Page 34 - Plastics News April 2018

FeAtures

plastic (Figure 2) improves product quality and reduces complex parts, especially those requiring tight
costs in seven key areas: tolerances and advanced molds or tooling. In
addition, high-pressure injection molding allows for
1. Decreased piece part prices. After covering initial
tooling costs, plastic piece part pricing is usually constructing thin-walled plastic parts that replace
much lower than the same part produced in metal, costlier, thicker and heavier features of die-cast metal
whether it is a stamped, cast, or a die-cast part, parts.
largely because the injection molding process has Prototyping, too, becomes easier with plastics.
faster cycle times (more parts made per machine hour). Inexpensive soft tools can be used to try out different
materials and finalize a design before making the
2. Elimination of time-consuming and costly secondary
operations. For example, plastic stock can be colored substantial upfront investment in hard tooling.
with color concentrates before molding, eliminating Prototype parts may also be easily machined out of
secondary painting operations. Similarly, a plastic plastic stock to reduce materials costs during initial
part can be textured or given various levels of polish design evaluations.
by treating the working surface of its injection mold. 6. Plastic materials can be quickly reused on-site. While
steel must be re-smelted at a reprocessing facility,
Assembly also offers opportunities for savings.
Fastening metal stampings or castings together is most thermoplastic scrap can be easily processed
expensive. They can often be replaced by a single into “regrind” and mixed with virgin material to save
injection molded part or one with snap-together time and up to 40 percent in materials cost. A word
components that eliminate welded joints and of caution: thermoplastics can be reprocessed, but
fasteners that slow production time and introduce thermoset materials cannot.
failure risks. 7. Improved product life. Most plastic materials have
greater chemical resistance than most metals. They
3. Reduced product weight and improved ease of
use. Weight reduction reduces shipping costs and do not rust or oxidize as metals do, and many are not
makes the product easier for the end-user to handle. affected by acids or base compounds.
In automotive applications, it helps lighten vehicles Plastics Offer Variety and Versatility
to save fuel.
There are more than 25,000 off-the shelf engineered
Table 1 compares the specific gravities of metals and plastic materials (Figure 3), each with its own application-
plastics with comparable mechanical properties: specific properties. If needed, higher-performance
Comparing Specific Gravity of Metals vs. blends and hybrids can be custom-designed to meet very
Plastics specific performance requirements. There are several
considerations needed to leverage the key characteristics
Aluminum 2.5-2.8 Polycarbonate 1.2-1.4 of plastics to best align the material with the project.
Brass 8.4-8.7 Nylon (most types) 1.2-1.7 These include:
Copper 8.8 Polyethylene .92-.95 • Bare material selection: Each base material has
Zinc 6.9-7.2 Polypropylene .90-1.04 its own set of advantages and disadvantages1.
Steels 7.7-7.83 ABS 1.02-1.4 To determine the ideal material type for your
application, identify critical project requirements
4. Greater product structural strength. Plastic parts such as chemical resistance, impact, flow, processing,
made with engineering-grade resins can actually be etc.
stronger than metal parts. In addition, molding in
features such as ribs, bosses and gussets as the part • Additives: Plastic additives are used to improve a
is produced further increases its structural strength material’s properties such as strength, rigidity, UV
without secondary operations. resistance and flame resistance.
Some of the most common additives include:
5. Increased product design options. Plastic’s form
flexibility makes it easier to design and produce

Plastics News April 2018 34