Basics of Injection Molding
Making polymers is a fantastic science.
Then there is the matter of shaping the plastic into useful objects....another
fantastic science. One of the most common methods of shaping plastic resins is a
process called injection molding. Injection molding is accomplished by large
machines called injection molding machines.
Resin is fed to the machine through the hopper.
Colorants are usually fed to the machine directly after the hopper. The resins enter
the injection barrel by gravity though the feed throat. Upon entrance into the
barrel, the resin is heated to the appropriate melting temperature.
The resin is injected into the mold by a reciprocating
screw or a ram injector. The reciprocating screw apparatus is shown above. The
reciprocating screw offers the advantage of being able to inject a smaller percentage of
the total shot (amount of melted resin in the barrel). The ram injector must
typically inject at least 20% of the total shot while a screw injector can inject as
little as 5% of the total shot. Essentially, the screw injector is better suited for
producing smaller parts.
The mold is the part of the machine that receives the plastic and
shapes it appropriately. The mold is cooled constantly to a temperature that allows
the resin to solidify and be cool to the touch. The mold plates are held together by
hydraulic or mechanical force. The clamping force is defined as the injection
pressure multiplied by the total cavity projected area. Typically molds are
overdesigned depending on the resin to be used. Each resin has a calculated
shrinkage value associated with in.
Some Typical Complications
Burned or Scorched Parts: Melt temperature may be
too high. Polymer may be becoming trapped and degrading in the injection nozzle.
Cycle time may be too long allowing the resin to overheat.
Warpage of Parts: Uneven surface temperature of the
molds. Non-uniform wall thickness of mold design.
Surface Imperfections: Melt temperature may be too
high causing resin decomposition and gas evolution (bubbles). Excessive moisture in
the resin. Low pressure causing incomplete filling of mold.
Incomplete Cavity Filling: Injection stroke may be
too small for mold (ie. not enough resin is being injected). Injection speed may be
too slow causing freezing before mold is filled.
Diagrams courtesy of Plastics: Materials and Processing,
Prentice Hall by A. Brent Strong |