Injection molding of thermoset and thermoplastics is the fastest growing element of the molding industry. New materials are being developed continuously; these and modified materials greatly enlarge the market for new plastics products. Molding machinery, mold engineering, product design, methods engineering, and automation have also been developed at a fast pace to keep up with materials developments. The mold designer must follow all of these developments in order to expand his understanding and experience.
The reciprocating-screw injection machine has made-the original plunger-type press obsolete in most cases, and it has been a tremendous help in expediting the growth of injection molding and the use of molded products.
The most widely used contemporary injection molding equipment includes the following basic types:
- The plunger-injection press utilizes a heating chamber and a plunger operation to force material into the mold .
- The reciprocating-screw press, often identified as an in-line press, utilizes a reciprocating screw to move and melt the granules of material as they are milled by the screw and passed through the heated injection cylinder. Most of the melting is achieved by mechanical working of the molding compound. Upon melting, the material builds up in front of the screw, forcing it to retract. At this point the screw stops and becomes the plunger, moving ahead to push the plasticized material into the mold.
- The two-stage screw press in most cases uses a fixed screw to plasticize the plastic granules and force the molten compound into a holding chamber from which it is transferred by a plunger into the mold.
- The rotating spreader is driven by a shaft cause it to revolve within the heating chamber, independent of the injection ram and thus to melt the plastic granules. Final filling of the mold is accomplished through the continuing movement of the injection ram.
An Election press consists of the clamping or movable end of the press which is moved by a hydraulic system, or by a toggle clamp arrangement actuated by a hydraulic system; the stationary end of the press provides nozzle protrusion and retention of the fixed half of the mold plus the plasticizing and material feed units.
Presses’ ate available with horizontal or vertical movement. Vertical movement presses are particularly desirable for insert or loose coring types of molding, The moving half of the press contains ejector or knockout systems for the most commonly used mold operations.
In addition to the mechanical motions, damping, and plasticizing functions, the press is equipped with numerous valves, timers, heating controls, and safety features for semiautomatic and completely automatic molding. It is designed and constructed to work continuously on fast cycles.
Mold designs must be achieved that meet the press requirements, material flow characteristics, speed, and cooling needs of the desired cycle.
Capacities of injection presses arc rated in ounces of polystyrene molded per cycle, or in cubic inches of material displacement. Caution must be used in converting the rated machine capacity to equivalents for the material that is to be used. This is a density differential correction.