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Injection Molding

Posted by Bob on August 18, 2016

In the last two weeks, we wrote about the 3 main types of molding thatInj 1.jpg 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.

Control.jpgAn 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 overview.jpglike 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. 

press.jpgThe 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

Bob Jackson, President
Stern Rubber Company
General Manager
Staples, Minnesota