All About Rubber Molding

Rubber molding refers to the various molding processes that turn uncured rubber compounds into complete rubber products. These materials possess characteristics that make them suitable for use in many products, including their chemical and environmental compatibility and their resilience and ability to return to their original molded shape without damage after being stretched or otherwise distorted. Rubber materials can be derived from natural or synthetic sources and are available in a wide range of formulations, each of which enhances or minimizes resulting material characteristics – these can be tailored according to the needs of the intended application.

The standard rubber molding process employs a steel or aluminum mold custom-designed to make the part of choice and into which raw material is placed or injected. Once the material finds the cavity that forms the parts geometry, the application of heat and pressure initiates the vulcanization process and forms the raw material into a finished product.

Depending on the material and intended use of the final product, manufacturers can employ different variations of the rubber molding process, including:

  • Compression molding
  • Transfer molding
  • Injection molding

Types of Rubber Molding

Before choosing a rubber molding process for your next part production project, it is important to understand the differences between the different process types, as each offers its own advantages and best use cases.

Below, we provide a more detailed breakdown of the three most common types of rubber molding.

Compression Molding

As suggested by its name, the compression molding method emphasizes the pressure aspect of rubber molding. Before beginning a compression molding operation, manufacturers first form the raw material into pre-forms that are in the general shape and form of the final product. These pre-forms have more material than is necessary for the end product, which ensures that they fill the entirety of the mold cavity during the molding process.

Once the pre-forms are ready, the compression molding process proceeds as follows:

  • Placement of the pre-form into the molding cavity
  • Application of pressure and heat to the pre-form, forcing it to fill and conform to the shape of the cavity (as the cavity fills, excess material leaks out of the cavity into the mold’s overflow grooves)
  • Curing of the pre-catalyzed material compound in the mold to cross link the molecules.
  • Manual removal of the molded part from the mold cavity
  • Hand removal of any excess material from the part.
  • Post molding operations such as trimming or nitrogen de-flashing.

Transfer Molding

Similar to compression molding, transfer molding requires the use of pre-formed raw material. However, the two molding methods differ in regard to the placement of the material during the molding operation. While in compression molding, the pre-form is placed directly into the molding cavity, in transfer molding, the pre-form is placed in a "pot"—i.e., a chamber—located above the molding cavity and below a plunger.

The stages of a typical transfer molding operation are as follows:

  • Creation of the pre-form
  • Placement of the pre-form into the "pot"
  • Application of pressure by the plunger (or ram) to the pre-form
  • Compression and transference of the pre-form into the molding cavity
  • Curing of the pre-catalyzed material compound in the mold to cross link the molecules.
  • Removal of the molded part from the molding cavity
  • Post molding operations such as trimming or nitrogen de-flashing.

Injection Molding

The rubber injection molding process employs the same basic principles as that of the plastic injection molding process from which it is derived. Similar to plastic injection molding, rubber injection molding heats the raw material. However, it places much greater emphasis on the liquefied material during the molding and cooling stages.

A typical rubber injection molding operation includes the following stages:

  • Creation of a standard or custom mold (must be capable of withstanding the significant pressures produced during the molding operation)
  • Feeding a rubber compound into the screw and barrel of a molding machine.
  • Processing of the catalyzed rubber compound in a screw and barrel on the molding machine.
  • High-pressure injection of the material into the molding cavity
  • Heating of the material under high pressures to maintain product shape
  • Removal of the molded form from the molding cavity

Although it does require an additional step for raw material processing this is done in the injection molding machine, which makes rubber injection molding one of the most efficient, versatile, and popular ways to form higher quantity requirements for molded rubber products.

Liquid Injection Molding of Silicone

The liquid injection molding process (LIM) employs a machine similar to plastic injection molding, however, it uses a closed loop material delivery system and the material being two parts is combined and then cooled prior to delivery to a mold mounted in the injection press.

A typical LIM molding operation includes the following stages:

  • Creation of a standard or custom mold (must be capable of withstanding the significant pressures produced during the molding operation)
  • Pumping two part silicone into a cooled mixing unit mounted on the molding press.
  • Processing of the silicone material in a cooled screw and barrel on the molding machine.
  • High-pressure injection of the material into the heated molding cavity.
  • Curing of the material under high pressures to maintain product shape.
  • Removal of the molded form from the molding cavity using an operator or automatic ejection system.

This process has a great deal of control on pressure, temperature, speed of injection, clamp tonnage and can be fully automated to produce very consistent difficult to make parts in high quantities without post molding operations and cost.

Rubber Molding Applications

Due to their precision and durability, custom rubber molded products find use in applications across a wide range of industries. Some common components by industry include:

  • Aerospace: seal and gaskets
  • Automotive: seals, gaskets, and electrical enclosure weather seals
  • Construction: architectural and window seals, fiberoptic sheathes, and outdoor electrical enclosures
  • Consumer and commercial goods: scratchproof feet for appliances
  • Food processing: oven, cart, kitchen, and dairy equipment seals
  • General industrial: backflow preventers, bellows, elastomeric check valves, electrical connectors seals, ordinance seals, and water purification and filtration seals
  • Medical: grommets, seals, surgical devices, fluid path components and grips.

Rubber Molding Benefits

Among the different molding methods, some are better suited for specific applications. For example, injection molding provides the following benefits:

1. Elimination of most labor saves time and cost and adds machine driven consistency.

The process of creating and placing pre-formed rubber in a mold is laborious and time-consuming and can significantly affect turnaround and production costs. Additionally, as the initial pre-form largely determines the weight and shape of the end product, it must be produced to precise and accurate specifications.

Compression and transfer molding require the use and placement of pre-formed materials in the molding cavity or pot. Rubber injection molding and liquid injection molding don’t. Instead, the method requires the raw material to be machine processed for direct injection into the molding cavity. This reduces the amount of time manufacturers and molding service providers need to invest in a project before starting the actual molding operation and eliminates the step of manually placing the material into the mold and may be able to eliminate removal of parts from a mold using labor. The machine cycle repeats consistently and is not technician dependent. A consistent processing cycle usually produces consistent repeatable products.

2. Machine driven processing of the raw material allows for faster production times.

The rubber molding processes require the control of both temperature and pressure. In injection molding operations, the raw material is processed before it is injected into the mold, which reduces its viscosity and allows it to flow more easily into the mold cavities. Additionally, the temperature control of the material jumpstarts the curing process, allowing for decreased cure times. Together, these qualities result in overall faster cycle and production times.

The precise nature of the LIM process and its closed loop system for material delivery and processing provide for very accurate control of material into what some people would consider to be impossible part geometries to create, faster cycle times, and repeatable parts.

3. Injection molding can be more cost-effective.

Compared to other molding methods, injection molding is more economical, especially for high volumes of medium to high complexity products. Some of the qualities that make injection molding more cost-effective include:

  • Faster production times (reducing production costs)
  • Lower error rates (reducing the number of runs and re-runs required to meet an order)
  • Less damage to molds during production (minimizing the need for mold replacement)
  • Less material waste (reducing overall material costs)

4. Compression and Transfer molding have advantages.

For the right part design, both of these processes have distinct advantages over injection molding.

  • Lower up-front tooling cost
  • Low set-up cost.
  • Smaller batches of material are available for lower up-front cost.
  • Both processes are capable of making accurate, excellent quality parts.
  • Insert molding
  • Many color options.
  • Custom compounding of materials.
  • Testing or manufacture of parts using liquid silicone rubber allows ramp up into liquid injection molding.

Contact Elastomer Technologies, Inc. for Your Rubber Molding Needs

Rubber molding is a highly versatile method used to create a wide range of durable parts and products for industrial and commercial applications. Part fabricators most commonly use compression molding, transfer molding, and injection molding methods to produce these molded rubber parts. Of the three methods, rubber injection molding offers greater efficiency, such as faster turnaround and lower material waste.

At Elastomer Technologies, Inc., we specialize in the design and fabrication of rubber molded parts and products. Our expert team has the skills and knowledge to provide you with long-lasting, high-performance molded rubber solutions tailored to your unique part needs.

To learn more about our rubber molding capabilities, including liquid injection molding, compression molding, and transfer molding, reach out to us today.