Injection molding is now the most prevalent molding method in the non-tire rubber industry. A pre-masticated rubber compound is placed in the injection machine’s integral extruder before being pumped through a relatively small orifice (mold bush) into the cavity feed (runner) system of the mold. The compound enters the mold cavity through a “gate” and during the process the rubber compound undergoes several elevations in temperature, thus changing its physical characteristics.
By the time the rubber compound is injected into the mold cavity, it will have reached a relatively high temperature (approximately 140º C / 285 º F), resulting in a curing time that’s drastically reduced from that of other processes. However, the process brings with it some quite difficult challenges for the mold release agent. Chem-Trend, a world leader in the development and manufacture of high performance release agents understands these challenges and has products to address them.
To produce high quality parts in an efficient manner, the individual components of the rubber injection molds need treating with a purpose applicable mold release agent. For most of the parts, only a semi-permanent release agent will be suitable. An injection mold can be treated extremely effectively with a base coat of release agent when it is has been removed and cleaned and remains in its disassembled state. Read more about cleaning and protecting rubber injection molds.
When utilizing a clean mold, there are multiple considerations to be made to support the production of clean parts in an efficient manner. These considerations can be broken down to the relevant sections of the mold, and help to illustrate the complexity, and influence, of mold release agent technology for the rubber industry.
Mold Entrance:
- To ensure that rubber flows easily to the mold cavity gates, the runner system and injection holes in the runner plate will need treating with a release agent to overcome their dryness.
- A release agent that forms a very tough film and is hard to disturb by flowing rubber compound is required. Typically, a semi-permanent release agent that will promote the easy flow of rubber during injection, and allow the cured runner system rubber to be removed easily between cycles, is best. The cured runner system needs to remain intact – otherwise picking individual rubber pips from injection holes can be time-consuming.
- The release agent must remain in place for many cycles and wear out slowly so that the touch-up interval is infrequent. Runner systems and injection holes can be hard to access in some molds, making mold turn-around times excessively long if the release agent does not work flawlessly time after time.
- There should be no contaminating transfer of the release agent through the runner system into the mold cavity. If it is necessary to use a highly lubricious mold release in order to de-mold the parts, it is ill-advised to use the same product on the runner system. Highly lubricious mold release agents can transfer into the mold cavity and become mixed with the rubber compound, causing knit-lines and even bond-line failure.
Inside the Mold:
- The mold cavity needs a release agent for two main reasons:
- Allow the parts to be de-molded easily
- Protect the mold surface from build-up or mold fouling
- Protection of the mold surface is especially important if the process involves rubber-to-metal bonding. Rubber-to-metal adhesives can contribute to mold fouling, since they do their job indiscriminately. If there is excessive adhesive on the metal parts, it will do its best to adhere to the metal mold. Also, if adhesive-coated metals are left in contact with, or in close proximity to, the hot mold surface for a longer period of time than necessary, the adhesive may sublime and form deposits on unprotected mold surfaces. The release agent used must provide an excellent barrier or protective film in these circumstances, enabling the mold to run for extended periods of time when using a semi-permanent release agent.
- All molding compounds will tend to leave deposits behind on a mold after each de-molding. As these deposits build up, release-ease diminishes. If deposits are allowed to build up until rubber sticks to the mold surface, it is too late. The adhered rubber will prevent the release agent from bonding to the tool surface and a useful releasing surface will not be formed, resulting in poor quality parts and increased scrap rates.
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