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Modern plastics as we know them today have their origin in the late
19th century when numerous European and American chemists experimented
with various types of rubber and residues from chemical mixtures.
In 1865 John W. Hyatt patented a process for combining cellulose nitrate
and camphor into a compound he termed "celluloid" which was
used as a replacement material for ivory in the production of billiard
balls. Celluloid is still used to this day for photographic film used
in the motion picture industry.
The first moldable material was introduced in 1907 by Leo Hendrick Baekeland
which was a phenolic material he named "Bakelite". Bakelite
was such a versatile and durable material that it could be used to produce
many useful domestic, industrial and military products.
Throughout the early 20th century many new plastic materials were developed
including the following important benchmarks: Rayon in 1891; Cellophane
in 1913; Nylon in 1920; Polyvinylchloride (PVC) in 1933; Teflon in 1938;
Polyethlene in 1933.
Since the 1950s, plastics have grown into a major industry experiencing
rapid growth which it still enjoys today with the continual development,
modification and refinement of materials. Southwest Plastics has been
serving our customer's molded plastic requirements since the early 1950s
and has enjoyed steady growth with this dynamic industry.
There are two basic families of plastics: Thermoplastic materials and
Thermoset materials.
Thermoplastics soften and melt upon the addition of heat. Upon removal
of heat they then again stiffen and harden into a new shape. The Thermoplastic
family is what most people relate to when they think about plastics because
of the many domestic products that are produced from them. What red blooded
American male does not have first hand experience with this family of
plastics when as a child built Revelle models of boats, cars or other
objects and then lit them on fire only to end up with an amorphous blob?
The injection molding technique was developed to efficiently process
this family of plastics into the many useful products we use everyday.
Some of the commonly used thermoplastics are polyethylene, polypropylene,
polystyrene, PVC, ABS and Nylon to name a few. There are countless other
engineered plastic materials and compounds to suit broad varieties of
applications. Southwest Plastics has extensive injection molding capabilities
for thermoplastic materials.
Thermosets harden and cure in the molding process as the molecules actually
crosslink in a chemical reaction and set into a final shape. No addition
of heat will soften or melt the material. As a result, thermosets have
their advantage in any high heat environment where softening or melting
cannot be tolerated. Domestically products such as ash trays, cooking
utensils and cookware handles are produced from thermosets. Industrially
thermosets are used in electrical, electronic and military applications.
Some examples of thermoset materials are phenolics, epoxys, melamines
and diallylphthalates.
Thermosets can be molded using the compression, transfer or injection
molding processes. Southwest Plastics has extensive capabilities in molding
thermoset materials using all three of these processes.
In order to transform plastic materials into usable shapes, many methods
of processing plastics have been developed including the following: Injection
molding, compression molding, transfer molding, blow molding, thermoforming,
vaccu-forming, extrusion, and blown film. Each of these processes require
specific expertice and skill. Southwest Plastics has expertise in COMPRESSION,
TRANSFER, and INJECTION molding.
Compression Molding is the first technique developed in the early 1900s
to mold Bakelite (phenolic) molding compounds. It is used today to process
many varieties of thermoset plastics and composites including phenolic,
epoxy, melamine, and diallyl phthalate. The compression molding press
is simply a vertically oriented hydraulic press. The compression mold
is clamped into the compression molding press, and is simple in that it
consists of two halves: the cavity and the force or core on the opposite
side. The mold is heated to the temperature as specified by the material
manufacturer which is usually around 300° F, and the material is placed
directly in the cavity. The mold is then closed under pressure compressing
the material and causing it to flow into the void between the cavity and
the core. The mold is held closed for several minutes (depending on the
material used and the wall thickness of the part) while the material is
curing or "setting" into the form of the mold surface. The mold
is then opened and the part ejected from the mold cavity. Compression
molding lends itself to relatively simple parts, but since there are no
sprues, runners or gates through which the material must flow, the material
can be heavily loaded with a composite of fillers and fibrous reinforcements
which remain in a random orientation in the molded part resulting in superior
strength properties.
Transfer Molding is another method for molding thermoset materials where
the design of the part is too complicated or unsuitable for compression
molding. Transfer molding is often used when metal inserts, studs, connector
pins wear plates, electronic components or other parts are molded into
the part. With transfer molding, the mold is clamped closed first. The
material charge is then loaded into a loading "pot" which is
connected to the cavity with a short runner. The material is then "transferred" from
the pot to the cavity via the runner under pressure exerted from a plunger
which is hydraulically actuated against the material which has been loaded
in the pot.
Injection molding is a high production method of producing a vast variety
of thermoplastic parts into shapes ranging from fairly simple to quite
complex. Injection mold cavities are fed with melted plastic material
which is forced under high pressure through a sprue which feeds a runner
system then through a gate into the cavity. The material is then cooled
to the point that the material is solid and then ejected from the mold
as a finished part. Certain thermoset materials can also be injection
molded after several modifications to the standard injection molding machine
are affected. The injection molding machine consists of two basic components:
the clamp unit and the injection unit. The clamp unit holds and clamps
the tool into position while the injection unit plasticizes and injects
the material into the mold which has been positioned by the clamp unit.
The clamp force must be sufficient to hold the mold closed against the
hydraulic pressure of the material being injected under high pressure
by the injection unit. The rule of thumb is that there must be three tons
of clamp force for every square inch of cavity projected area in the mold.
Both units are coordinated by a computer controller on the machine which
is programmed to efficiently to produce high quantities of consistent
quality parts on an automated cycle.
Southwest Plastics has been in business for over 50 years and has extensive
experience in both thermoset and thermoplastic materials. We have a state
of the art facility and offer molding services using compression, transfer
and injection molding techniques. Let us work with you on your next project
so we can become your "plastics partner".
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