Rubber Molding Process

Rubber injection molding is an ideal process to produce precision rubber molded components and rubber-to-metal bonded products. The rubber injection molding process is a rubber molding process based off a process that was intended for molding plastics. The difference in the processes is that rubber is cured under significantly more pressure per square inch of molds cavity surface. Rubber injection molding began in the mid-1960s.

This rubber molding process is selected for rubber parts that require high volumes, tight tolerances, over-molding or precision applications. Rubber injection molding delivers speed and precision not found in other rubber manufacturing processes.  It is an optimal manufacturing process for products specified with rubber compounds that have quicker rubber cure times. Efficiency is gained by a higher level of automation, strict control over flash and reduced waste from scrap parts.

Rubber Injection Molding Process Description

Rubber injection molding successfully alters the plastics process by heating the rubber and placing it under significantly more pressure per square inch of cavity surface in molding.  Through various innovations, injection molding has become one of the most efficient ways to create molded rubber products in many cases.

The process of injection and injection-transfer molding starts with efficient material preparation. To begin, the material is mixed in bulk and then stripped immediately into continuous approx. 1.25″ wide & .375″ strips. The strips of uncured rubber stock are then fed from a hopper into the rubber molding injection equipment via a conveyance channel.

A large auger type screw moves the stock into a barrel that holds a specific amount of rubber stock.  As it turns and moves through the channel into the injection unit, the material is heated.  As the rubber stock is heated, it plasticizes to a target temperature.  The plasticized rubber material is then injected into a heated mold cavity held under high-pressure.  A runner and gate system is used similarly to the plastic injection molding process.  The temperature and pressure activate the cure of the rubber compound, vulcanizing it.  Once the rubber reaches and required level of cure, it is allowed to cool and reach a solid state within the mold. The molds open and parts are removed or ejected ready for the next cycle.

Rubber compression molding is an ideal process for low to medium volume production of rubber products.  This rubber molding process can produce a diverse range of precision rubber molded components.  Allowing for the affordable production of large, intricate products. It is often used to produce environmental seal products such as rubber o-rings, seals and gaskets.

The Rubber Compression Molding Process

Compression molding is a process that involves taking a rubber compound or mixed raw material and creating “pre-forms” in the basic shape of the end product.

This rubber molding process uses a preformed piece of uncured rubber that is placed in an open mold cavity.  The mold is preheated to an elevated temperature.  As the mold closes, the material is compressed and flows to fill the rubber mold cavity.  A combination of high-pressure and elevated temperatures activates the curing of the rubber compound, aka vulcanization.  The optimal level of cure is achieved, the part hardens and cools then the mold is opened and part removed.  The cycle repeats with the next rubber preform inserted into the rubber mold.

The transfer molding process can be used to produce both solid rubber parts and parts that require bonding rubber to a metal or non-metallic component. Examples of parts manufactured with transfer molding include vibration isolators and shock absorbers.

The Rubber Transfer Molding Process

Similar to compression molding, transfer molding requires raw material preparation into pellets and/or pre-forms.  Rather than placing the preform in the mold cavity, the preformed material is placed in a “pot” located between the top plate and a plunger.  In the pot, the material is compressed by the heated plunger and forced (transferred) through sprues into the cavity below.  The plunger is kept in place until the preform takes on the shape of the mold and the cure is reached, vulcanized.  The part is allowed to harden, cool, then is ejected. The cull pad, any cured rubber left in the pot, is removed, mold cavities are cleaned and the next molding cycle begins.

When used for rubber to metal bonding, the transfer molding process requires the components are loaded into a heated mold either by hand or using a loading fixture.  Material is loaded into the well pot of the transfer mold.  The mold closes and the heated rubber is then transferred through the runner and gate sprues into the mold cavities.  Under pressure and elevated temperatures, the rubber cures/vulcanizes and parts are removed.

The rubber transfer mold consists of a piston, well pot, sprue plate, cavity plate, and a base plate with a knock-out system used to eject the parts after molding.