| replaced by a single injection molded part incorporating the features of the total assembly. If
multiple assemblies are required, the plastic parts can have snap-together features to eliminate
any fasteners. Injection molded parts can shorten the time to the market place in new product
development programs because of elimination of sub-assembly tooling or fixtures.
If heavy metal shakes, rattles and rolls, then plastic twists and shouts. Plastics are easily
processed into complex shapes that would be impossible for metal because plastic materials
have non-Newtonian flow behavior. This means that the viscosity (resistance to flow) will
decrease when the flow rate increases. The flow rate is increased by increasing the injection
pressures. The standard injection molding pressures are 20,000 to 30,000 Psi. This capability
allows plastics to be made to flow to produce thin walled parts with uniform wall dimensions
replacing the more costly thicker-walled design features of most metal parts.
6. SAVE DOLLARS BY RE-USING MATERIALS
Any way you look at it, recycling makes sense. Re-using materials by adding regrind (ground up
runners and scrap parts) to virgin materials generates even more cost savings.
Plastic materials fall into two basic types of process groups: Thermoset and
Thermoplastic. Thermoset (often called compression molding) is like working with epoxy.
Once the material has been heated and formed in a mold, it is set. The material cannot
again be processed; it is literally a reaction by temperature or thermally set. Examples of
thermoset materials are Alkyd, Polyesters, Melamine and Phenolic. Most injection molding plastics are thermoplastics; that is they can be reprocessed.
Thermoplastics fall into two distinctive molecular groups: amorphous and crystalline.
Amorphous materials when processed act like honey; that is they never really melt, they
just soften and are formed under pressure. Crystalline materials act like solder or ice.
They have a specific melt temperature and remain a solid until this temperature is
reached. After the melt temperature is achieved, the materials flow very easily with very
low viscosities. When the material is cooled to a temperature below the melt temperature,
the material hardens to a solid form.
|
Amorphous Materials
Acrylic
ABS
Polystyrene
PVC
Polycarbonate |
Crystalline Materials
Nylon
Polypropylene
Acteal
Polyester
Polyethylene |
Comparison
Characteristics:
Shrinkage
Ease of flow
Dimensional control of parts |
.004 - .012/in/in
relatively stiff flowing
easier to maintain close
dimensional tolerances |
.012 - .025/in/in
easy above melting
temperature more difficult to
maintain close dimensional
control |
7. INCREASE PRODUCT LIFE
The Tin Man needed more than a brain to last - he needed an endless supply of oil as well.
Replace the environmental vulnerability of metals with the durability and longevity of
plastics. Most plastic materials have greater chemical resistance than most metals. Plastics
do not rust or oxidize as metals do and most are not affected, as are metals, by acids or
base compounds.
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