Injection Molding

SUMMARY
Advantages:
Light weight, thin wall part
Versatile, almost limitless, part design
Wide selection of materials for the proper properties
Very good durability and impact strength (dependent upon material selection)
High yield likely resulting in lower part cost
Disadvantages
High tooling cost that must be offset by a large number of parts required for the
application
Oversized parts usually not available except on specialized equipment (such as
General Motors Saturn automobile fenders)
TYPICAL APPLICATIONS
Material handling applications
Toys
Automotive parts
Recreational products
Sporting goods
Furniture
DVD’s and CD’s
PC’s and peripherals

Injection molding of plastics is the most common form of plastic fabrication available. Injection molding is the process in which thermoplastic material is fed into a heated barrel, mixed, and forced into a mold cavity where it cools and hardens to the configuration of the mold cavity. Dozens of materials, and derivatives for custom compounding, are available to produce remarkable products that often combine numerous metal parts into one complete unit. A wide variety of colors are available to provide class A finished surfaces that do not require post mold secondary finishing.

Typical injection molded parts have ribs, sockets, and possibly metal inserts (called insert molding) molded directly into the part. Unlike Structural Foam molding, thin wall parts are the norm for injection molding with wall thickness usually no greater than 1/8” or less. This thin wall, along with rapid, high pressure injection of the molten material, results in very rapid cycle times, often referred to as “yield” or PPH (Parts Per Hour). Thin wall, commodity parts, such as polyethylene lids for margarine tubs, would have numerous “cavities” in each mold and may mold a dozen or more lids every 5-7 seconds! Some other high volume applications include DVD’s and CD’s as well as just about any plastic part found in consumer or commercial applications.

To produce parts with such volume requires a mold that can withstand the internal pressures that is part and parcel with injection molding. The molds are almost always high tensile steel, although aluminum may be used in low volume, or gas-assist, applications. Gas-assist is a relatively new process, within the past twenty years, that mimics structural foam in producing large parts, with normal wall thickness common to injection molding and beneficial low molding pressures. Part surface is much improved over structural foam molding eliminating the surface swirl marks and providing much improved cycle times (referred to as “yield”).

Injection molding additionally offers the ability to include “overmolding” the resins over a substrate (such as fabric) to produce headliners for automobiles. The injection molding process is quite versatile and is usually only limited by budgetary constraints.

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