Rubber Injection Molding
Injection molding rubber was originally an extension from the plastics industry in the early to mid 1960s. After overcoming the initial issues of temperature (plastics is cooled when molding and rubber is heated) and pressure (rubber injection molding requires significantly more pressure per square inch of cavity surface), the process has become the most efficient way to mold rubber in most cases.
Injection and injection-transfer molding start with more efficient material preparation. The material is mixed, typically in 500 pound batches, and then stripped immediately after being mixed, into continuous strips measuring approximately 1.25" wide and 0.375" thick. This strip is fed into a screw which charges a barrel as needed with a pre-defined amount of material. When the mold is closed, the material in the barrel is injected into the mold cavities and cured.
There are many advantages to this process:
- Complete elimination of pre-forms, a labor-intensive step that can introduce variability in pre-form weight and shape resulting in variability of the finished product.
- Complete elimination of operator placement of pre-forms. In many cases, the operator has to "strategically" place the pre-forms in either the cavity (compression molding) or the pot (transfer molding) to insure quality output.
- The injection screw pre-heats the material before forcing it into
the cavities. This decreases the viscosity of the material, allowing
it to flow more easily into the cavities. The other advantage is the
potential for decreased cure time for two reasons:
- more rapid cavity filling due to lower viscosity
- the material is well on it's way to being cured as a result of the heat added during the screw charging and shear created during injection.
Injection-transfer molding is a combination of injection and transfer molding that Custom Rubber Corp. uses quite frequently to maximize the number of cavities in a given footprint. The injection machine is used to fill the pot with a pre-defined amount of pre-heated rubber. The one disadvantage with this process is the added material waste in the transfer pad.
Custom Rubber Corp. is an active member of the RMA (Rubber Manufacturer's Association) and one benefit to that membership is access to industry-specific operator training for all of our manufacturing methods. If you are interested in seeing a preview of the type of training all of our operators receive, you can visit the online site and learn a little about molded rubber.