1.0 Objective
1.1 To develop practical skills in compression molding technique for polymer materials.
1.2 To investigate the effect of cure time and cure temperature to the compressed polymer product.
2.0 Introduction
Compression molding is designed to facilitate the replacement of metal compound with polymer and other composite. A compression molding machine is a kind of press which is oriented vertically with two molding halves (top and bottom halves). The method used in compression molding is polymer is being preheated and placed into an open, heated mould cavity. After the preheating process, the mould will close with a top plug and pressure is applied to force the material to contact all areas of the mould. The process (heat and pressure) is maintained until the polymer cured whether in room temperature or at the elevated temperature.
Figure 1 : Compression Molding Process
This molding process can be employed with thermosets, thermoplastics and advanced composite thermoplastics but thermosets are the most used material in this molding process. Compression molding is a high volume and pressure molding method, which is suitable for producing complex and high strength products.Compression molding process has high production rate since the mold cycle time is short, other than that it can produce a good flexibility in parts design and the raw material wastage also minimum.
3.0 Component and Equipment
Temperature, time and pressure required was set for curing or hardening of sample for the two platens hot press
Rectangular mould was pre-heat by placing it on the lower platens of hot press until the hot press temperature achieved.
Heated mould was removed fromlower platens and plastics resin was placed in the mould cavity.
Line the mould plate with plastics sheet for both side to get smooth surface of final product.
Press start button of the machine to move up lower platens until the mould nearly touch the upper platens.Sample was pre-heat for 5 minutes, then the press is closed
completely for an additional 8 minutes
Return button was pressed to allow lower platens return to its original position when compression process was completed.
When cure was completed, the cured sample was immidiately taken out and cooled at room temperature using water or cooling machine.
Cured sample was trimmed and cutted into required shape.
5.0 Result and Discussion Table 1 : TPS : PLA
TPS : PLA [20:80]
Melting Temperature 180 Cᵒ
Pre-Heat Time 6 min
Test Time 6 min
Cooling Time 3 min
Table 2 : HDPE
HDPE
Melting Temperature 180 Cᵒ
Pre-Heat Time 3 min
Test Time 4 min
Cooling Time 5 min
In this lab, we are using HDPE and TPS:PLA sample to be test with compression molding machine. TPS:PLA sample had same melting temperature with HDPE which is 180 Cᵒ but it have a slight difference in pre-heat time, pressure time and the cooling time. This might be caused by the difference in properties for each sample. TPS:PLA have longer pre-heat time than HDPE which is 3 minutes more than HDPE, the test time for TPS:PLA also longer than HDPE by more than 2 minute than HDPE. It is different with cooling time, the cooling time required for HDPE is longer than TPS:PLA.
7.0 Lab Question
1. Define the method of compression molding.
The basis of this molding is the female mold and a male mold plunger or can be known as platen and for the male it is usually using hydraulic system which come together with a pre-heated, carefully measured quantity of sample between them and partially cured. Pressure is applied after those processes and the pressure applied might reach hundred tons. Cooling need to be included in the process before the mold is opened so that the component is not ejected. After the cooling process, the mold can be open to see the product produced from the sample used. Excessive material or known as flash may be found after the mold is open, the flash need to be removed.
2. Determine four primary factors in a successful compression molding process. a) Amount of materials.
b) Heating time and technique. c) Force applied to the mold. d) Cooling time and technique.
3. What are the pros and cons of compression molding.
PROS CONS
Good surface finish with different texture and styling can be achieved.
Due to expensive machinery and parts, the initial capital investment associated with compression molding is high.
Raw material wastage is minimum. Uneven parting lines
Maintenance cost is low. There is limitation on mold depth Extra features like inserts, bosses and
attachment can be molded in during the processing.
Sometimes secondary processing (trimming, machining) of product is required after compression molding.
High part uniformity is achieved with compression molding process.
The process is suitable for high production volume. It is not economical for making small number of parts or for prototyping applications.
Production rate is high as the mold cycle time is in few minutes.
Twisting and shrinkage in product is reduced therefore dimensional accuracy
8.0 Reference
1. Sperling, L.H. (2001). Introduction to Physical Polymer Science. New York: Wiley- Interscience.
2. Strong, A.B. (2000). Plastics: Materials and Processing. 2nd. ed. New Jersey: Prentice- Hall.
3. http://www.plasticmoulding.ca/techniques/compression moulding.htm
4. http://dtresource.com/compression-moulding.html
5. http://www.plasticmoulding.ca/techniques/compression_moulding.htm
