made per machine hour).
2. Elimination of time-consuming and costly
secondary operations. For example, plastic stock can be
colored with color concentrates before molding, eliminating
secondary painting operations. Similarly, a plastic part can
be textured or given various levels of polish by treating the
working surface of its injection mold.
Assembly also offers opportunities for savings. Fastening
metal stampings or castings together is expensive. They
can often be replaced by a single injection molded part, or
one with snap-together components that eliminate welded
joints and fasteners that slow production time and introduce
3. Reduced product weight and improved ease of
use. Weight reduction reduces shipping costs and makes
the product easier for the end-user to handle. In automotive
applications, it helps lighten vehicles to save fuel.
Table 1 compares the specific gravities of metals and
plastics with comparable mechanical properties:
4. Greater product structural strength. Plastic
parts made with engineering-grade resins can actually be
stronger than metal parts. In addition, molding in features
such as ribs, bosses and gussets as the part is produced
further increases its structural strength without secondary
5. Increased product design options. Plastic’s form
flexibility makes it easier to design and produce complex
parts, especially those requiring tight tolerances and
advanced molds or tooling. In addition, high-pressure
injection molding processes can produce thin-walled plastic
parts that replace costlier, thicker and heavier features of
die-cast metal parts.
Prototyping, too, becomes easier with plastics.
Inexpensive soft tools can be used to try out different
materials and finalize a design before making the substantial
upfront investment in hard tooling. Prototype parts may also
be easily machined out of plastic stock to reduce materials
costs during initial design evaluations.
6. Plastic materials can be quickly reused on-site.
While steel must be re-smelted at a reprocessing facility,
most thermoplastic scrap can be easily processed into
“regrind” and mixed with virgin material to save time and
up to 40 percent in materials cost. A word of caution:
thermoplastics can be reprocessed, but thermoset materials
7. Improved product life. Most plastic materials have
greater chemical resistance than most metals. They do not
rust or oxidize as metals do, and many are not affected by
acids or base compounds.
Plastics Offer Variety and Versatility
There are more than 25,000 off-the shelf engineered
plastic materials (Figure 3), each with its own application-
specific properties. If needed, higher-performance blends
and hybrids can be custom-designed to meet very specific
performance requirements. There are several considerations
needed to leverage the key characteristics of plastics to
best align the material with the project. These include:
• Bare material selection: Each base material has its own
set of advantages and disadvantages1. To determine the
ideal material type for your application, identify critical
project requirements such as chemical resistance, impact,
flow, processing, etc..
• Additives: Plastic additives are used to improve a
material’s properties such as strength, rigidity, UV
resistance and flame resistance. Some of the most
common additives include:
• Long glass fiber additives improve stiffness and
strength, increase temperature performance up to
150°C, and create a moderate surface appearance.
• Short-glass fiber additives improve stiffness, increase
temperature performance and improve appearance over
long glass additives. Glass content of 30% or less allows
parts to look comparable to unreinforced plastic parts.
• Carbon and stainless steel fillers improve conductive
and/or shielding properties.
Figure 2. Injection molding is faster, more efficient and able to meet higher
tolerances compared to making automotive die cast parts.
Figure 3. One of the greatest advantages of plastics is design engineers
can select from more than 25,000 engineered materials for automotive