Molding for Medical Device Assembly: Glossary of Injection Molding Terms
Chilling: the action of removing heat from the melt to convert it from a liquid consistency back to its original rigid state. As the material cools, it also shrinks. This stage can consume 70 percent to 80 percent of the total cycle time; it is largely dependent upon the wall thickness of the part.
Clamp force: the force required by the clamp to keep the mold closed during the injection process.
Clamping unit: a device that holds the two halves of a mold in correct alignment and keeps the mold closed during injection by applying a clamping force sufficient to resist the injection force. It typically consists of a fixed platen and a movable platen, which are attached to tie bars.
Ejection: the removal of the cooled, molded part from the mold cavity and from any cores or inserts. As the mold opens, ejector bars or ejector pins make contact with the ejection mechanism and push the part out of the mold.
Ejector bars: pins that are pushed into a mold cavity from the rear as the mold opens to force the finished part out of the mold.
Flash: any excess material that is formed with and attached to the part or component along a seam or mold parting line.
Injection: the stage during which the melt is introduced into a mold to completely fill a cavity or cavities.
Injection-molding machine: a device that consists of two basic components: a plastic injection unit and a mold-clamping unit. The machines can be either electrically or hydraulically actuated.
Injection unit: a device that acts like an extruder. It consists of a barrel that is fed from one end by a hopper containing a supply of plastic pellets.
In-mold assembly: a process in which parts are molded and assembled in the mold. The combination of these two traditionally separate processes can reduce production cost, while improving quality and productivity. In-mold assembly eliminates traditional joining processes, such as bonding or welding.
Melt flow rate: a measure of the molten viscosity of a polymer determined by the weight of polymer extruded through an orifice under specified conditions of pressure and temperature.
Metal injection molding: a process similar to plastic injection molding that can produce many of the same shapes and configurations. Metal powder is mixed with a thermoplastic binder and molded into a cavity. With metal injection molding, a wide variety of flexible designs, component integration and high-performance characteristics are possible.
Micromolding: a process similar to traditional injection molding that allows manufacturers to produce tiny parts for microelectromechanical, microfluidic and nanotechnology applications. Micro parts typically have tolerances of 0.0001 to 0.0002-inch. Their geometry can only be seen by microscope. Micro parts feature wall thicknesses as low as 0.0015-inch and overall part weight as low as 0.00012-gram.
Moldability: the characteristics of being easy to mold without rupturing or developing flaws due to movement of the polymer during gelation.
Multi-shot molding: the injection of two or three materials, in sequence, into a single mold during a single molding cycle.
Nozzle: a hollow metal hose screwed into the extrusion end of the heating cylinder of an injection machine, designed to form a seal under pressure between the cylinder and the mold.
Plasticizing: the conversion of a polymer material from its normal hard, granular form at room temperature to the liquid consistency necessary for injection at its correct melt temperature.
Platen: the structural part of an injection-molding machine to which the mold is mounted.
Shot: the complete amount of melt injected during a molding cycle.
Shrinkage: a process that occurs when plastic cools in a mold, because polymers have high thermal expansion coefficients. Some thermoplastics contract up to 10 percent. However, fillers added to plastic tend to reduce shrinkage. To compensate for shrinkage, the dimensions of the mold cavity must be made larger than the specified part dimensions.
Sprue: the feed opening provided in injection molding between the nozzle and cavity.
Thermoplastic: a polymer that melts or flows when heated.
Thermoset: a polymer that does not melt when heated.
Tie-bar spacing: the space between the horizontal tie-bars on an injection-molding machine. This limits the size of molds that can be placed between the tie-bars.