Compression Molding Processes

Compression molding is one of the most common plastic shaping processes that they generally apply to thermosetting polymers. There are various advantages and disadvantages of this process over plastic injection processes. In here;

  • A general explanation of compression molding processes,
  • Materials used and parts produced in this process,
  • Advantages and disadvantages of these processes.

Working Principle of Compression Molding Process

In compression molding processes, we shape thermosetting materials between two halves of the mold that constitute the cavity of the produced product. We place a piece of plastic material on the bottom half of the mold and the upper half presses on this plastic material.

Compression molding process.
Illustration of the compression molding process.

They preheat the bottom half generally. And the piece of plastic material that they place inside the mold, is also heated with such systems. The piece of the initial charge of material can be in the form of pellets or a semi-molten state. We can make the heating of the initial plastic piece with heated barrels which includes screws inside it, just like plastic injection molding, infrared heater, or convection ovens. Pre-heating of charge shortens the process cycle time.

Adjustment of required curing time for thermoplastic material is also a very important parameter. Curing of thermoset material takes place inside mold which is not open. We use hydraulic systems generally to push or pull the two halves of mold in these processes.

Types of Compression Molding Machines

Compression molding machine.
A typical compression molding machine.

In general, we can classify the types of these machines that we use automation or not. According to this aspect, there are three types of machines available.

  • Full Automatic Molding Machines: These machines are fully automatic in terms of the process cycle. All the process steps such as loading the charge, compression, and de-attaching the part from the mold completed by automatic systems.
  • Semi-Automatic Molding Machines: In these types of these machines, operators perform the loading and de-attaching generally by the operator. Compression of mold halves done in specific time intervals.
  • Hand-held Molding Machines: Operators are performing all the process cycles. Loading of plastic charge, de-attaching it, etc.

Which Materials does the Compression Molding Process Use?

As we stated above, these processes generally use thermosetting polymer parts for production. Urea-formaldehyde, urethanes, epoxies, and phenolics are the most used types of thermosetting polymers in compression molding processes.

Electrical parts and electrical plug coatings are the most important products that they produce with these processes. They produce plates and pot handles also with thermosetting polymers in compression molding machines.

Advantages and Disadvantages of Compression Molding Process

The most important advantage of these processes is that they produce simple parts generally. So, the tooling required to obtain mold structures is also simple. Because of that, the use of compression molding processes is cheaper if we compare it with other plastic shaping processes.

The produced scrap is nearly zero in these machines, which means there are no waste materials taking place from the production process. Waste means additional cost to complete the mechanism.

Because of the uniformity of parts and curing inside these molds, lesser residual stresses occur inside produced parts.

The main disadvantage of the compression molding process is the long process cycle times if we compare it with other processes. Curing of thermosetting leads these longer times for these processes.

Compression Molding of Polymer Matrix Composites

Compression molding is a very common application in the production of conventional plastic parts. We use these characteristics also in composite part production. But these composite parts must include thermosetting matrix material to establish the compression molding process.

The difference between these plastic matrix composite processes from the conventional one is the heated molding halves that are supporting the occurrence of required curing. 

You know that also, composites must include reinforcement phases which are generally fibers, flakes, or particles. So, we must establish a very optimized flow to obtain the intended structures of composites. 

FRP compression.
A very big FRP compression molding machine(Image

Different Methods of PMC Compression Molding Processes

In general, there are three types of these processes that they use to produce different types of polymer matrix composites. 

Electric Reservoir Molding

Electric reservoir compression molding is a special process to produce thin foam plates that include thin cloth surface layers. Like in the conventional process, there are two halves of molds. 

The foam core which they impregnate with resin on the lower half of the compression mold. They place this foam core also between two thin dry fiber walls to obtain a sandwich structure. 

When they press the upper half of the mold on the lower half with moderate pressures(Generally around 0.5-1 mPa), the resin inside the foam core is released to wet the dry outside fiber layers. After the curing of the resin, we obtain completely thin sandwich structural layers.

Molding of FRP Compounds

They produce FRP compounds as starting material for secondary applications like this process. According to the type of the starting FRP compounds, we can call these processes, BMC molding, TMC molding, or SMC molding. These are the general types of FRP compounds. 

These FRP compounds are cut into specific sizes, and they are formed with secondary processes to obtain the required shapes. 

Preform Compression Molding

In this kind of this process, precut mats are placed in the lower half of the mold. Resin is also placed as pellets or sheets. With the application of compression of the upper half, the resin is flown between the mat layers. 

With the application of required heat, the resin is cured and fiber-reinforced molding parts are produced. 


In the plastic industry, the use of thermoplastic polymers is more common. So, this process is a special process to produce thermosetting parts, around other plastic shaping processes used.

Also, if we think about all the composite products, engineers can develop new production techniques to obtain the required composite products like above. 

Do not forget to leave your comments and questions below about the compression molding process and its parameters.

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What is compression molding?

It is a process that two of the high-temperature metal halves to create molding compounds for the liquid thermosetting materials. So, we produce thermoset parts from these machines. They are very important in the production of the parts from these materials. Also, the curing of the material is very important in the two halves of this process. So, we need a specific time after the compression takes place on these machines.

What are the steps of the compression molding process?

The general steps are;
– Firstly, we place the material inside the molding halves to create the part.
– Then we heat the material to soften and it takes the shape of the mold.
– And then the hydraulic system presses the two halves of the compression molding machine.
– After a while, the material takes the shape of the cavity. And curing takes place for the solidification of the thermoset material.
– Finally, when the material solidifies, the ejectors are ejecting the material from the mold.

What are the three advantages of compression molding?

There are various kinds of advantages of these processes. The main advantage is, we can easily obtain the parts from thermoset materials in terms of serial production. So, it will be very easy for the serial production of the thermoset materials.
The second important advantage is, we use only one mold to produce numerous parts. And this is a very important aspect.
The third advantage is, the total cycles to produce thermoset parts are very rapid if we compare it with the other processes.


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