Four Considerations in Designing Molded Rubber Parts Versus Molded Plastic Parts

1. The Part’s Function and Application

As with any design, form ever follows function, to borrow a phrase from architect Louis Sullivan. The application of the molded part in the final product, its operating environment and its appearance must be balanced with cost considerations, volume requirements, and molding production constraints.

For example, the need for flexibility or rigidity in the part may dictate the material selected. Rubber has a wide hardness range from very soft to very hard. Plastic may be a better option when an extremely hard or rigid material is needed, while rubber is typically more flexible than most types of plastics. With its ability to flex and return to its original configuration, rubber molded parts are superior for dynamic sealing applications. Rubber also delivers high performance for applications where acoustic or vibration-absorbing properties are important.

2. The Operating Environment for the Part

If the part will be operating in a harsh environment, such as an under-the-hood application in a truck or automobile, the material selected must be able to withstand a wide temperature range, from – 40 to + 400 degrees (F), over a long period of time. Several rubber materials are able to function consistently at such temperature ranges, as well as stand up to exposure to hydrocarbons and other chemicals, while general-purpose plastics do not perform as well in extreme heat or cold. Typically, thermoset elastomers are the best choice when difficult temperature, chemical, environmental, and physical conditions are present, while plastics can be a good choice when operating conditions are more moderate.

3. The Aesthetics of the Part

For many consumer products, the appearance of the part is critical. As a result, plastic is a more popular choice than rubber for aesthetic purposes. Plastic is easily pigmented, and most types of rubber are not. For some applications, a hybrid of rubber and plastic – thermoplastic elastomers or TPE – is chosen for its rubber-like ability to stretch and return to its original form, as well as its ability to be molded into different textures. However, some types of thermoplastics used in consumer applications, such as smartphones, have the downside of high brittleness and can be easily broken.

4. Production Cost and Volume Considerations

When low costs and high production volumes are required, plastic usually delivers faster cycle times and considerably lower material costs than rubber, which is partly due to the different processes involved. The cycle time for plastic molding is much faster than rubber, measured in seconds for plastic versus minutes for rubber. However, many types of rubber are relatively inexpensive and high cavitation in the mold can often offset the cost of longer cycle times. Product designers may wish to request quotes from both rubber and plastic molding firms for comparison purposes.

The first step in rubber molding is for a chemist to develop a custom rubber compound for the part and its operating environment. For example, if the rubber is to be over-molded to a metal or plastic substrate, the compound may require a bonding agent. The rubber molding engineers will then recommend a specific molding process and design a mold for the part.

The types of rubber molding processes include:

• Compression molding
• Transfer molding
• Liquid silicone injection molding
• Over-molding: rubber to metal or rubber to plastic insert bonding

Rubber molding works by adding precise amounts of heat and pressure to cross-link or cure the part. Once a rubber part is cured, the rubber has gone through a chemical change. The chemical cross-linking that takes place gives the rubber the ability to flex but still retain its original shape.

The mold for a rubber part is typically less expensive to produce than for a plastic part. Due to rubber’s flexibility, rubber parts are easier to release from the mold than plastic, often using such methods as compressed air to blow the part off the mold. Rubber’s shrink rate is unidirectional, which allows for greater consistency, and large thicknesses can be achieved.

Plastic parts are made by injection molding, which involves melting the plastic, injecting it into a mold, and allowing it to cool. Unlike rubber, which is permanently cured during molding, plastic maintains its original properties. Therefore, thermoplastics can be recycled and reused.

Complex plastic parts require a more intricate and expensive mold design, which allows the part to be released without breakage. Plastic molding also must take into consideration shrink rate variances during cooling. The thickness of the part may be a challenge for molding, as thicker plastic parts are more difficult to mold and may require ribs or brackets to lend structural support to the part.

Industrial designers who are not familiar with molding processes may present a design for a plastic part and ask that it be made from rubber instead, or vice versa. However, because the materials and the processes for producing molded parts are so different, the part may need to be redesigned to better suit the material and the molding process. For example, plastic parts typically require a draft angle to allow the rigid part to be de-molded without breaking, while most rubber parts do not require this geometry.

In developing a new product, the manufacturer’s engineers can benefit by consulting with the molding firm’s engineers early on in the process, to ensure that the part can be manufactured efficiently. By discussing the part’s function, application and operating environment, the choice of the optimal material will become clear.