 BASIC DETAILS OF THE PLASTIC INJECTION MOLDING PROCESS.
Plastic resins are manufactured with an extremely wide range of physical properties to meet both low and high performance standards of modern plastic products. These resins are then delivered to several types of processors who form the resins into the desired end product.
Some of these processes include:
INJECTION MOLDING (high definition, high performance parts, ie. Cell phones)
BLOW MOLDING (bottles)
THERMOFORMING (deli food containers)
ROTATIONAL MOLDING (hallow balls)
REACTION INJECTION MOLDING (large thick parts, ie. Pallets)
CALENDERING (shower curtains)
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| Injection molders usually receive the resin in pellet form. Plastics are unique in that a wide variety of chemical and mechanical properties can be built into the resin prior to it being formed into a product. This internal structure variation means resin selection for a specific use is critical. Coloring may be added by the supplier or at the molding machine (often called a press) in either pellet, powder, or liquid form. Some resins require drying just before processing. The resin is normally transferred to a hopper above the press. The injection unit includes a specialized screw inside a cylinder called a barrel. An opening at the back end of the barrel allows the resin to enter from the hopper. The front end of the screw is equipped with components that, as it rotates, allow the resin to accumulate in front of the screw forcing the screw backward inside the barrel. The size of this accumulation of resin, called a shot, is controlled by the distance the screw is allowed to retract inside the barrel. The resin is transformed from a solid state to a thick viscous liquid state by the frictional heat of the screw compressing the resin within the barrel and by electrical heater bands surrounding the barrel. |
 Each half of the mold is clamped to a steel plate in the press called a platen. These platens are held tightly together by the press during plastic injection. Following injection and cooling the platens are separated far enough for an ejection system to force the part out of the mold, whereupon the platens are closed and the cycle repeated. Cycle times will range from a few seconds to a minute or more thus are the major processing cost factor. Cycle time control begins with part design and is influenced by resin selection, mold design, material, and construction, along with molding technique.
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