Injection Rubber Molding

Hawthorne Rubber Mfg. Corp. uses compression, transfer, and injection molds. On our quotations, we may propose more than one type of mold. To clarify these different molding options, their advantages, and disadvantages, we have put together three education/information web pages on molds. The following covers the most sophisticated type of mold: injection.

Figure 1 is an example of a basic vertical type three plate multi-cavity injection mold; however, you are not limited to three plates. The mold does not require heater elements or temperature controllers. The injection press fully controls the molding temperature it is running in.

Molding Procedure: The press opens the mold. It remains in the press. Center plates are separated by the stripper bolts and hang from the top plate. (However, some core bars or plates may be designed to be removed to retrieve the finished molded parts.) When bonding to metal inserts, they are placed into the cavities at this time. The mold is closed by the press, and the fully automated injection cycle begins. A large ram or screw forces preheated uncured rubber through the injection nozzle, through the mold’s runner system, down through the “sprues,” and into the cavities. The uncured rubber is then forced into the shape of the cavity in the mold. A slight excess of material flows out of the cavity, along with the gates and vents. The mold remains closed until the rubber is cured, completing the cycle.

Figure 2 is an example of a basic horizontal type two plate multi-cavity injection mold. In general, you are limited to two plates. Again the mold does not require heater elements or temperature controllers. The injection press fully controls the molding temperature it is running in.

Molding Procedure: The molding procedures for horizontal and vertical molds are very similar. The main differences are:

The lack of sprues in the two-plate design. Cavities are normally filled from an injection point along the cavity parting line.
Often, parts can be ejected and swept out of the mold by the press brushes.
Utilizing the press brushes and ejector system, molds can complete the cure, eject the cull, runner, and parts, brush off any remaining rubber and begin the next cycle without the intervention of the press operator.
Less rubber lost to the injection runner system.

General Comments: Injection molds share many advantages and disadvantages that transfer molds do over compression molds. Additional rubber is consumed in the “cull,” “runner,” and sprues when filling the cavities. However, because the cavity plates start closed, less rubber escapes the cavity, limiting excess flash. This also makes it well suited for molding delicately shaped parts or securing inserts embedded in a product. The uncured rubber is automatically fed into the injection cylinder, preheated, and then accurately metered into the mold by monitoring and controlling the injection time, temperature and pressure. Because many different colored materials are run through an injection cylinder, it will take several cycles to wipe out any prior pigments when molding light-colored compounds while setting up the job. The transfer and compression mold are more suited for short runs of colored and translucent compounds. Injection molds have the shortest cycle and cure times. This is due to a greater level of automation, the preheating of the rubber in the injection cylinder, and a rapid transfer of heat to the rubber while being forced through the runners and sprues. Due to the more complex injection mold design, it is more expensive to purchase than a transfer or compression mold, but it may be better suited for your product design and the size of the order.