Different Types of Rubber Moulding
Multiple rubber moulding processes have been devised over the years as a result of unique manufacturing situations having different requirements and limitations. Hence, in order to choose the best rubber moulding method that will yield the desired result, it is important to understand the process, the pros, and the cons of each.
Three commonly used types of rubber moulding are:
- Compression moulding
- Injection moulding
- Transfer moulding.
There is also blow and vacuum moulding, however, these are mostly used with plastics.
Compression moulding is the oldest and simplest of the three methods. Unvulcanised rubber is placed into a two-part clamshell style mould which is then closed and heated. As the rubber in the mould heats up, it expands and fills the mould cavity. When the correct pressure and temperature are reached, the rubber vulcanises or cures, meaning the rubber will keep its shape as it cools and is removed from the mould. See Figure 1.
The advantages of compression moulding are:
- Low levels of wastage
- Simpler tooling and more cost-effective when compared to injection moulding.
- Most suitable for parts with a large cross-sectional area
- Greater capacity to process stiff, high durometer materials.
The disadvantages of compression moulding are:
- More difficult to maintain consistency of finished parts.
- Slower processing times.
The injection moulding process is used to produce products that require tight tolerances with high precision and accuracy. Raw rubber is heated until it is molten, meaning that it will flow for easily into the mould cavity, and is supplied to the mould through a nozzle and barrel assembly that provides a continuous flow. The preheating process shortens curing time and allows for higher curing temperatures. Loss of material, or flash, is limited by tightly clamping the mould. The liquified rubber fills the mould cavity and packs tightly to the walls of the mould to form the desired shape. Originally this method was limited to the plastic industry but in the 1960s manufacturers started use this method for rubber parts as well. See Figure 2.
There are a range of benefits that come from rubber injection moulding, such as:
- Minimal material wastage
- Fast cycle times
- Fast production time
- High dimensional tolerances.
However, as with any manufacturing process, there are some disadvantages:
- Increased setup costs
- Restrictions on the design of the tool and ultimately the finished part
- The requirement of more expensive tooling and machinery.
Transfer moulding has similarities to both compression moulding and injection moulding. Initially, the unvulcanised material is compressed, very much like compression moulding, but that compression forces the material through a small channel into multiple closed moulds which moulds the part, like injection moulding. Transfer moulding is a highly efficient process with short tool loading times. The process allows for high production runs though parts do not have tight tolerances. The material cures during the process, after which the mould is split, and the parts are released. See Figure 3.
The advantages of transfer moulding are:
- Transfer moulding is more accurate and consistent than compression moulding.
- There is a short manufacturing phase than compression moulding because multiple cavities can be used.
The disadvantages of transfer moulding are:
- There is more chance of material wastage.
- Inserted transfer moulding tools require repair time, removal, and reset maintenance.
- Designing of the intricate moulds can be costly.