Special Injection Molding Cycles

There are many designs of special injection molding machines. The need for insert molding has led to the development of a shuttle table machine. This is a vertical machine with one core plate attached to the movable platen. Two identical cavities are mounted on the shuttle table. While one set of cavities is molding, the empty mold is loaded with inserts. When the first molding cycle is completed, the mold slides out and the second mold slides under the force and the first cycle molded part is removed and loaded with inserts. 

With backpressure the process is performed in conventional injection molding machines (IMMs) (Chapter 4). The cover stock is inserted and located in an open mold. A shear edge mold permits draw-in of the cover stock during the closing cycle to avoid wrinkles and damage by stretching of the fabric. Molds require special attention. They generally use a hot runner system with its shut-off nozzle(s). All mold elements such as ejector, core pulls, and slides have to be on the injection side mold half. 

Inductive heating is thus another possibility of temperature control in injection molds. It can always be considered for special requirements or when conventional options are not available or only in combination with significant increases in cycle time, such as in the variothermal process, which is frequently used for the manufacturing of micro components or microstructures. 

A major constraint in the design of preforms is that they must reliably be injection-molded, preferably in the shortest possible cycle time. The finish, the open end that frequently includes a thread portion, needs special considerations as it is likely to be the thinnest section of the preform and as it is the last to fill while including critical dimensions. 

Fig.16 shows a typical apparatus for gas injection. An ordinary injection molding machine is equipped with a specially designed valve nozzle and a gas unit which generates and accumulates pressurized gas. The gas is introduced to the mold and recovered, synchronized with the molding cycles under the control of a sequencer. 

---