The different types of rubber moulding processes

The different types of rubber moulding processes

From bumpers to bushings, suction cups to seals, products made from rubber are commonplace in nearly every industry. Their flexibility, sealing, and insulation properties means that rubber is still an important material in the hi-tech era in which we live. But how are rubber products made? How do manufacturers ensure that rubber products have the tolerances and durability needed? Let’s have a look at the different processes used to mould rubber.

What is Rubber Moulding?

Rubber moulding is the process of transforming uncured rubber, synthetic or natural, into a product by transferring, compressing, or injecting the un-cured rubber into a metal mould. Pressure and heat are applied, creating a chemical reaction, known as curing or vulcanisation. This causes the polymer chains in the material to link together, giving the strength and flexibility we associate with rubber goods.

It was the self-taught chemist Charles Goodyear, who in 1844 received a patent for the process of vulcanising. When rubber is vulcanised, it retains its elastic properties, but becomes stronger and less sticky. Goodyear’s vulcanised rubber could be reinforced by adding other materials such as soot. Goodyear discovered the process by accident when a mixture of rubber and sulphur landed on the stove in his workshop, and he noticed the change in the material’s properties.

The three common processes used in rubber moulding

There are three types of rubber moulding processes which are commonly used today:

  1. Injection moulding
  2. Compression moulding
  3. Transfer moulding

Each process has advantages and disadvantages, as we will discover.

Injection moulding

Rubber injection moulding is a process where a heated rubber compound, or material, is injected under high pressure into a closed mould. The process is very similar to the more well-known plastic injection moulding technique.

Image showing the injection moulding process

Uncured rubber is fed into the injection moulder via a hopper, where it enters a screw chamber and is heated and compressed as it moves towards the mould. As the heated and compressed rubber nears the mould, it is in a highly malleable state, allowing it to flow into the mould and fill it completely. The hot mould then vulcanises the rubber. After a set period the mould can be opened, and the formed rubber released.

Injection moulding has the advantage of being able to produce highly accurate and geometrically complex items. The relatively high cost of tooling makes It good for the mass production of products, but less well suited for smaller runs, or prototyping work.

Compression moulding

Compression moulding is the original way to mould rubber, and is a much simpler process. Heat and pressure are again utilised to change the properties of the rubber.

Image showing the compression moulding process

The mould comes in two halves, a lower fixed half, and an upper movable portion. The mould is heated and the uncured rubber, known as the ‘blank’, is placed in the lower mould. The upper mould is placed on top, and pressure is applied. The pressure exerted on the charge by the blank causes it to retain the shape of the mould. The heat applied valcanises  and changes the characteristics of the rubber.

The main advantage of compression moulding comes from the simpler, and lower cost tooling that is required. The tolerances of the final rubber product are not as fine as those manufactured using injection moulding, and some finishing may be required to remove flashes and parting lines.

Transfer moulding

The final moulding technique combines elements of both injection and compression moulding. The mould consists of two halves, as with compression moulding, however there is an open-ended cavity, known as the ‘well’, into which the rubber is placed. The well is connected to the main mould shape via a network of channels.

Image showing the transfer moulding process

The rubber in the well is pushed by a plunger to apply pressure to ensure the rubber flows into the mould. The rubber then cures and solidifies. Finally, the end product is ejected from the mould.

The process gives similar advantages to that of injection moulding, including very tight tolerances and geometrically complex parts. The process is simpler than injection moulding, and the tooling cost is lower too. Transfer moulding is very suitable for making a range of rubber components.

About Checkmate Flexible Engineering

CFE is a specialist engineering company whose core capabilities are the specification, design and building of fabrications from elastomers, including rubber. We offer a complete service from conception to manufacture. Our products include dunnage bags, specialist seals, peristaltic hoses, tube press pressure bags, inflatable walkways, and bespoke compound. Clients include the UK MOD, Government Research Establishments, international shipping companies, and NATO forces.

If we can help you with your flexible engineering requirements, please get in touch. We would love to hear from you.