The Protolis guide to compression molding

How does compression molding work?

Compression molding is a straightforward process where pre-measured rubber material is placed into a heated mold, compressed under pressure, and cured into its final shape. Its simple tooling makes it not only reliable for production but also excellent for rapid prototyping, helping teams validate designs quickly before scaling.

Explore more information about rapid prototyping in Protolis: Rapid prototyping

• The compression molding process

The compression molding process depends on five critical steps that must be carefully followed to ensure consistent product quality. From machine and material preparation to compression, deflashing, and inspection, each stage helps prevent defects and ensures the final parts meet required specifications. Precision in these steps is essential for reliable, high-quality results.

1. Machine preparation: The process starts with the compression molding machine, where the mold is preheated to optimize the curing of the rubber. The temperature settings and compression time are crucial as they affect the quality and characteristics of the final product.
2. Material preparation: Rubber preforms are precisely weighed and checked before they are placed in the mold. This is important to prevent defects like weak parts due to underfilling or excess material (flash) due to overfilling.
3. Compression process: In this central phase, the preform is placed into the heated mold, which is then closed to compress the material. Heat and pressure are applied, causing the thermoset to conform to the mold’s shape. During this stage, any trapped air is also expelled in a degassing step to prevent voids or weak spots in the final product.
4. Deflashing: After the thermoset material has cured, the mold is opened, and the excessive material is trimmed away. This step ensures that the final product meets precise dimensional requirements.
5. QC inspection and final product: The last step involves a thorough inspection of each part to ensure they meet the required specifications. Parts that pass the inspection are then packaged and prepared for dispatch.

This method is valued for its simplicity and cost-effectiveness. It is especially suitable for producing larger volumes of standard products. However, by adjusting the mold design and processing conditions, it can also be adapted for smaller batches or more customized specifications.

• Types of rubber compression molds 

While the overall process is consistent, there are three main types of molds commonly used in the industry: flash, positive, and semi-positive. Each has its own advantages depending on part design, production volume, and quality expectations.

1. Flash-type mold: Flash-type molds are the most commonly used for compression molding. The mold plates close with a small clearance, allowing excess material to escape and form flash at the parting line. This flash is removed after molding. Because they are simple to manufacture and tolerant of variations in material volume, flash molds are ideal for prototyping and small batch production. They allow for quick iterations, lower tooling costs, and fast turnaround without compromising functional performance. 
2. Positive Mold: Positive molds are built for precision. The cavity is fully enclosed, with no room for flash. The rubber charge must be exactly measured and positioned. This approach produces clean, consistent parts with minimal post-processing. However, it is less forgiving, slower to set up, and more expensive to tool. Positive molds are generally reserved for applications requiring high dimensional stability or cosmetic perfection, typically at later stages of development.
3. Semi-Positive Mold: Semi-positive molds offer a compromise. They allow limited, controlled flash to escape into overflow grooves. This makes them more flexible than positive molds, while still delivering a cleaner finish than flash molds. They are a solid choice for medium-volume production when a cleaner part is needed, but without the cost or rigidity of fully enclosed tooling.

• Key equipments for compression molding

The successful execution of compression molding depends not only on process control but also on the use of specialized machinery designed to deliver precision, consistency, and efficiency. Each piece of equipment within a molding facility contributes to the reliability of production and the quality of the final components.

Compression molding relies on several types of machinery to ensure the efficient production of rubber parts. Core equipment includes:

• Large tonnage Press: These presses vary widely, with capacities ranging from 50 to 1,500 tons, accommodating different sizes and complexities of molded products.
• Heating chamber: A dedicated chamber or oven heats the material to the required temperature for molding.
• Mold: The mold comprises two halves—upper and lower—that are precision machined to form the desired product shape.
• Automation systems: These include conveyor systems, hoppers, robotic arms, temperature control units, and mechanical ejection systems, all designed to streamline the molding process.
• Air: Compressed air systems are employed for cleaning the mold cavities and part surfaces, ensuring they are free from debris.
• Heating: Long, slender cylindrical electric resistors are used to maintain consistent heating.

Together, these tools play a vital role in the setup and operation of a compression molding facility, enhancing both productivity and product quality.

Continue your exploration of compression molding in other chapters.