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Stern Rubber Company’s expertise in injection molding goes back as far as the process itself. Injection molding of rubber has its roots in the molding of plastics. In the 1960’s, engineers developed a way to use the same principles in molding plastics to mold rubber, making rubber injection molding one of the most efficient rubber manufacturing processes. The rubber injection molding process works significantly heating the rubber and using a very high pressure to inject the rubber compound into the mold.

Rubber Injection Molding Process

Injection molding rubber is quite similar to molding other materials but typically requires higher pressures. Steps in the process include the following:

Mold Design and Preparation

Planning a rubber injection molding project requires the same design and material considerations as any injection molding process. Molds must allow liquid rubber to rapidly and completely fill the cavity, cool effectively, and be fully ejected without damage. Designs that avoid thin walls and geometries that make ejection difficult are typically more successful. Molds or dies are fabricated to produce each unique design. Proper mold maintenance is essential to ensure consistent production quality, as it helps prevent defects, optimize fill, cooling, and ejection processes, and extends the lifespan of molds used in rubber injection molding projects.

Material Preparation and Injection

Rubber formulations should be chosen to meet the specific part requirements. Some rubbers commonly used include:

  • Butyl Rubber
  • EPDM
  • Hydrogenated NBR
  • Natural rubber
  • Neoprene
  • Nitrile (Buna-N or NBR)
  • Styrene-Butadiene Rubber (SBR)

A compound of rubber and additives to meet functional or appearance requirements is preheated and thoroughly mixed to achieve a consistent blend. A release agent is applied to the mold, and the mold plates close tightly to prevent leakage (flash). A precise amount of this liquid rubber compound is forced at high pressure and temperature into the mold.

Cooling, Demolding, and Finishing

Combined heat and pressure generate changes in the rubber as it fills the mold cavity. Rapid cooling is typically achieved with cold water channels around the mold. When a part is adequately cooled, it is released or pushed out by ejector pins and the cycle begins again.

Even with an ideal mold design and effective release agents, injection-molded rubber parts may still require finishing steps such as trimming, deflashing, and deburring. Finally, parts are inspected to ensure they meet internal and customer quality standards and requirements.

Benefits of Rubber Injection Molding

Injection Molding

Rubber is a tough, resilient material that performs well in a wide range of temperatures and challenging applications. With injection molding, durable rubber parts with customized shapes and details can be manufactured to tight tolerances at affordable prices. The benefits of rubber injection molding include the following:

Precision and Consistency

Molds for injection molding allow manufacturers to maintain consistency and precision over even very extended production runs, even with tight tolerances, intricate details, and complex geometries. The accurate nature of injection molding also produces minimal waste which can often be reused.

Efficiency and Cost-Effectiveness

With minimal waste, automated processes, and rapid cycle times, injection molding allows vast numbers of identical parts to be produced quickly. Longer production runs improve efficiency, further reducing costs per part. Injection molding is one of the fastest, most affordable methods of generating large volumes of identical parts in short timeframes.

Design Flexibility and Customization

Injection molding mold designs for rubber can be as intricate and complex as those for molding thermoplastics and other polymers. Inserts, overmolding, and multiple materials are all easily achieved with consistent, quality results. Molds, rubber formulations, and molding processes can all be fully customized to accommodate any customer requirements.

Rubber Injection Molding by Stern Rubber

At Stern Rubber, our injection molding services are one of the most cost-effective solutions we offer for the rapid production of high volumes of identical, precise parts and components. Serving industries as varied as agriculture, mining, manufacturing, and water supply and delivery, our team has decades of experience in all kinds of rubber manufacturing. Contact us to learn more about how our full capabilities can satisfy your next project.

In the last two weeks, we wrote about the 3 main types of molding that we do at Stern Rubber Company: compression, transfer and injection.  This week, we will discuss injection molding in detail.
Injection molding is different from transfer or compression molding, in that it takes a specific injection molding press.  This type of molding is somewhat similar to compression and transfer, as they all run at high hydraulic pressure, and have electrically heated platens that are run in the 300 to 400 degrees Fahrenheit range.
An injection press has an injection unit that is usually mounted on the top of the press.  There is a screw extruder that takes in the rubber material and pushes it into the injection ram.  The material is warmed in the extruder and the injector.  The ram then shoots the rubber into the mold through a runner system that is cut into the mold.  A computerized control system controls the entire molding process, from the volume of material, to the injection and clamp pressures, and the cure time.  The controls on the newer machines monitor and track any number of parameters and can do SPC calculations on those parameters.
Injection molding is good for medium to high volume parts.  It is not usually good for low volume parts, as there is a longer set up time to get the mold mounted in the press, and there is material that is wasted that has to be ran into the injection unit to push out the previous material.
Injection molding is good for rubber to metal bonding, just like compression and transfer molding.
Here at Stern Rubber, we currently have Desma and Rutil injection presses.  A few other brands include Maplan and Rep.
Like compression and transfer, injection molds are made from steel or aluminum.  The shape of the part is cut into the mold.  The main difference between injection tooling and the other types is that the tools have a runner system where the rubber is fed from the injector into the cavities.
The tool is held closed for the given cure time, and when the tool opens, the runner system is removed, and the parts are removed.
The advantages to injection molding are reduced cure times, less flash to remove after molding, and the ability to hold tighter tolerances.  The shorter cure times are possible due to the pre-heating of the material in the screw and injector.  Because the tool is clamped shut before the material in injected in, it is possible to mold parts with much less extra flash that has to be removed, and allows for less variation in size, so tighter tolerances.
The main disadvantages include longer set up times, and more material used at start up and in the runner system.
If you have want to learn more about this process, or think you have a project that will be a good fit for injection molding, please contact us at sales@sternrubber.com.