Compression molding is a manufacturing process used to create various products and components, primarily from thermosetting materials such as composites, rubber or resin material. It offers a variety of unique options for manufacturing functional prototypes as well as for low-volume products.

What is Compression Molding?

Compression molding has become an essential technology in many industries, contributing to the development of modern products that we use every day. The technique involves compressing the raw material within a heated mold tool, usually under heat and pressure, until it takes the desired shape. The molded parts are durable, economical and suitable for many applications.

How Does Compression Molding Work?

This molding process uses heat and pressure to form the part’s final desired shape. It involves a measured amount of molding material (preheated) called “charge” which is compressed into the desired form using two heated molds. These molds usually have a pre-heated cavity and a vertical press mechanism to effectively construct and maintain the required tooling.

The excess material seeps out of the mold in the form of a flash (the excess plastic that is usually forced out of the mold cavity at the parting line between the two parts of the mold).

This molding process works as follows:

• Step 1 – Create molds: Produce tooling using methods such as machining, die casting, or 3D printing.
• Step 2 – Set up the machine: Clean the mold, turn on the heat, and complete other necessary set-up procedures based on the machine or device in use.
• Step 3 – Prepare charge: Choose the material and determine the appropriate amount of material. Avoid using too much charge, as this can cause excess material to seep out of the mold, leading to flash that requires manual cutting.
• Step 4 – Insert charge: Position the charge in the centre of the bottom mold.
• Step 5 – Compress part: Close the top mold, apply pressure, and allow time for the piece to form. Some manufacturers use heat during compression, which can quicken production by softening raw materials.
• Step 6 – Release part: Remove the finished piece from the mold.
• Step 7 – Clean part: Remove any resin flash along the edges manually, and clean the part as needed before final assembly.

Types of Compression Molding

There are several types of molding techniques, and each method differs in terms of its application and the materials used.

Industrial Hydraulic Compression Molding

Large industrial compression molding machines are ideal for commercial large-batch part production. They employ hydraulic lift systems that make it possible to attain a high level of pressure to manufacture and produce large, uniform parts. Workers have full control and monitoring over the process from a separate control panel.

Benchtop Presses

Smaller benchtop compression machines are best to manufacture small to midsize parts. Benchtop presses usually take up less space compared to large industrial compression machines but is best for large runs. They have multiple access points that give engineers and operators the flexibility to easily remove finished parts and the produced flash. Also, the smaller size and footprint make it ideal to employ these machines in smaller production facilities or warehouses.

Cold (Heatless) Compression Molding

This technology is usually done by using heat but on the other hand, there is also cold (heatless) compression where the charge or raw materials are generally preheated before being put into the mold. For softer plastics, even heating is not needed. The engineers and manufacturers can control the procedure and curing and also shorten the production cycles by the timely introduction of heat.

Wet Compression Molding

The technique of wet compression molding is used to combine epoxies, resins and many other liquids with solid fabrics. Wet compression machines are very common in the production lines of car manufacturers like BMW where they produce strong and durable products in less time and low cost.

Suitable Materials for Compression Molding

Plastics, thermoplastics and thermosets are compatible with the compression molding process. Thermoplastic materials are plastics that on heating become soft and flexible and harden when cooled. They show no changes in the chemical and mechanical properties even after hardening and hence they can be heated and cooled (or reshaped) numerous times.

Whereas for thermoset materials, the characteristics are different. They become irreversibly cured on exposure to heat and all their inherent properties are changed and permanently cured to stay in the given shape. Hence, they can only be molded once and cannot be repeated or undone. 

Materials used commonly in compression molding include:

• Epoxy
• High-density polyethylene (HDPE)
• Polyether ether ketone (PEEK)
• Polyphenylene sulphide (PPS)
• Polytetrafluoroethylene (PTFE)
• Silicone
• Urethane

Advantages of The Compression Molding Process

Let’s take a deep dive into the compression molding advantages.

Cost-Effectiveness

Compression molding can be termed the most cost-effective manufacturing method for the production of simple, flat and large parts. Small design complications like pockets and curves are acceptable whereas much more complicated designs are often not possible with compression molding. 

Since it employs low pressures, tool costs are cheaper and the molds last long without any defects like warping eliminating the need to be replaced. The main problem of low cycle times can be countered by producing molds with multiple cavities to produce more parts during the same cycle.

Produces Strong Parts

The process works by placing the charge into the mold as opposed to injection molding where the charge is injected into the mold, which theoretically eliminates the flow and knit lines and defects like short shots. The structural stability of the compression molded parts is also higher compared to injection molding.

Manufacturing with composite materials is easily possible with compression molding. Composite parts offer unique properties along with being strong and durable.

Flexibility in Design

One of the chief advantages of compression molding is its ability to offer cheap prototyping. For example, cheap molds can be easily designed on CAD software and 3D printed. Later, this cheap mold can be used to perform compression molding for research and prototyping before the actual mold is built. This way, it can be more advantageous to tool designers and engineers. This manufacturing process is also a good solution to produce consistently coloured rubber parts.

Limitations of Compression Molding

Although this technology has many benefits, it also has a few limitations:

• Not suitable for complex parts. When the design is complex, for example, the design has intricate details or slant angles, it’s advised not to use compression molding as it cannot achieve those. 
• Slow cycle time. It lasts for minutes compared to injection molding which is usually seconds. The associated labour time is also increased because of the slow cycle time and hence the cost of labour.
• Creation of flash and burrs. They need to be removed manually which results in extra time and material wastage.

Applications of Compression Molding Manufacturing Process

Compression molded parts offer a plethora of applications in every commercial industry:

• Automotive parts: Many interior parts of cars and bikes can be produced using compression molding. Big parts like tractor bumpers and engine components using plastic are produced cheaply with the help of compression molding. 
• Computer, gaming devices and consoles: The keyboards for computers, monitor frames and controllers for video games and buttons can be compression molded. 
• Kitchenware: Plastic dishes like plates, cups and cutlery can be compression molded. Especially the parts that are produced using melamine are usually compression molded.  
• Electrical components: Electrical sockets, switches, faceplates, and metering devices are usually manufactured with this technology.
• Medical and dental device parts: Many plastic and silicon medical and dental equipment used in daily life, like syringe stoppers, equipment handles, and respirator masks are manufactured using this process.

What is the Difference Between Injection Molding and Compression Molding?

The main difference in operation relies on the fact that, in injection molding, the charge is introduced as pellets into the injection mold and on the other hand, in compression molding, the mold is closed or pressed against the charge to get its shape. 

Manufacturers use both technologies these days but for different types of parts and have different applications. Sometimes there are big and ultra-large parts for which an injection mold cannot be produced. That is when a compression mold is ideal. On the contrary, for complex molds, injection molding is an ideal option. If the design is simpler, compression molded solutions remain cheaper.

Because the material is loaded into the mold cavity directly, there is no limitation on the weight of the part that can be made other than the size of press and tonnage required, whereas injection molding presses are limited in the weight of part they can produce by the volume of the injection barrel that fills the mold.

In terms of speed of manufacturing, injection molding has a shorter cycle time and is hence faster and more cost-effective for large batch production compared to slow compression molding. On the other hand, the compression molding technique uses low pressure and is also cheap for the same reason. The wastage is also reduced and can be advantageous when working with costly materials. 

In conclusion, for high-volume production and complex parts, injection molded parts are ideal whereas, for medium to small batches and simple designs, compression molding is recommended.

Conclusion

Compression molding is a manufacturing process that wastes less material and has a lower cost tooling than injection molding since no runners, sprue, or gates are required to form the part. Consequently, this technique produces an aesthetically and functionally superior surface finish. This makes it the go-to technique for large flat and curved components.

The simplicity of compression molding machines makes the process an attractive option for functional prototyping and low-volume manufacturing. In addition, compression molding is the best option when the part material used is impossible to produce using other molding techniques and custom composites are fabricated.

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