Achieving optimal flashless molding is a balancing act involving proper tooling, the right size press, precise process development and a tailored compound, according to speakers and exhibitors at the recent International Rubber Molding Conference in Cleveland.
On the materials side, the proper compound depends on the right balance of four variables: plasticity/viscosity, cure rate, hot tear strength and part release, said Ray C. Hetherington, technical director of the ITW Southland division of Illinois Tool Works Inc. in Virginia Beach, Va.
Hetherington said in general the compound for flashless molding must exhibit fast mold cavity fill time and fast cure rate once in the mold, while emitting low volatile organic compounds and meeting the customer's specifications. Processors look for fill times of two to six seconds and a cure time fast enough to keep the material from flowing during the cure, the speaker said.
Controlling plasticity or viscosity is critical, he said. As an example, he showed a comparison between two organic compounds with identical cure rates but with vastly differing viscosity-the compound with the higher centipoise took nearly three times as long to fill the mold.
As for the other variables, Hetherington said faster cure rates will lessen the chance the material will flash upon filling the cavity; appropriate hot tear strength is necessary for automated de-molding and low reject rates; and an internal mold release-one in the compound-is recommended to prevent mold release buildup and to reduce costs.
The technical director said tools must be designed and built specifically for flashless molding, exhibiting the proper surface flatness and finish to maintain part integrity.
In addition, the material delivery system must deliver consistent material shot size and tightly controlled cavity fill and back pressure.
His comments were echoed by John W. Roembke, president of Roembke Mfg. & Design Inc. in Ossian, Ind., an exhibitor at the conference.
To offer his customers elements of both these variables, Roembke-a specialist in flashless mold design and production since 1977-has teamed up with Kipe Molds Inc. of Placentia, Calif., an innovator in valve gate technology for liquid silicone.
Tapping into Kipe's expertise, Roembke Mfg. is working to adapt the principles of pneumatic valve gating technology to standard elastomers such as EPDM, fluoroelastomers, acrylics, silicone and others, enhancing the value-added aspect of the company's business, John Roembke said.
On the mold front, Roembke Mfg. is able to produce positive vented molds with tolerances down to +/- 0.02 of an inch, he said, allowing processors to operate their presses at higher pressures and/or use longer cure times.