In several applications, we combine the strength and durability of machined metals with the cost effective use of overmolded plastics. Transfer molding, or overmolding, is typically more labor intensive than operating an unattended injection molding machine, but has advantages when plastics can be used for less critical areas of the component design. The use of overmolding in a plumbing application uses brass for the mechanical functionality, and adds a threaded fitting which would be expensive if fabricated by other means. (See below) AMFAS has also overmolded clear plastics with opaque resin for lighting applications that provide a clear lens in a single integrated piece. Many applications can be found which use molded inserts for screw threads for strength or metal inserts to improve wear surfaces when plastic may not provide the durability required.
Most plastic resins today are thermoplastics which allow excellent performance for most applications. AMFAS offers thermoset resins for high temperature, high dielectric, or optimum burn/smoke applications. These resins physically change composition when molded and typically are labor intensive to produce.
The process of molding is relatively simple to understand. Most molding machines operate using the same principles. Plastic pellets are loaded into a funnel-shaped hopper, which then feed into a screw that is inside a barrel. The turning of the screw moves/mixes the plastic from the hopper toward the mold in use. Heaters on the barrel add heat to the plastic until it melts inside the barrel. To inject the melted plastic, the screw is pushed toward the mold using hydraulics, or electric motors, pushing the molten plastic into the mold at high pressures. The plastic compresses as it moves through the runners and gate into the mold. The plastic expands once inside the mold cavity filling even the smallest feature of the molded part. The plastic then cools inside the mold which opens when the part has cooled enough to be handled. The entire process is computer controlled, producing consistent high quality parts.
The tonnage of the machine is often used to describe the size of the part that can be molded, because it is a critical part of the process. The high pressure used during the process of injection tries to push the mold open. The clamping tonnage keeps the mold closed and sealed during injection. Typically, the mold is clamped with 3X to 5X the amount of force generated by the high pressure injection. This insures that the mold remains firmly clamped, and does not open which might result in flash around the edges or sealoffs.
At AMFAS, we also address the shot size of the machine being used. Many suppliers do not review this, which results in an inferior product. Also, all plastics degrade quickly at high temperatures, burning and becoming brittle. They may discolor as well. We insure that the shot size is 30-60% of the machine capacity to insure our parts are of the highest quality and that the plastic resin has not been degraded during the molding process.
AMFAS partners with suppliers that demonstrate the best process control and commitment to providing consistent quality.