Thermoforming techniques
Thermoforming is the simplest and most used process to form an acrylic sheet. Being a thermoplastic material it softens and it is easy to handle and can take any form when heated at suitable temperature and time.
As it cools it recovers its rigidity and keeps the form it was exposed to. The cost of equipment and molds is relatively low and bi or tri-dimensional forms can be obtained by means of a great variety of processes.
This is a bending process that can be achieved through two methods:
Lineal heating bending.
A Chemcast acrylic sheet is heated on a lineal resistor, bending at the desired angle.
To bend, remove the protector paper of the bending line (the rest of the paper may be left to protect the areas that are not to be worked on), then place the sheet on the sup-ports with the bending line directly on the heating line, bending on the heated side.
Heating time varies according to the thickness of a sheet. To bend an acrylic sheet over 0.16” thick it should be heated on both sides to obtain a suitable bend. Heat the sheet until it gets soft on the bending zone. Do not try to bend the sheet before it is well heat-ed, this may cause irregular or creased corners.
Heat carefully, irregular heating may cause arching on the bending line. Some times this is hard to avoid, specially on pieces over 24” long. Arching may be diminished fas-tening the recently formed material with some clamps or a template until it cools.
Templates can be made of wood, fixed or adjustable.
With suitable heating, clean shining corners are obtained
Place the sheet on the support with the fold-ing line directly on the heating line
Use fixed or
adjustable templates to keep the piece at the desired angle
YES No
Acrylic Acrylic
Electric resistor Top
Butts Support with
adjustable hinge at any angle
Cold forming
Chemcast acrylics sheet can be cold formed on curved frames, as long as the radius of the curve is 180 times bigger than the thickness of the material used.
Formula: R (radius) = 180 X T (Thickness of material in inches.)
Three-dimensional thermoforming (with molds).
R=180 X E
The procedures for tri-dimensional forming in general, require using vacuum, pressured air, mechanical equipment, or a combination of these to mold Chemcast acrylic sheets to a desired form. These techniques are described next:
Free or gravity shaping
This method is the simplest of all, because once the material is softened, the sheet is placed on the mold and the material adopts the form by its own weight. The edges of the material can be fastened to the mold to avoid waves that tend to occur when cooling.
Mechanical forming with matrix and male mold.
A Chemcast acrylic sheet can be formed pressing the annealed material between the male mold and the matrix, to produce pieces of very accurate dimensions. This procedure requires excellent finishing of the molds to reduce their marks to a
min-Acrylic
Mold
Frame
Male mold
Free, pressure or vacuum forming The pieces that require optical clarity like skylights, helicopter cabins, etc., can be formed without mold, Chemcast acrylic can be vacuum or pressured air formed.
The form of the finished piece is given by the form and size of the ring that fixes it to the frame and by the given height.
However, these forms are limited to spherical outlines or bubbles freely formed. Vacuum is better for this kind of forming, or pressure if it is over 1 atmos-phere.
Vacuum and pressure forming, matrix.
This procedure allows forming pieces, on 1 piece molds whose form requires more accuracy than the ones vacuum formed.
However, high pressure leaves marks of the mold on the piece. As high pressure is required, the molds should be of metal, epoxy resins or other materials that can stand high pressure without deforming.
Good finishing of the molds is a must to obtain quality pieces.
Pressure forming with the help of a pis-ton and matrix
The technique of piston help is used to reduce thinning at the bottom of the formed pieces. The piston stretches the material before pressure is applied. Piston speed of 6.6 yd./min., is required, it may damage the material at initial contact.
Forming pressure 6.16 pounds/in2
Presión de aire
Vacío
Vacuum with return and male mold forming.
This technique is useful to form pieces that require uniform thickness on the walls and fewer forming marks. An annealed sheet is stretched in a vacuum box until it reaches the necessary depth for the mold; once it is inside it, vacuum is freed gradually so that the acrylic returns to its original form meeting it.
More defined forms can be obtained if at the point of returning, vacuum is applied to the male mold
Pressure forming with the help of a pis-ton, matrix and vacuum.
This is the most sophisticated of all, since it is a combination of almost all the oth-ers, it is generally used for very deep thermoforming which requires more con-trolled thickness and when breaking is possible because of excessive molding depth.
Vacío
Vacío
Vacío
In this section we will try to expand the techniques mentioned before. Although these examples are designed for infrared heating equipments, it is possible to apply them to the conventional molding systems.
Infrared heating furnace
molding techniques
Vacuum forming, matrix and mechanical support
Vacuum forming, matrix and mechanical support Vacuum forming, matrix Pressured air pre-stretching, mechanical support and vacuum
Pressured air pre-stretching, matrix, mechanical support and vacuum
Pressured air stretching, mechanical support and vacuum
Vacuum forming, matrix, mechanical support and pressured air.
Free pressured air forming Free pressured air forming