Molded Rubber Parts: Choose the Right Process

As a leading provider of automotive and industrial rubber parts, Canada Rubber Group Inc. is all too aware of the challenge of supplying high-quality molded rubber parts at competitive prices. How molded rubber parts are produced is a key determinant of their quality and cost.

Processes for Producing Molded Rubber Parts

Essentially, there are three ways to produce molded rubber parts for automotive and other applications. These are:

  1. Transfer molding

  2. Compression molding

  3. Injection molding

Transfer molding utilizes a closed mold system. Uncured rubber is forced from the top transfer pot to the lower closed mold through pressure exerted by a closed ram. Uncured rubber forced into the lower mold by the plunger is then held under high temperature and pressure to activate the cure system in the rubber compound. A cycle time in which to achieve a complete cure is established and the material remains in the lower mold for the entire cycle. At the end of the cycle, completed parts are removed or ejected from the lower mold and the cycle begins anew.

Transfer molding is a good molding process for medium to high production volumes, where tight tolerances are required and where the final applications requires over-molding with coloured or translucent materials. A downside is that tooling costs can be higher with transfer molding versus compression tooling.

Compression molding is similar to transfer molding except that preformed uncured rubber (charge) is placed directly into the lower mold cavity. The upper mold is then closed upon the lower mold, heating and compressing the preform to fill the mold cavities. The compressed compound is held in cycle for a duration of time sufficient to achieve an optimal cure.  At the end of the cycle, completed parts are removed or ejected from the cavities and the cycle begins anew.

Because it uses compound which has been preformed and must be manually loaded, compression molding may entail higher labor costs. In addition, cycle times with compression molding can be longer. The advantage of compression molding is the tooling is economical versus transfer or injection molding.  Tools frequently have cavities that number in the hundreds for high-volume orders.  Therefore, each cure cycle will generate hundreds of parts.

Injection molding of rubber works best for higher-volume production and where tight tolerances and over-molding may be required. In injection molding, a controlled amount of rubber ribbon stock or pellets is fed into the injection unit. In the injection unit, the compound is heated to a specified temperature. Once the specified temperature is achieved, the material is then injected into the mold cavity where it is held under temperature and pressure in cycle to achieve a complete cure. At the end of the cycle, completed parts are removed or ejected from the cavities and the cycle begins anew.

Because injection molding can be a highly automated process, piece price costs can be lower than with either transfer or compression molding. Injection molding is also highly suited for parts that must be consistently produced to very high tolerances – the consistency and repeatability of injection molding is a decided advantage. In addition, injection molding is the preferred process for parts possessing complex geometries and shapes. Finally, injection molding works well in parts production which requires metal-to-rubber bonding. The disadvantage of injection molding is in that the tooling costs are significantly higher than that of compression or transfer molding.  Low-volume orders may not justify the expenditure in an expensive automated tool.

Molded Rubber Parts from CRGI

At Canada Rubber Group Inc., we supply a variety of molded rubber parts. We can supply parts produced by either transfer molding, compression molding, or injection molding as the application and production volume warrants. Alternatively, we can recommend the preferred molding process to ensure that parts are produced with the highest quality at the lowest cost. If required, we can also supply molded rubber parts which require metal-to-rubber bonding.

Molded rubber parts that we have supplied include parts for the following applications:

  • Automotive applications: bumpers; pedal pads; grommets; plenum plugs; seals and miscellaneous parts for reducing noise, vibration and harshness (NVH).

  • General industrial use: wobble joints; molded rings and shapes; custom molded parts for miscellaneous industrial applications, including parts for reducing noise, vibration and harshness (NVH).

To find out more about the molded rubber parts we can supply, please contact our sales department at