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 transfer molds.
Molding Procedure: The mold is removed from the press and opened. A precut “slug” of uncured rubber, at a set weight, is placed into the transfer “pot.” When bonding to a metal insert, it is first placed into the cavity. The loaded mold is then placed back into the press, and the press is then closed. The pressure forcing the plates together distributes (transfers) the precut slug of uncured rubber in the pot 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.
General Comments: Unlike the compression molds, transfer molds consume additional rubber when filling the cavity in the “pot” and “sprues”. However, because the cavity plates start out closed, less rubber escapes the cavity, thereby limiting excess flash. This also makes it well suited for molding delicately shaped parts or securing inserts that are embedded in a product. Since you work with a single “slug” of uncured rubber and not several individual precut or shaped “slugs” of rubber for each cavity, the transfer mold is ideal for colored and translucent compounds. Transfer molds have a shorter cycle and cure time than compression molds. This is due to a rapid transfer of heat to the rubber, while being forced from the “pot”, through the “sprue” and into the cavity. It is also faster to load just one “slug” of rubber when running a multi-cavity mold. Due to the more complex design of the transfer mold, it is more expensive to purchase than a compression mold, but it may be better suited for your product design.