectures, seminars, courses, etc., would be useful to increase thermoform-ers´ general knowledge, and would further advance this well established industry. Many of the thermoformed products in use at present have been manufactured to replace their original use forms. This has taken place so fast that those original ones have been almost forgotten. For example: it is not easy to remember in what ham-burgers were packed before the arrival of the one piece polystyrene package or what kind of material lined the interior of refrigerators.
The following list begins with the area with the most number of thermoformed pieces and continues in a decreasing order up to the one with the fewer pieces.
Packaging industry
Since the beginning of the thermoforming process, the packaging industry has been the most benefited due to the high productivity and benefits (cost-profit) that it offers.
History of thermoforming industry Manufacturing thermoformed products
At present, most of the packaging equipments (blister) are high speed automatically sustained. These equipments are called "form-fill-seal" and are used to pack cosmet-ics, cold cuts, sodas, candies, stationery, etc.
Take away food industry
In the growing "take away food" industry, a great deal of thermoformed products are used, ranging from a complete meal container (divided containers), to hamburgers and sandwich packages, sodas, etc.
Usually, that industry requires printed thermoformed packages. This printing can be made before or after thermoforming. Some examples of this are trays, cups, sandwich, hamburger, hot-dog packages, etc.
Food packaging industry
Supermarkets are the great consumers of thermoformed containers. The materials used are low-cost thermoplastics. These are designed to be piled or placed in differ-ent forms. Examples: meat, fruit, eggs and vegetables containers.
Transport
Public and private transport such as bus, train subway, plane, car, etc., has within its equipment many thermoformed plastic parts. Most of these are used for inside finish-ing or non-structural exterior parts. In others, they are used for seats, backs and arms of seats, fronts of doors, service tables, wind-shields, instrument protectors, guards, spoilers, etc.
Signaling and advertisements
These are usually made of acrylic and can consist of only one piece and can be very large. Transparent (clear) acrylic is generally used and it is painted on the inside using acrylic based paint.
The use of acrylics for exteriors makes advertisements weather resistant and they vir-tually need no maintenance; furthermore, they can stand extreme cold or hot weather conditions. Exterior lighted bill-boards, interior advertisements, signaling in public places, offices, etc., are some examples.
Household products
There is a great deal of products that have thermoformed parts; actually, they are pro-duced in great quantities. They can be found in cabinet, washing machines, dish washers, dryers, refrigerators, air conditioning outlets, humidifiers, T.V. and radio cab-inets, etc.
users like hospitals, nurseries, schools fairs and others, there are the military sector and international aid organizations. Some examples of products are: trays, cups and plates.
Medical industry
The medical industry requires a great variety of products and sterilized packaging for hospitals, clinics and doctors´ offices. The specifications for these products are usual-ly very strict and recycling materials is unacceptable.
The use of acrylic , since it is physiologically harmless, is growing every day. Some examples are: chirurgical equipment, syringes and needles, chirurgical tables, cabi-nets, incubators, dentists´ seats and exercise platforms.
Agriculture and horticulture
Commercialization of decoration plants in supermarkets and specialized shops has generated, for some time, the need to make flower pots and small containers, includ-ing with multiple divisions for exhibitinclud-ing and sellinclud-ing. This kind of containers are made of recycled plastic at low cost. Flower pots, different size and divided containers, small green houses, trays for growing seeds, planting containers, etc., are some examples.
Constructión and housing
For some years, construction industry has used thermoformed products, which have become quickly popular. Thermoformed parts have easily replaced a lot of products. Actually, there are products that cannot be manufactured any other way, such as sky-lights or cannon arches. In this sector, acrylic is used a lot because of its weather resistant properties and its thermoforming quality.
Examples of these are: skylights, cannon arches, hydro-massage tubs, bath modules, wash basins, bathroom screens and cabinets, tables, chairs, lamp stands, kitchen items, stairs, frontages, partings, windows, aquariums, etc.
Luggage
Some luggage manufacturers are deciding in favor of using the thermoforming process, since it has advantages over the injection products. Because it is molded effortlessly, the possibility of thermoformed products fracturing is reduced. Examples: all kinds of suitcases, briefcases, etc.
Photography equipment
One of the oldest thermoformed products is the tray used for developing photos, also flash bulbs (metallic reflector) and the magazine for standing cameras, even though its manufacturing requires a precision thermoforming technique.
Suitable polymers for thermoforming
Basically, every thermoplastic polymer is suitable for the thermoforming process. Those materials, when exposed to heating, show an elasticity, hardness, and resist-ance capacity, under load variation in their module. With an increased temperature over the H.D.T., the material will tend to become rubber-like, having as critical value the temperature of annealing of the thermoplastic polymer. This can be seen in the rapid bending of the hot sheet, when the force of gravity is strong enough to cause this deformity.
Table 1 shows the suitable and most common polymers for thermoforming, as well as their temperature. POLYMERS HEATING DEFLECTION TEMPERATURE AT 264 PSI (ºF) AT 66 PSI (ºF) WITHOUT CHARGE (ºF) SHEET TEMP. (ºF) MOLD TEMP. (ºF) AID TEMP (ºF) THERMOFORMING TEMPERATURE Extruded acrylic Cell-cast acrylic Cellulose acetobutyrate High density polyethylene Polypropylene
Polystyrene
High impact polystyrene SAN ABS Polyvinyl chloride (RV.C.) Polycarbonate 201.2 204.8 149-167 131-149 158-203 185-203 212 167-239 158 266 208.4 230 167-176 140-176 230-239 158-212 194-203 221 176-248 167 248 248-302 212 284 212 248 203 230 320 275-347 320-356 284-320 293-374 293-392 284-338 338-356 428-446 248-356 275-347 356-446 149-167 149-167 203 113-149 113-149 158-185 113 203-248 338 194 194 194 176 284
One of the least considered aspects in thermoforming practice, is that of the ther-mal properties of polymers which is one of the most relevant and critical aspects of the process. Wholly understanding these factors will reduce the risk of long pre-pro-duction run or bad adjusting of the product to the outline.
When we talk about thermal properties, it is indispensable to establish the concepts related to this topic. First, it must be remembered that energy often dissipates
Thermal properties
Specific heat and thermal conductivity are two of the physical properties of polymers that are extensively used in thermoforming.
In the thermal phenomenon debate some terms and concepts must be included. The first thermal property is temperature. Temperature is the measurement of the degree of "heat" or "cold" in an object. A temperature scale must be established, water properties have been taken as a parameter, specially the degree of ice fusion and water boiling. There are three scales to measure the temperature of a substance: the scale in centigrade degrees (°C), Fahrenheit (°F), and Kelvin (°K), the first two are the most commonly used. Heat is simply a form of energy, therefore, the suitable physics unit to measure heat is the same as the one for mechanical energy and it is the joule (J). As in the case of tem-perature, water is used as parameter of substance to define the heat unit. The amount of heat needed to raise the temperature of 2.2 pounds of water by one degree [at pres-ent it is taken as 58.1ºF to 59.9 ºF (14.5 °C to 15.5 °C) is defined as 1 calorie (cal)]. When a calorie is added to 2.2 pounds of water, the water temperature increases 33.8 degree, for example: if the same amount of heat is added to the same amount of methyl-alcohol, the temperature rises about 35.06 degrees, or if 1 cal. is added to 2.2 pounds of aluminum, the temperature of the metal rises about 41 degrees. In fact, each sub-stance will respond differently when exposed to heat. The amount of heat needed to raise 33.8 degree in 2.2 pounds substance is called specific heat of that substance. Water works as a parameter and it has been determined as 1 cal./pounds, and it is taken as a basis to compare every material. Excepting water, most materials have a specific heat, lower than plastics.
Thermal conductivity is one of the three ways by which heat energy can be transferred from one place to another; it results from the molecular movement and therefore, it needs the presence of matter. Heat energy is transferred by collisions where the rapid movement of atoms and molecules of the hotter object transfers part of the energy to the colder object or the one with a slower movement of atoms and molecules. When a substance is heated, it expands, heat increases the volume of a substance and dimin-ishes its density. The thermal conductivity of acrylic is 0.0005 cal./seg. cm2
Thermal expansion derives from increasing the temperature of a substance, and as a consequence it expands, actually, almost every substance: solid, liquid or gas has the property to increase its size, as its temperature rises. As for thermoforming, when a polymer is heated the mobility of molecular chains increases, therefore, they tend to separate from each other, increasing the volume and area of the polymer. This proper-ty is extremely important especially in thermoformed pieces, which are exposed to sudden changes of temperature or weather conditions. In thermoforming, the plastic sheet is expanded more rapidly than the metal frame, creating some wrinkles near the frame, which disappear when the sheet contracts. The numeric values of the coeffi-cients for heating and cooling are identical; this means that it takes the same time for
Temperature Heat measurement Specific heat Thermal conductivity Thermal expansion
a piece to get hot as to get cool. It must be taken into consideration that there might be problems when the thermoformed parts have to be within a very close dimensional tolerance. There might be other kinds of problems when there is shrinkage in a male mold, making it difficult to remove the part from the mold. The thermal expansion coef-ficient of acrylic is 0.00009 cm./cm./°C.
Heating plastics
In the thermoforming process, the heating operation is one of the longest stages in which there might be present the most difficulties and material and human resources waste. That is why this chapter is devoted to heat transfer, aiming at trying to clarify phenomena that might occur in plastics heating operation.
Although scientists have divided heat transfer into three different phenomena: con-duction, convection and radiation, in practice, the three phenomena are concurrent.
Conduction
This is heat transfer from one part of a body to another part of the same body, or from one body to another which is in physical contact with it, without a substantial dis-placement of the particles of the body.
Convection
This is heat transfer from one point to another, in a fluid, gas or liquid (by mixing one part of the fluid with another). In natural convection, the movement of the fluid totally derives from the difference in density as a result of different temperatures. In the forced convection, which is the one we are interested in, the movement is produced by mechanical means. When velocity is relatively low, it must be noted that free convec-tion factors, such as different temperature and density, may have an important influence.
Radiation
This is heat transfer from one body to another that is not in contact with it, by means of a wavy movement through space.
For the purposes of thermoforming process, three media for heat transfer are considered: A) Contact with a solid, liquid or hot gas.
B) Infrared radiation.
C) Internal excitation or by microwaves.
The first two ones are very much used in plastic thermoforming and for several of them the scope of temperature is between 120°C and 205°C (250°F and 400°F).
Heat transfer: conduction, convection and radiation
Plastics are poor heat conductors; therefore, thick sheets need a considerably long time to heat. In table 8, there are some thermal properties of some materials to be com-pared. In plastic thermoforming the method and size of the heating equipment must be taken into consideration.
Heating a sheet on both sides (sandwich-like heating) helps to reduce the time taken in this operation. In some cases, heating time can be reduced if the sheet is pre-heat-ed and kept at a mpre-heat-edium temperature; however, this is rarely done with less than 6mm. thick materials.
In addition, the amount of heat required to raise the temperature of plastics is high, compared with any other material; except water. To estimate the needed heat for a sheet, the following formula can be used.
Required heat = Length X width X thickness X density of material X (specific heat X dif-ferent temperature + fusion heat)
Thermal properties of plastics
MATERIALS SPECIFICGRAVITY g/cm3 SPECIFIC HEAT Btu/ Ib 0F FUSION HEAT Btu/lb THERMAL CONDUCTIVITY Btu ft/sq ft hr 0F THERMAL COEFFICIENT of LINEAL in/in 0F10-5 Air Water Ice Soft wood Hard wood Phenol R. Epoxy R. Polyethylene Acrylic Polycarbonate Graphite Glass Quartz Aluminum Steel Copper 0.0012 1 0.92 0.5 0.7 1.5 1.6-2.1 0.96 1.19 1.2 1.5 2.5 2.8 2.7 7.8 8.8 144 144 55 171 171 88 0.24 1 0.5 0.4 0.4 0.3 0.3 0.37 0.35 0.30 0.20 0.20 0.20 0.23 0.10 0.092 0.014 0.343 1.26 0.052 0.094 0.2 0.1-0.8 0.28 0.108 0.112 87 0.59 4 y 8 90 27 227 2.8 1.5 1.5 3-5 1.5-2.8 7 3.5 3.7 0.44 0.5 0.4 y 0.7 1.35 0.84 0.92 Table 8: Thermal properties of some materials.
Heat transfer
media
For practical purposes we will divide the media for heat transfer into 4 types:
Heating by contact
The fastest heating method is placing a plastic sheet directly in contact with a hot metal sheet. It is specially used in mass production of small and thin items.
Heating by immersion
With this method, a plastic sheet is immersed in some liquid that transmits heat as evenly and quickly as possible, but its use is restricted to molding parts out of huge or very thick sheets, since handling and cleaning of the piece are very difficult
Heating by convection
Furnaces with air convection are widely used, because they provide even heating and can, to a certain degree, dry some materials that contain some degree of moisture. These furnaces provide a huge safety margin as for time variations in thermoforming cycles.
Important note:
All the above mentioned heating media require a considerable amount of time to pre-heat the equipment.
Infrared heating:
This method can supply instant heating and therefore, its exposition cycles are very short, and sometimes it takes only a few seconds. The main sources of this kind of energy are:
-Quartz lamps that emit in the visible and near infrared.
-Ceramic or metal resistors that emit more energy in the far infrared.
The surface of these radiation heaters can be between 599 ºF and 1301 (315°C and 705°C). It must be noticed that at the highest temperatures, the mass of radiation occurs at shorter wave lengths. On the other hand, at lower temperatures, radiation expands on longer wave lengths; and this is extremely important, since each plastic absorbs infrared radiation in different areas. Only the radiation absorbed is used to heat plastic directly.
Internal heating
This method has not had enough application in thermoforming because the equipment used is very expensive. Besides, it is not suitable for every plastic, and cooling time is very long. It is useful in forming processes where localized heating is required on a spe-cific area of the material. For example, when forming edges of material which has a high loss factor, such as P.V.C.
In certain applications, thermoformed products show uneven parts, even when a sheet has been uniformly heated. Heterogeneous shrinkage of a sheet is due to the very design of that part. In those special cases, controlling heat by section will give more uniform wall areas. This procedure is called shading or screening and it consists in placing a non-flammable filter to regulate heat (a wire net, asbestos, etc.) between the sheet and the source of heat, this will reduce the flow of heat to certain areas of the material, and will prevent excessive stretching on that area.
In more sophisticated equipments, at present, there are electronic controls and ceram-ic parabolceram-ic elements that allow variability when heating different areas of the sheet. Before we start with temperatures and forming cycles, we will establish some termi-nology:
a) Temperature to remove items off a mold b) Operation: bottom limit
c) Normal temperature to form d) Operation: top limit
Temperature to remove item off a mold
It is the temperature at which an item can be removed off the mold without distortion. Some times an item can be removed at higher temperature if cooling devices are used.
Operation bottom limit
This represents the lowest temperature at which the material can be formed without internal effort. This means that the plastic sheet must touch each corner of the mold before it reaches its bottom limit. The material processed under this limit will show internal effort that later will cause distortions, glow loss, cracking and other physical changes in the finished product.
Normal temperature to form
This is the temperature at which a sheet must be formed in a normal operation. It must cover the whole sheet. Shallow thermoformed items with the aid of air or vacuum will allow a bit lower temperatures, and this translates into shorter cycles. On the other hand, deep forming requires high temperatures, as well as for pre-stretching opera-tions, details or intricate radiuses.
Operation top limit
Under this temperature a thermoplastic sheet begins to degrade, and it also turns too fluid and cannot be handled. These temperatures can be exceeded, but only with mod-ified formulations that improve the physical conditions of the sheet. Injection and
extru-Temperatures and forming
General recommendations
a) The characteristics of a finished product are determined by the kind of thermoform- ing technique used.
b) The material must be heated evenly at the annealing and forming point, before it cools below its molding temperature.
c) Acrylic must cool slowly and evenly while it is in the mold.
d) The formed piece must be cool before any finishing is done, like spraying paint or serigraphy.
e) In the design of a piece, a 2% shrinkage in both directions and a 4% increase in thickness must be taken into consideration, as well as a 0.6% contraction at 1% when cooling
Temperatures and forming cycles
As it was previously mentioned, one of the most important steps of the thermoforming process is determining the right temperature of the material. For acrylic, the right selec-tion of annealing or normal temperature will prevent:
At a low temperature:
Internal effort concentrates in the thermoformed piece which later, under sudden envi-ronmental temperature changes, will emerge as fissures or cracking.
At high temperature:
Bubbles and mold marks, due to extreme heating.
Table 9 shows the ranging temperatures for Plastiglas acrylic sheet, for general use, and Sensacryl FP¨, deep molding sheet.
Table 9 KIND OF MATERIAL BOTTOM LIMIT (OF) TOP LIMIT (OF) TEMPERATURE RANGE
Plastiglas (general use) Sensacryl (deep molding)
320 356
356 392
In Mexico, due to the high cost of electricity, it is more common to use a convection furnace with pressured air re-circulation by means of gas, for which a very practical for-mula is very useful to determine the permanence time for an acrylic sheet, taking into consideration the annealing temperature range previously adjusted.
Formula: 53.3 X E (inches) = T (min.)
Where : 53.3 = Factor, E = Thickness of material, T = time.
This formula can be used for thin (0.04 to 0.24 inches) Chemcast sheets. For thicker sheets, the factor has to be changed as follows:
Formula: 3 X E (inches) = T (min). Ex: 53.3 X 0.118 = 6.30 min.
As it has already been mentioned, there are variables that may modify these formulas, such as: environmental temperature of the place where the furnace is located, cure (especially in extreme weather conditions), material thickness fluctuation and the con-ditions of the equipment among other things.
Forming temperature
Every thermoplastic material has a process specific temperature. These ranges apply without taking into consideration the way the material will be processed. The most used materials compared with acrylic are mentioned in table 10:
Table 10, Ranges of forming temperature
MATERIAL SHEET TEMP. (0F ) BOTTOM LIMIT (0F ) TOP LIMIT (0F ) MOLD TEMP. (0F ) MECHANICAL SUPPORT TEMP . (°F) REMOVAL TEMP. (0F ) NORMAL (0F ) Acrylic CHEMCAST Sensacryl FP ABS Polycarbonate AD Polyethylene 320- 356 356-392 257-356 392-482 320-428 320 356 257 392 320 338 374 329 455 374 356 392 356 482 428 210 248 338 248 266 185 284 185 149-167 158-176 158-185 194-248 194-212
Another important factor in the thermoforming process, is establishing the right tem-perature for plastic material. You must bear in mind that apart from the heat trans-mission medium, a sheet must be heated at the recommended range of temperature (annealing range), besides, a sheet has to be heated in an evenly way.
In practice, it is not easy to accurately establish the temperature of the sheet, even
EstablishIng the right temperature of the material
process (annealing point), is one of the cues to establish the right temperature. Some controls for infrared radiation thermoforming equipment have been developed, where a sheet is fastened horizontally, and the "yielding" or "bending" phenomenon is used, and photo-electric cells control heating time and/or temperature.
Clamp
Frame
Vacuum box
Photo-electric cells
Solenoid valve controlled by photoelectric cells.
However, this criterion cannot be applied indiscriminately to every plastic, since some materials may over-heat before they begin to yield or bend. Although a range of tem-perature is established, the expected temtem-perature of a sheet may not be achieved; this may be caused by:
a) Fluctuations in the thickness of the material
b) Temperature changes in the equipment and/or environment c) Minimum fluctuations in the line voltage (in infrared equipment).
d) The regulator of the pressured air circulation gas equipment may not be the right one, there is not enough gas pressure, the burner is not the right one or it may be blocked with soot, etc.
There are cone formed pyrometers, infrared radiation or gas (hot air) heating tablets, that can render a more accurate measurement. Although probably, the best way to measure the temperature of a sheet is by means of an infrared pistol, which measures by zones; though the equipment is expensive, it is the only one that measures the tem-perature of a sheet accurately and reliably.
Thermoforming equipments
Originally, convection furnaces were the first equipments to heat plastic sheets that were going to be thermoformed, and up to now, that kind of heating is still preferred for sheets of different thickness, and for temperature even distribution.
Heat can be applied with gas or electric resistor units. To produce air circulation from 4,500 to 6,100 cm3/min. (150 to 200 feet3/min), pressured air re-circulation and deflec-tors are crucial to get homogeneous temperatures. The furnace temperature must be adjusted to the plastic forming temperature.
Infrared radiation heating, compared with oil immersion or contact heating (the two lat-ter very limited in practice), is extremely rapid. For example, a 3.0 mm sheet heating time by infrared radiation can be achieved in one min. at about 10 watts/inch2. Because infrared radiation heating takes very little time, heat energy absorbed by a sheet may cause over-heating, that may even affect the degrading of the material (bubbles or burning) if it is not controlled. It is important to consider that in long runs, the furnace temperature has to be gradually reduced.
In some cases, when the product has intricate or very deep sections, there is the risk of the thickness of the material considerably thinning; in this case screens must be used (they may be made of perforated plate or metallic display) to prevent over-heat-ing.
The elements of infrared radiation can be obtained in a very wide range of designs, according to their importance they are:
1.- Tungsten filaments in quartz tubes or lamps, temperature 3992 ºF (2,200 °C). 2.- Spring- like nichrome resistor on refractory ceramic bases.
3.- Nichrome resistors protected by plate or stainless steel tubes.
There are manufacturers who make infrared radiation thermoforming machines in a wide variety of sizes, capacity, degree of automation and versatility.
The specifications to acquire a thermoforming machine vary depending on the finished product that you want to get and therefore, it is necessary to consider:
Voltage, wattage, amperage, useful area of forming, number of heaters (lower and upper), controls to regulate temperatures by zones, degree of automation, capacity to
Gas furnaces with pressured air circulation
accept mechanical support, type of sheet fastening device, (clamps, mechanical, pneumatic, etc.), ventilators to cool the product, general dimensions, production capacity, cost- profit.
This kind of furnace supplies uniform heat and constant temperature, with a minimum risk of over-heating an acrylic sheet. Electric ventilators must be used to force hot air circulation on the acrylic sheet at a speed about 4,500 to 6,100 cm3/min., and devices to distribute the air in every zone of the furnace.
Gas furnaces need heat inter-changers to prevent accumulation of soot due to the gas flow, as well as controls to interrupt the gas flow, when necessary.
Electric furnaces can be heated, using sets of 1000 watts resistors. When using a fur-nace with a 10 m3 capacity, about 25,000 power watts will be consumed and half of this will be used to compensate heat loss due to leakage, insulating transmission and the use of doors. A minimum 2" thick insulation is advised and the doors of the fur-nace should be as narrow as possible, to reduce most of the temperature loss. Automatic devices must be used to strictly control temperature between 32 ºF and 482 ºF (0 °C and 250 °C). To get a more uniform sheet heating, it is important to hang it vertically, and this can be done with a system that fastens the material all along with clamps or canals with springs which move on wheels that slide on rails, like the ones used for closets.
Basic criteria to construct a gas furnace with pressured air circulation.
The best advice in this case, is asking any industrial furnace manufacturer to build one with the mentioned characteristics, since the construction of one, specially the heating and operation systems, is very risky for anybody who has only little knowl-edge on the subject.
This kind of equipment must be approved by specialists in gas installations, it also has to be registered before the corresponding authorities.
It is also relevant to point out that the information provided here, is only related to the metallic structure and fastening system for acrylic sheets. A furnace construction can be divided into the following sub-systems:
A) Structure
B) Fastening acrylic sheet C) Electric system
D) Gas installation E) Controls
Recommendations to build a furnace
Building the structure with commercial iron tubular of 11/2" X 11/2" or 2 X 2". a) Cut it according to the measurements and requirements of design.
b) Weld the lateral walls.
c) Weld the upper wall, the lower one and the back one; to join them with the lateral ones, and build the whole structure.
d) Line the inner part of the structure with a black plate cal. 18 and weld it or rivet it with "pop".
e) Cover the holes (thickness of the tubular) with a rigid sheet of glass fiber to get ther-mal insulation, code RF-4100, or a similar one.
f) Line the exterior with a black plate cal. 18 and rivet it with "pop" or weld it.
g) Make the doors with a structure of tubular PTR 1" X 1", and follow the same instruc-tions as for the walls, they should be shorter to leave room for the rails.
h) Attach the doors to the furnace with hinges.
i) Put the closet-type rails, they should be twice as long as the furnace. They are fixed with screws on the upper part of the furnace. Once they are fixed to the furnace and the furnace on its place where it will operate, using bearings fasten the rails to the
ceil-ing or structure of the place.
GAS STRUCTURE WITH AIR RE-CIRCULATION
Closet-type rails
Plate "U" bearings of 1/4”
The electric ventilator is placed in this section to force the air
Every joint must be welded with electric welding Rectangular tubular profile
1/4” iron plate
Iron hinge
Spring
Washer
Nut
Type C profile cal.# 18
Acrylic sheet 1/4” crossbar
handle
5/16" Cold rolled bar
FASTENING SYSTEM FOR ACRYLIC SHEETS
FURNACE FRONT VIEW AND DOOR DETAIL AND RAILING SYSTEM
Steel cable to fix it to the ceiling of the place.
Hook formed 1/2" cold-rolled bar.
Joint of the fastening system for acrylic sheets.
1 1/2” x 11/2” iron angle
1/2” cold rolled bar. 1 3/4” x 2” (1500 rail)
closet- type profile No. 50 wheels
It is normally used in automatic thermoforming machines, heating a sheet by means of radiation at a speed 3 to 10 times faster than in a pressured air circulation furnace, thus, with very short heating cycles. It should be noted that the ratio temperature/time becomes critical and it is harder to heat the material uniformly.
Infrared heating furnace
LATERAL VIEW AND DETAIL OF THE FURNACE DOOR AND RAILING SYSTEM Steel cable to fix it to the
ceiling of the place 1 1/2” x 1 1/2”
iron angle
2 1/2” x 2 1/2” iron angle
1 3/4” x 2” (riel 1500) closet -type profile
No. 50 wheel
Infrared energy is absorbed by the acrylic surface exposed, rapidly reaching tempera-tures over 356 ºF (180 °C), that later on, is transmitted to the center of the material due to temperature conduction.
Infrared radiation heating can be obtained using tubular metal elements, spring elec-tric resistors, or by grouping infrared light lamps. To get a more uniform heating distri-bution, a net or metallic mesh can be placed among the heating elements and the material which can work to expand the temperature. It is also convenient to place an infrared heating plate, about 12”from the material and 20” from the bottom plate.
To regulate energy input into the equip-ment, we recommend using devices such as different transformers or percentage meters that will help to control tempera-ture. Planning electric energy charges and great capacity equipment is also advis-able, an electric sub-station will also be needed.
An electric resistor can only be used to make bends in a straight line; to achieve this, you also need a spring type electric resistor (20) or armored type (about 1KW X 1.2 m.). Lineal resistors are made of wire, inside Pyrex ceramic tubes. The material must not be in contact with the tube to avoid marks on the surface. A distance of 6 mm. from the tube to the material is recommended to get uniform heating on thin material.
When more than 3.0 mm thick material is going to be heated with this procedure, the resistors should be placed on both sides of it. In the next picture, it is shown how an asbestos plate bender at the beginning of production will provide a suitable bend, but as production advances, the heating area expands making a bigger radius bend, that is why a resistor with water re-circulation is much better for acrylic bending.
Lineal heating electric resistors
Acrylic Sheet Heating zone
Electric resistor
Asbestos plate
Acrylic Sheet Heating zone
Electric resistor Asbesto s plate
Basic criteria to build a lineal heating electric resistor.
Bi-dimensional thermoforming or lineal bending, can be made with a spring type resis-tor or a tubular one. Building these equipments is conditioned to thickness, kind of bending and volume to be produced. Generally, a 1.32 yd. long resistor is the most common, though a 24” one is also acceptable, the specifications for this resistor are 1Kw for each 1.32 yd., thus, with a rule of three consume can be deduced both for a longer or a shorter resistor.
Acrylic benders are more common than the ones built with asbestos plates on the lat-eral walls, these are suitable as long as you do not have to produce a huge volume, since when asbestos plates are exposed to the same infrared radiation they tend to get hot and therefore, the heating area will expand changing a piece production standard. In other words, at the beginning of production, there will be small radiuses and as production advances, the heating area will be wider creating a bigger radius.
An electric resistor bender with water re-circulation will be more effective and produce better quality bent pieces. This equipment needs tubular profiles that allow water re-circulation, which will keep the surface cool and will only allow a heating zone. The required materials to build this kind of bender are listed below.
It is important to include a rheostat to control temperature intensity on an acrylic sheet, since it will provide the suitable pace of production and, obviously, it will reduce costs of electric energy.
ASBESTOS PLATE FOLDER WATER RE-CIRCULATION FOLDER
• Spring-like, tubular or nichrome tape resistor
• No. 16 or 18 cable with glass fiber insulator
• Terminals.
• 2 X 14 Heavy duty cable
• Plug
• 500, 1000, 2000 or 3000 watts dimmer
•1/8", 3/16" o 1/4" asbestos plate
• Spring-like, tubular or nichrome tape resistor
• No. 16 or 18 cable with glass fiber insulator
• Terminals
• 2 X 14 Heavy duty cable
• Plug
• 500, 1000, 2000 or 3000 watts dimmer
•3/4" x, 3/4" aluminum tubular profile
• 6.6 yd. hose
• Clamps
• 10 to 20 lt. container
Complementary equipment: vacuum, pressured
air and mechanical forces
The thermoforming process consists in heating and softening a sheet of any kind of thermoplastic material and making it adopt the form of the corresponding mold to get an almost finished product with a particular form.
Some times, an external force has to be used to turn a flat sheet into a different form and to make it copy the outline and details of the mold. The level of energy or use of this force must be adjusted, so that the plastic sheet can be easily forced to take another form.
The most common used forming forces in the thermoforming process are: vacuum or pressured air, mechanical forces and the combination of these three. Choosing a form-ing force in the formform-ing process generally depends on the size of the product, the vol-ume to be produced and the speed of the forming cycles.
In addition, the following factors must be considered, since any of these can make a difference in selecting the forming force:
a) Intrinsic limitations of each thermoplastic material b) Construction and material of the mold
c) Thermoforming equipment available
The oldest method to form a plastic sheet into a utilitarian piece is vacuum forming. The original description of the thermoforming process was precisely "vacuum-forming". The basic principle of the vacuum-forming process is having a softened thermoplastic sheet in a mold perfectly sealed and where the air inside is evacuated by the vacuum force or suction. As the air is evacuated from the mold, it creates a negative pressure
Vacuum forming
on the surface of the sheet and therefore, natural atmospheric pressure yields, forc-ing the hot sheet to take the place of the empty spaces, as it can be seen in the picture.
Acrylic sheet
Vacuum equipment
There is a great variety of vacuum pumps: reciprocal piston, diaphragm, blades, eccen-tric rotor, etc. All these provide a good vacuum but cannot evacuate great volumes of air at high speed; that is why a stock tank has to be connected to be used as "vacu-um acc"vacu-umulator". On the other hand, there are compressors that can evacuate a great volume of air but are limited for vacuum force.
A suitable vacuum system needs a pump that can displace from 28 to 29" Hg or from 0.5 to absolute 1 Psi (710 to 735 mm of Hg.) in the stock tank before the forming cycle. The line, duct or pipe between the stock tank and the mold should be as short as pos-sible with a minimum of angles. It is important to eliminate air leaking due to damaged piping, perforated hoses, loose couples or nipples, as well as unnecessary valves. Rapid action or globe valves should be used. Vacuum pumps are available in one or two steps. A two step vacuum pump can evacuate pressures below 10 Psi; displace-ment capacity or evacuation for a one step pump is reduced by half. Table 11 shows vacuum pumps typical capacities
Table 11: Vacuum pump typical specifications
SPECIFICATIONS VACUUM THEORETICAL CAPACITY
No. OF CYLIN-DERS 1 2 2 2 2 3 DIAMETER (inches) 3.04 3.04 4.08 5.08 5.6 5.6 RUN (inches) 2.8 2.8 2.8 3.2 4.08 4.08 POWER NEEDED (Kw) 0.56 0.74 1.48 2.2/3.7 3.7 5.6 DIAMETER OF PIPING OUTLET 19 25 32 38 52 52 TWO STEPS (yd3/min) ----0.280 0.498 0.935 1.54 3.08 ONE STEP (yd3/min) 0.280 0.561 0.996 1.87 3.08 4.64 SPEED (RPM) 800 800 800 750 900 900 Vacuum tanks
Excepting some vacuum equipments, most have a stock tank. Bearing in mind that work pressure is about 10 Psi (about 21 inches Hg/530 mm. Hg) vacuum, then the vol-ume of the tank should be 2.5 times bigger than the volvol-ume between the molds, the vacuum box and the piping. Doubling the volume of the stock tank (along with other similar conditions) pressure can be increased 15% (11.5 Psi), according to what is established, the theoretical limit for the vacuum forming process is only 14.5 Psi.
Stock tank (400 lt.)
2” flexible hose
globe valve
Solenoid valve Air deflector
Bearings In many cases, a rapid displacement of vacuum is very important. This can only be made by placing the vacuum tank as near the mold as possible and reducing the pip-ing friction as much as possible, which can be done by:
a) A bigger piping diameter.
b) Piping with wide curves, avoiding 90° angles.
c) Changes in the transversal section of the piping (diameter changes).
Many equipments in the market do not meet these requirements. In general, the piping must be 1" diameter to displace 1 ft3 of air, for big pieces a 2" or 3" diameter is suit-able. There should also be a flexible plastic hose internally reinforced with wire or a similar material that prevents it form collapsing; it should be connected between the mold and the piping, as shown in the picture.
Vacuum forces, applications.
In general, pumps work constantly to keep vacuum in the stock tank, there is a varia-tion on the vacuum-meter readings in each cycle. The vacuum generated on the formed part must be kept enough time to cool and stand the internal force of the mate-rial which will tend to keep the original form, causing waves and bending.
As a general rule, the faster the vacuum is made the better the piece will be formed. Occasionally, slow forming speed for deep forming pieces or intricate sections is
rec-In operations where vacuum force is replaced by pressured air, it should be considered that it is harder to seal the mold satisfactorily. The forming force can easily multiply up to 10 times if the pressured air is at 100 Psi. However, the molds can stand such pres-sure very few times.
To form by using pressured air, it is necessary to take as many precautions as possi-ble. A regular size mold requires a closing pressure of some tons, which obviously a common vise (type "C") cannot stand. Then, various clamps or rapid action fasteners, which are very useful in this case, should be used. With the pressure exerted, a badly built mold may explode like a bomb. An aluminum or machine finished metal mold is a good choice; resin or wooden molds must not be used unless they are reinforced with metal.
Pressure forming equipment must be stronger than the vacuum forming one. It must have a similar tank for the compressor as well. Piping does not need strict specifica-tions since pressure drop is not considerable. If in a piping pressure drops 5 Psi, pres-sure loss in the system will be 10 Psi, 50% of the prespres-sure. But if the prespres-sure system is 100 Psi, it will be 5%. A valve to reduce pressure and a manometer should be also installed, as well as a baffle or filter at the entrance of the mold, so that cold air is never in direct contact with a hot sheet. Some times, heaters should be incorporated to the air system, since they will help in great blows, which must be kept hot until a piece is formed on the mold.
If possible, there should also be filters to eliminate water that tends to condense in the system and in the long run can make the equipment rusty, in addition, combined with air particles, it can block air ventilation orifices in the molds. Periodical maintenance is a must.
Pressured air forming.
When needed, the mold should have ori-fices to eliminate the air caught inside and avoid wrinkles or deficient forming. Pressured air forming has become popu-lar, specially for small pieces. The advan-tages of this method are: improvement on dimensional tolerance, forming speed can be considerably increased and fine details are better defined.
Acrylic Acrylic Mold Mold Vacuum orifices Air exhaust Vacuum Pressured air
Mechanical forming
The thermoforming process is not limited to pneumatic techniques. There are sev-eral mechanical forces that can be applied. The simplest form of mechanical forming is used for bi-dimensional form-ing. In this case, a heated sheet is placed on the surface of a curved mold which is usually a smooth surface and gravity is enough to curve the sheet; the edge of the sheet should be fastened to keep it in position until it cools. That is the case for
the manufacturing of the cannon arch whose sides are tightly fastened and there is not thickness variation.
Mechanical forming, matrix and male mold.
Matrix-male molding is used, among other things, to shape complicated pieces. In this molding technique, a heated sheet is shaped between 2 opposing but similarly outlined molds (matrix-male). When the molds are joined, the outlines force the sheet to take the same shape, in the space left between the two molds. Any protuberance on the male mold, mechanically, will force the plastic into the counterpart (matrix). For big or medium production, mechanical equipment is used to close the molds; in other cases, the movement is created by servomotors. If both molds have a controlled temperature, cooling time can be reduced.
There are three basic criteria to achieve good thermo-shaping performance when using this technique.
The first, is applied force, regardless of its source (pneumatic, hydraulic or mechani-cal), it must be strong enough to make plastic deform, of course, a huge surface or an intricate mold will need a bigger pressure force.
The second refers to suitable elimination of the air caught inside. The pressure exert-ed between the two molds causes that air gets caught between them and the sheet, and air must be removed to shape the piece well. Boring some holes in one or the two molds in the areas where this anomaly is spotted, can eliminate the air.
The third is related to the depth limit of stretching, that derives from the forces used in the process. It can be easily understood that maximum stretching is only successful when the mold has exit angles bigger than 5° and very big and smooth curve radius-es, the angles close to 90° may diminish stretching and even tear the plastic material.
Mechanically forming with matrix-male molds does not only depend on the forces used, usually, this kind of forming can be combined with vacuum, pressured air or both at the same time. Therefore, the matrix-male mold does not have to coincide accu-rately, the male mold may be relatively inferior in dimensions and have a substantially different form from the matrix.
When male molds are made like this, they can act as "pushers" of a plastic sheet. This kind of support is called mechanical support, because it presses the softened materi-al into the matrix. The purpose of this support is to stretch the materimateri-al so that the finmateri-al form is accomplished in combination of vacuum and/or pressured air.
Using mechanical support in the process has the advantage of a better distribution of the thickness of a product, than using any other process. Many variations in the process can be obtained combining these techniques. Those variations can be vacu-um pressure changes, vacuvacu-um or pressure application time, mold closing speed time or forming cycles.
Usually, mechanical supports are made of wood. Hard or tropical wood is the most used to make supports. In some cases, pieces of other plastic material such as: nylon, rigid polyurethane, acrylic, aluminum or steel, which are easily machine finished, can be incorporated.
If production volume requires it, a cooling and/or heating system can be incorporated. The decision to heat or cool the support, must be made from the beginning of the design, since later on it will be harder if not impossible to try to adapt a heating ele-ment, that is why required machine finishing should be made to incorporate the sys-tem.
When a support is very cold, a sheet will surely get cold on it. Cooling usually takes place between the points of a support and a sheet and the sheet and the mold. In extreme cases, the sheet may shrink on the support during the forming.
The form of a support has a determining influence on the wall or thickness of a fin-ished piece. In the next picture, there are three different kinds of support.
Combined techniques
Mechanical support
Flat surfaced and blunt edged support
Tin-like support Sphere-like support Flat surfaced and blunt edged support
This allows a sheet to stretch between the support and the edge of the mold, and meanwhile, the part of the sheet in contact with the edge of the support gets cool. A piece formed this way will have a thick bottom and thin walls
Tin-like support
In this second alternative, a sheet is in contact with the support and cools fast only on the perimeter of the support. Stretching is similar to that of the flat support, but the central area of the support allows extra stretching.
Sphere-like support
On the other hand, in this case, only a small area is in contact with the support. There might be a significant stretching as the support moves forward, therefore, the area of the perimeter between the edge and the support decreases.
Choosing the type of thermoforming technique Criteria to design thermoformed products
Thermoforming molds
One of the most important aspects to be taken into consideration in thermoforming pieces is the thermoforming technique to be used. Depending on the characteristics of the product if the wrong technique is used, there may be problems before you can get a piece with the specifications initially determined, finished. And many times the oper-ation will fail, with the consequence of a waste of time, money and resources. Thus, before manufacturing a mold, the following should be considered:
1.- Form and dimensions of the piece. 2.- Desired aspect.
3.- Thermoforming technique.
Based on these factors, you can plan and anticipate possible defects in the pieces. In this chapter all the variables that emerge when a thermoforming mold has to be man-ufactured, are analyzed.
It must be mentioned that: products made using thermoforming technique, though this technique is versatile and flexible, regarding aspect and characteristics, differ from products manufactured using injection molding. In the following comparative table the basic differences can be analyzed. To conclude, to design thermoformed pieces the following criteria must be established:
1. - Thinning of material should be considered, this mostly depends on form, size and technique used (chapter 8). Generally, thinning of material is directly proportional to the height of a piece.
2.- A 3° and 5° exit angle of the mold should be considered.
3. - It must be taken into consideration that a piece will contract 0.6 to 1% when it cools. 4. - In general, the surface of a thermoformed piece will be smooth, though some
tex-tures can be obtained.
5.- In designing a piece, big radiuses should be included; there may be edges but they can tear the material.
VARIABLES PROCESO
INJECTIÓN TERMOFORMING
Thickness Mold exit angles Molding temperature Dimensional tolerance Inserts Surface finishing Production Mold
May create ribbings, all types of holes, coils, etc.
Scrap, material waste Radius
Time to make a piece (design, mold, tests).
Subsequent treatment and finishing
Constant 0.5° to 1°
392ºF-464ºF (200°C – 240°C) Excellent
Possible insertion of elements in other materials. Smooth surfaces or any other
texture can be obtained. High production, hundreds or
thousands of pieces a day. Steel with alloys or expensive
treatment, complex design, matrix-male mold.
Yes.
Very little, recoverable. Must blunt edges, about 1.5
thickness of material. From 3 to 6 months. Any treatment or finishing,
paint-ing, hot-stamppaint-ing, serigraphy, metallization, etc.
Variable 3° - 5°
320ºF-356ºF (160°C – 180°C) Relatively good, not for accuracy.
Mold surface can be prepared for inserts
Only smooth surfaces, some shallow textures Medium, some dozens a day. Variety of materials, rather low
cost, simple design, may use matrix-male mold.
No.
Depends on the shape, about 25% waste and recoverable. Larger radiuses, 0.4” to 2”
need-ed. Depending on shape and depth.
Maximum 1 month. Any treatment or finishing,
paint-ing, hot-stamppaint-ing, serigraphy, metallization, etc. Table 12 Basic differences between Injection and thermo-shaping processes.
The following criteria are key factors to successfully produce thermoformed pieces. They are the core of any development, but it is also vital to thoroughly analyze these concepts and later we will see in detail each consideration in the design of molds. Then, these basic criteria and considerations will be the fundamental parameters to manufacture thermoforming molds, regardless of their complexity. It should be noted that when these molds are manufactured, the following concepts must be assessed. 1. - Form and dimensions of the piece.
2.- Aspect of the piece.
3.- Estimated production volume.
Probably the most important of these concepts is the estimated production volume, since it will depend on the definition of the kind of mold, material, finishing, thermo-forming technique, etc. Next, the model designs are shown:
Criteria to design thermoforming molds
1. - A male mold is easier to use, less expensive and more suitable to form deep pieces. In general, a matrix should not be used to form pieces deeper than half the width of the piece. The matrix is used when the concave face of the fin-ished piece must not be in contact with the mold.
2.- The molds must have enough vacuum orifices so that an annealed sheet can conform to the critical parts of the mold, the vacuum orifices have to be made in the deepest parts and areas where air is caught, and must be small enough not to leave marks (1/32" to 1/8" diameter). Vacuum can be more effective if the hole is enlarged from the inside.
3.- There must be ducts that allow water or oil circulation through the mold when temperature control in it is needed.
4. - When the dimensions of a formed piece are critical, molds must be built big-ger to compensate for the contraction of the material.
Expected contraction from molding tem-perature to environment temtem-perature is 1% maximum.
5.-A slight curving of the flat big areas of the mold will allow flat areas when the material cools.
6. - Pieces with 90° walls cannot be obtained; the mold must have an exit angle of at least 3°.
7. - Edges should be blunt, since vertex form accumulates internal efforts. A piece will be more resistant designing blunt edges and corners.
8.- The thin or weak parts can be rein-forced with reinforcement ribs, which will also reinforce big flat areas.
9.- If it is necessary to mold using a per-manent incrustation, you should consid-er: the difference between the expansion coefficient and the various materials, oth-erwise, there can be a failure due to a forced insert, because of different expan-sions and contractions of the materials in contact.
10.- The surface of the molds can be lined with cotton flannel, felt, velvet, suede, etc, to diminish mold marks. The most com-mon is cotton flannel.
Considerations in the design of thermoforming molds
One of the advantages of the thermoforming process is the diversity and kinds of molds that can be made at a very low cost and relatively fast, being highly accepted for other applications, over other processes.
Usually and unlike injection molds, only half the mold is needed and it depends on the form of the product, desired aspect and chosen technique (may be male mold or matrix).
Choosing the right one is much more important when the part to be thermoformed is very deep. When the pieces are shallow, profiles are small or when thinning is irrele-vant, choosing will depend on the aspect of the piece. If details of the mold are impor-tant, then the side of a plastic sheet in contact with the mold surface should be the front of the piece.
Some times, a bigger radius or smooth aspect is desirable if a sheet of material shows a nice surface, then the surface which does not touch the mold will be the front of the piece, besides, a dimensional control closer to the surface of the mold can be obtained.
Thinning of the material
Under every condition of thermoforming when pieces are formed of a plastic sheet, the area of the surface will get bigger, there will be some stretching and the material will get thinner.
One of the decisive factors of this thinning is the ratio, generally defined as maximum depth or height ratio with a minimum space through the opening. To estimate this thin-ning, the area of the available sheet to be thermoformed must be determined and divid-ed into the area of the finishdivid-ed piece, including waste. It is always desirable that the
molds and thermoformed pieces have generous curving radiuses. Theoretically, there is a formula to determine the thinning percentage of the material, considering that the material is uniformly annealed and stretched.
A=3 B=4 C=2 D=1 E=1 A=3 B=4 C=2 D=1 E=1
In practice, with a micrometer or calibrator you can determine thickness directly on the thermoformed piece, cutting small pieces on different sections. Other methods use translucent sheets and correlate color intensity vs. thinning of the sheet. Thickness can also be determined making squares with an oil marker on the sheet before thermoform-ing it and observthermoform-ing stretchthermoform-ing of the material.
Thinning % = Final thickness of the material = =
Original thickness of material
available area of a sheet total area of shaped piece A X B
A X B X E (2C + 2D)
One should consider the possibility of wrinkling on some critical areas or on the bottom of a male mold or matrix. If an annealed sheet cannot contract from the dimension A to E, excess material will cre-ate wrinkles.
In a matrix the opposite happens, the sheet will expand to the 4 vertexes of the mold surface, becoming very thin. This
Next, some techniques to prevent wrinkling are shown:
When low molding temperature is used, a sheet will keep a greater tenacity and elas-ticity. For big pieces, molding time and temperature should be increased on difficult zones to be thermoformed, minimizing this kind of defect. For deep molding sheets, because of their partially cross-linked structure, they tend to minimize wrinkling. When there are many molds, there should be enough room to prevent wrinkling, a distance 1.75 times the height of a piece, is suitable.
Dimensional shrinking and tolerance.
Dimensional shrinkage and tolerance in thermoforming vary for pieces formed on matrix or male mold. On a male mold, shrinkage can be reduced if the piece cools most of the time on the mold. If cooling reaches environmental temperature on the mold, shrinkage will be minimum. Thus, the internal dimension of the piece will be very close to the one of the mold, but then a production cycle will not be productive.
However, the fact is that a piece must be removed from the male mold when it is still hot, otherwise removal will be difficult. This is exactly thermal shrinkage, which is the proportional difference between the environmental temperature and the one at the time of removal. Thus, to keep the specified dimension of a piece, the model must be slight-ly bigger.
On the other hand, a piece formed in a matrix will begin shrinking as soon as the tem-perature of the material is below the one of forming. To keep a close continuous toler-ance, the mold dimension must be considerably increased and vacuum pressure kept during the whole operation.
As a guideline it can be assumed that shrinkage on male molds it is .127 mm/mm (0.005 in/in) and in a matrix it is bigger. For acrylic, polycarbonate, thermoplastic poly-ester and oriented polystyrene .203 mm/mm (0.008 in/in) can be considered. Anyway, one should be cautious about these values, since the following conditions can signifi-cantly alter them.
1.-Mold temperature: a difference of 15°F (10°C) can change shrinkage over 0.001 in/in. (0254 mm/mm).
2.- Size and thickness: this refers to the exit angle limited by the mold and the effect of greater thickness regarding temperature profile.
3. - Final use temperature: Due to expansion and contraction proportional to lineal expansion coefficient, a thermoformed piece will keep on varying with
environmen-tal temperature changes.
4.- Use extreme conditions: Shrinking can reach top values after the first exposition to the highest temperature of use.
5.- Molecular orientation: There might be bigger shrinkage related to the molecular ori-entation of the material.
Some times, to prevent distortion and shrinkage, cooling templates are needed until a piece reaches the environmental temperature. Further more, the pieces thermoformed at a temperature below the one specified, tend to go back to their original state due to the plastic memory of the material. It is advised to monitor shrinkage and deformation during production.
Aspect of the mold.
It must be clarified that the surfaces obtained by injection and extrusion processes cannot be reproduced by conventional thermoforming techniques. Even highly brilliant materials may lose their glow during the process. In addition, they tend to emphasize mark and waving when they touch a cold mold and undergo thickness changes. A change of thickness will cause small distortions. Thus, cleaning the working area is a must. All the outlines should be rounded, actually, a mold with big radiuses will bene-fit the thermoforming operation, since the material will tend to stretch better
If you want a sheet to copy details of a mold, like non-skid textures or similar ones, those detail should be at least three times bigger than the thickness of the material. Actually, it is better to have a not so smooth molding surface, this way, the piece will not copy the mistakes of the mold. It may even be sand-blasted with glass fiber micro spheres or an abrasive material. This way you can eliminate the air caught between the mold and the piece. Some times it is a good idea to sand the surface using rough sandpaper, this helps at the time of removal, to break the vacuum between the mold and the piece.
Superficie lisa, bien pulida Superficie áspera
Vacuum bores
When using thermoforming techniques with vacuum or pressured air, it is very impor-tant to eliminate most of the air between a mold and a sheet in a minimum of time. Depending on the kind of mold, 1/2" or 1" orifices can be used, as in the case of ther-moformed skylights, up to homogenous distribution in all the vertexes of the mold.
These pictures show the distribution of the vacuum pressured air bores, typical for pressure-free forming molds, male mold and matrix
Metallic frame
1/2” or 1” piping
Acrylic
Base
In general, the diameter of vacuum bores should be slightly smaller than the thickness of the material. As a starting point, the vacuum bores will have a diameter equivalent to the final thickness of a thermoformed piece. This rule does not apply when the material is very thin or very thick, or when the marks of these orifices are irrelevant. It can be considered that a suitable range is from 1/32" to 1/8" diameter. To eliminate a
Another function of a mold is to contribute along with a frame to stabilize the position of a sheet and provide good sealing all around the mold. In some cases, a canal around the piece is helpful, exactly on the external zone of the cutting line.
Mold cooling
Some times when production runs are very long, the mold should have a cooling sys-tem, generally copper piping is used. It should be placed adequately and have enough capacity to carry a considerable volume of water or refrigerant. A relationship between the temperature of the sheet and the mold should be established so that the material does not get too cold and it does not thermoform below the bottom limit of the mold-ing temperature.
There are different methods to cool a mold, for example, when there are critical mold-ing zones, plastic or poly-tetra-fluorine-ethylene inserts can be incorporated. In some cases, a plastic covering can be applied to reduce thermal conductivity, or even after thermoforming, pressured air can be injected through the bores or holes. Three cool-ing systems are shown in the next picture: First an undulated coolcool-ing system, the sec-ond is a branch system and the third is an external multiple alternative flow branch system with 2 inputs and 2 outputs.
Widened bores on the inside
Increased diameter bore
great volume of air, 1/8" or _" diameter holes can be drilled. Depending on the manu-facture of the mold, the bores can be widened on the inside of the mold, as shown in the picture. To reduce the time to eliminate the volume of air round a softened sheet and a vacuum box, the space can be refilled with polystyrene foam balls or polyurethane pieces. Undulated system Branch system External multiple alternative flow branch
Mold supports
As it has been mentioned before, when thermoforming a piece the material always gets thinner. Molding supports are used to get a better distribution of material in a thermo-formed piece. Their purpose is to stretch a softened sheet, as a pre-forming. This tech-nique is very important, specially with very deep pieces. In general terms, the molding supports can be made of the same material as molds. There are three categories of mold supports:
Metallic supports
Usually they are made of iron or aluminum, must be very smooth, with radius on the edges. The range of temperature is 10 to 15°C (10°F) below the temperature of the material, if their temperature is too high the sheet will stick to them.
Thermal material supports
These are made of wood, plastic or metal and they are built under the principle of a good thermal insulator. The surface may be of soft wood, plastics like nylon, or anoth-er thanoth-ermofixed, synthetic foam or any othanoth-er matanoth-erial including soft flannel.
Skeleton type support
Skeleton or frame type supports are only rounded bars welded forming intersections, which should be totally rounded to avoid tearing the material.
Support dimensions are related to the size of a piece, since they have a great influ-ence on the thickness distribution of the material. It must be noted that in some cases, by only changing the depth penetration of a support (75% depth of the piece), the thickness of the material between the faces and the surface can be controlled. Therefore, the equipment must have the required depth adjustment capacity, pene-tration power and speed.
Materials used.
Unlike other plastic molding processes, such as injection or compression, thermo-forming has the advantage of using relatively low pressure and temperature. That is why a great variety of materials can be used. Usually, wooden molds can be used, they are ideal for low production and as wood has a low thermal conductivity, it helps the annealed sheet not to cool quickly at first contact, but these molds are not good for medium or high production. Manufacturing molds with phenol laminates are better because they are not seriously affected by heat or humidity.
There are also molds made of mineral or metallic charges and polyester or epoxy or rigid polyurethane resins. These are easy to remove off a mold and may even have a mold with multiple cavities. The thermal properties of epoxy and polyester resins make them suitable for medium production. Copper piping can be used as cooling system to better control the mold temperature, but even then, it is not enough for high production.
Materials used to manufacture thermoforming molds
