Compression Molding

How Does Compression Molding Work?

01 Mold Design

The design of tools for compression molding are similar to other types of molding: Design for flash by including overflow grooves or a flash design to allow excess material to escape. The mold must have a cavity to form the external shape and a core for the internal shape, with a parting line where they meet.

02 The Compression Molding Process

Measure, weigh and shape the material (called “the charge”) and place it in the lower half of a heated mold cavity. Close the mold and apply pressure, which forces the material to spread evenly throughout the cavity. Heat and pressure are held for a specific time to allow the material to cure or solidify. The mold is opened and the finished part is ejected from the mold, often using an ejector pin.

03 Post-Processing

Compression molded parts often contain flash, which needs to be removed. A secondary curing process may be needed for some materials to fully harden the part and achieve its final properties, especially for thermoset materials. Holes, threads, or other features can be added using CNC machining. Heat staking of inserts and ultrasonic welding are common processes used to join compression molded parts to other components.

Is Compression Molding the Right Choice for my Application?

Advantages

Lower Tooling Cost

Cost is one of the driving factors behind the popularity of compression molding. Compression mold tooling is much cheaper than tooling for a comparable process like injection molding. Unlike injection molding, compression molds do not require the gates and runners.
Cost-Effective for Short Production Runs

The lower tooling and setup costs make compression molding economical for smaller production quantities.
Improved Dimensional Stability

Compression molding avoids the flow and knit lines that can cause defects (common in injection molded parts), leading to greater dimensional stability and lower warping. There’s also less need for post-processing to remove marks left by gating and knit lines.
Strong & Durable Parts

The high pressure and heat of compression molding create dense, solid parts with high structural stability and excellent mechanical properties.
Insert Molding

Compression molding can easily incorporate other materials or components, such as metal inserts, during molding.

Disadvantages

Longer Cycle Times

The process requires extended time for heating and cooling, making it significantly slower than methods like injection molding.
Hard to Automate

Compression molding is less automated than other processes, requiring more manpower for loading material and removing flash, which increases labor costs.
Potential for Defects

Compression molding can be susceptible to defects like voids or air bubbles if the material doesn’t flow evenly.
Limited Complexity

Using compression molding. It’s difficult to produce parts with highly complex or intricate geometries, thin walls or sharp details.
Lower Precision

Compared to other types of molding, compression molding may be subject to dimensional inconsistencies. That means it’s not suitable for applications requiring very tight tolerances.

What Industries Utilize Silicone Molding?

Compression molding is favored by many industries for parts that are large, complex, or require high strength, consistent performance and durability.

Aerospace

Compression molding is commonly used to build structural elements like beams, channels and stringers. They’re often incorporated into satellites and launch vehicles. Seals, o-rings and integrated fasteners are also used in this demanding industry.

Medical

The durability of compression molded parts make them a preferred solution in many types of medical products, including respirator masks and components, prosthetic limb parts, diagnostic equipment housings and implantable devices, such as vascular grafts.

Automotive

Tough, durable compression molded parts are typically used on exterior and interior body panels like fenders and bumpers, engine and under-the-hood components, dashboards and interior trim, and seals and gaskets.

Electrical

Compression molded parts are used in many consumer devices, including appliance housings and control panels, electrical sockets and switches, and insulators for wires and circuit boards.

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