Rubber components and products are created through the moulding process, where the rubber material is shaped and formed. The moulding process not only shapes the parts but also subjects the rubber material to chemical reactions that toughen and harden the polymer chains of the raw rubber. This chemical reaction causes the long polymer chains to cross-link, preventing them from moving independently. As a result, the rubber material can expand and contract under stress.

Most rubber moulding processes involve heat to reduce curing time. The three most commonly used types of rubber moulding are compression, injection, and transfer moulding. Additionally, extrusion is another method used to manufacture rubber parts and components, but it does not require the use of a mould.

Before starting the moulding process, there are several necessary steps that need to be taken, which will be discussed in this blog.

Compression Moulding

Compression moulding is a process that entails placing pre-formed rubber raw or unvulcanized materials into a mould cavity for curing. This method was first used by Harvey Firestone in the 1890s when he produced the first modern rubber product in his home oven. Compression moulding is the original method of rubber moulding, and it has not undergone significant changes in the past 130 years.

How it Works: 

The compression moulding process typically involves mixing raw rubber materials, usually in batches of 500 pounds. The rubber compound or mixed raw material is then shaped into a pre-form that matches the desired shape of the final product. The pre-forms are loaded into an open mould, which is formed by two halves of the same mould that come together to create a closed space.

Once the pre-forms are loaded into the mould, the mould is closed, and heat and pressure are applied to the mould to cure the rubber. The rubber material takes the shape of the mould cavity during the curing process, and the pressure applied ensures that the rubber is compressed uniformly, creating a dense and strong final product.

After the curing process is complete, the mould is opened, and the finished product is demoulded. This is typically done by hand, where the moulded rubber is carefully removed from the mould to reveal the final product.

Pros of Rubber Compression Moulding

• Less waste
• Reduced tooling costs
• Improved processing of stiff, high durometer materials
• Quick lead times

Cons of Rubber Compression Moulding

• Added labor is needed to pre-form the material
• Slower cycle times due to manual demoulding

Compression Moulding Applications

Compression moulding is widely used for manufacturing a variety of rubber components and products. Here are some common applications of compression moulding:

• Automotive components: Compression moulding is used to produce various automotive components such as gaskets, seals, hoses, and vibration dampers.

• Industrial seals: The process is also used to manufacture industrial seals that are used in various applications, including oil and gas, chemical, and food processing industries.

• Electrical insulation materials: Compression moulding is used to manufacture electrical insulation materials such as electrical connectors, circuit breakers, and switches.

Transfer Moulding

Transfer moulding is a moulding process where a preformed material is inserted into a moulding chamber and then forced into a cavity for curing. This process is similar to compression moulding in that it also involves placing preformed rubber materials into a mould, but transfer moulding utilizes more complex moulds and tooling methods.

How it Works

In transfer moulding, the raw material is first mixed in batches of around 500 pounds. Then, the pre-formed rubber material is loaded into a “pot” in the moulding chamber. From there, the material is forced into the part cavity through “sprues” by a “plunger.”

During the curing process, heat and pressure are applied to the rubber material, causing it to take the shape of the mould cavity. Once the curing process is complete, the final part is de-moulded.

Pros of Rubber Transfer moulding

• Higher accuracy and consistency in the final product.
• Tighter dimensional tolerance in the final product
• Simpler and fewer pre-forms are required, as one pre-form can fill multiple cavities
• Easier to produce colored rubber parts
• Reduced processing time due to the ability to use multiple cavities

Cons of Rubber Transfer moulding

• Slightly higher tooling costs compared to other rubber moulding methods.
• Increased waste due to the flash pad or excess rubber left in the pot after the transfer, which must be recycled or discarded.

Transfer moulding Applications

Transfer moulding is commonly used for creating parts that require high accuracy and have lower volume requirements. It is also used for:

• Bonding rubber to delicate metal parts
• Creating composite seals with insert moulding
• Utilizing mould designs involving multiple cavities

Injection moulding

Injection rubber moulding is a process that involves injecting molten polymer materials under high pressures and speeds into closed moulds. This method was developed in the 1960s as an extension of the plastic industry. Despite advancements in rubber moulding technology, injection moulding remains one of the most efficient ways to mould rubber in many cases.

How Rubber Injection moulding Works

The injection rubber moulding process involves injecting molten polymer materials under high pressures and speeds into closed moulds. The raw material is first mixed, typically in 500-pound batches, and then stripped immediately after mixing into continuous strips measuring approximately 1.25″ wide and 0.375″ thick. The strip is then fed into a screw on the injection moulding machine, which charges a barrel as needed with a predefined amount of material. The mould is then closed, and the material in the barrel is injected into the mould cavities. Heat and pressure are applied to vulcanize the rubber, and the final part is de-moulded. 

Pros of Rubber Injection moulding

• High production rates
• Ability to use a variety of rubber materials such as silicone, neoprene, nitrile, and EPDM
• Minimal waste
• Fast cycle and production time
• Complete elimination of time and labor required for making pre-forms and operator placement of pre-forms

Cons of Rubber Injection moulding

• The method is not effective for lower production quantities
• The setup and changeover costs can be extremely high

Injection moulding Applications

While rubber injection moulding is not recommended for prototyping due to the high initial costs associated with creating moulds, it is an excellent choice for mass-producing a wide range of rubber products, such as electronic components, seals, gaskets, and consumer goods. Injection-moulded rubber products are widely used in industries such as aerospace, medical, and automotive.

Which Type Of Rubber moulding Is Right For Your Application?

The choice of custom rubber moulding process depends on several factors, such as the type of rubber material, the final product requirements, batch size, production rate, and cost. Each process has its advantages and disadvantages, and the appropriate process should be selected based on the specific needs of the application.

Flextron has great expertise in reusing existing tooling or finding the best cost-effective manufacturing procedures for your new part.