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A History Of Plastics
From its birth in the late 1800’s, to recent developments
and applications, the injection molding industry has grown at a fast and steady rate. It
has evolved from producing combs and buttons to molding products for all production fields
including automotive, medical, aerospace, and consumer goods, as well as toys, plumbing,
packaging and construction. The following timetable shows some of the important dates
regarding the evolution of the injection molding industry:
Table Of Evolution
1868 - John Wesley Hyatt injection molds Celluloid billiard
balls.
1872 - John and Isaiah Hyatt patent the injection molding
machine.
1937 - Society of Plastics Industry founded.
1938 - Dow invents polystyrene (still one of the most popular
materials).
1940 - World War II events create large demand for plastic
products.
1941 - Society of Plastics Engineers founded.
1942 - D.M.E. introduces stock mold base components.
1946 - James Hendry builds first screw injection molding
machine.
1955 - General Electric begins marketing Polycarbonate.
1959 - DuPont introduces Acetal homopolymer.
1969 - Plastics land on the moon.
1972 - The first parts removal robot is installed on a molding
machine.
1979 - Plastic production surpasses steel production.
1980 - Apple uses ABS in the Apple IIE computer.
1982 - The JARVIK-7 plastic heart keeps Barney Clark alive.
1985 - Japanese firm introduces all-electric molding machine.
1988 - Recycling of plastic comes to age.
1990 - Aluminum molds make their mark in production injection
molding.
1994 - Cincinnati-Milacron sells first all-electric machine in
U.S.
Evolution of Screw And Evaluation Vs. Plunger
The machine that the Hyatt brothers invented was primitive, but
performed well for the situation they were presented. It was simple in that it acted like
a large hypodermic needle and contained a basic plunger to inject the plastic through a
heated cylinder into a mold. In 1946, James Hendry began marketing his recently patented
screw injection machine. This auger design replaced the conventional Hyatt plunger device
and revolutionized the processing of plastics. Screw machines now account for
approximately 95% of all injection machines.
The auger design of the screw creates a mixing action when new
material is being readied for injection. The screw is inside the heating cylinder and,
when activated, mixes the plastic well, creating a homogenized blend of material. This is
especially useful when colors are being molded or when regrind is being mixed with virgin
material. After mixing, the screw stops turning and the entire screw pushes forward,
acting like a plunger for injecting material into a mold.
Another advantage of using the screw technology is a reduction
of energy requirements. The injection cylinder that holds the plastic that is being
readied for the next cycle has a series of electrical heater bands around the outside.
When energized, these bands heat up the cylinder to the point of softening the plastic.
However, because the screw generates friction when it turns within the cylinder, heat is
generated. Thus the material is also heated from the inside out which results is less heat
required from the electrical heater bands to soften the plastic to the correct
temperature.
Although the screw machine is the most popular, there is still a
place for the plunger type machine. A plunger does not rotate. It simply pushes material
ahead, then retracts for the next cycle. It, too, resides within a heated cylinder.
Because there is no rotating, there is no shearing or mixing action. So, in a plunger
machine the necessary heating action is provided solely by the external heater bands
because there is no friction from the plunger as there is from the screw. Also, if two
different colored materials are placed together in the heated cylinder they are not
blended together. The plunger simply injects the materials at the same time. If the two
colors are, for instance, white and black, the resultant molded part will take on a
marbled appearance with definite swirls of black and white through the part. This may be a
desired finish for a particular product, such as lamp bases or furniture, and the use of a
plunger machine allows that finish to be molded into the product. Use of a screw machine
would result in a single color (gray) product being molded because the two colors would be
well mixed prior to injecting.
The injection molding industry has made a huge impact on our
lives. Starting in the workshop of the two Hyatt brothers, it has become a major focus for
manufacturing of products from toys to medical devices, and everything in between. The
future holds only great promise for more productive, cost effective methods of producing
more products using this technology. Improved methods, materials, processing, and tooling
will increase the advantages for product designers and manufacturers who choose plastic
injection molding as their primary method of manufacturing.
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