Most solid plastic materials are produced by a process called injection molding. During the
process of plastic injection molding, plastic resin pellets are heated until they melt. The
melted liquid plastic is introduced, under pressure (injected), into a mold. The mold may
be made up of any metal such as steel or aluminum. The molten form is then allowed to cool
down and set into a solid form. The plastic material thus formed is then retrieved out of
the mold.
The injection-molding process has been around for nearly 150 years. Reciprocating screw
injection molding machines were introduced in the 1960s and are still used today. The
injection molding machine comprises an injection unit, where material is prepared for
injection into the mold, and a clamping unit, where the injected plastic is captured in the
mold under conditions of temperature and pressure to form the finished product.
The actual process of plastic molding is just an expansion of this basic process. The
plastic goes into a barrel or chamber by gravity or is force-fed. As it moves down, the
increasing temperature melts the plastic. Then, the molten plastic is forcibly injected
into the mold under the barrel with an appropriate shape. As the plastic cools, it
solidifies. The plastic molded like this has a reverse shape of that of the mold. A variety
of shapes can be produced by the process. The process of plastic molding is cheap due to
the simplicity involved, and the quality of the plastic material is modifiable by changing
the factors involved in the custom injection molding basics process:
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The pressure of injection can be varied to change the hardness of the final product.
Injection pressure causes the material to flow. Pressure increases as mold filling becomes
more complex. There is a direct relationship between injection pressure and injection line
pressure.
The thickness of the mold affects the quality of the part produced. On average, the minimum
wall thickness of an injection molded part ranges from 2mm to 4mm (.080 inch to .160 inch).
Parts with uniform walls thickness allow the mold cavity to fill more precisely since the
molten plastic does not have to be forced through varying restrictions as it fills.
The temperature for melting and cooling determine the quality of the plastic formed. In
plastic injection molding, the temperature of the melt in the cavity is generally between
200 and 300 degrees, the melt is formed in the cavity, cooled, and solidified into a
product profile.
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