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Home Process Equipment The Mold Material TroubleShooting
The Basic Injection Molding Process.
The injection molding process is the process where we melt a
thermoplastic
polymer, give it the desired shape and cool it down to solidify
the material
and produce a useable part. The steps are to melt our plastic,
inject it
under high pressure in a mold, cool it down and remove it from
the mold.
Melt. Shape. Cool Eject!
The injection molding process can be divided in 4 seperate
phases:
1. The Injection Phase
2. Holding or Packing Pressure Phase
3. Cooling & Plasticizing Phase
4. Ejection Phase
What happens exactly during those four phases of the molding
cycle?
At the start of each new molding cycle the screw is in the back
position and the
material is kept at a temperature suitable for the processed
material by the
heater bands around the barrel. The screw is at such a position
that there is
sufficient material in front of the screw to fill the tool
cavity for a single shot
plus a small amount to be able to apply the pressure in the
holding phase.
Material suppliers provide you with a general guideline how to
set up your barrel
temperatures with a given material. It is very important to
realize that the
actual melt temperature can significantly differ from the barrel
temperature,
by as much as 50 C based on my personal experience. This is due
to the fact
that other parameters will also influence the actual melt
temperature as is
further explained on the Melt Temperature and How It Is Achieved
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Last Updates!
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Regards,
Jan van Poppel
Injectionmoldingexpertise.com 2012
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At the start of the injection phase a pressure is applied to the
screw moving it
forwards. The check or back flow valve at the tip of the screw
closes as a result
of this movement and enables the screw to act as a plunger to
push the molten
material (the melt) in the mold cavity.
The speed of the screw moving forward can be controlled by
controlling the
speed of the screw or by maintaining a certain barrel pressure.
Most injection
molding machines allow you to vary the speed or pressure during
injection
creating an injection speed profile.
The injection phase ends when we have filled the 95 99 % of
cavity of the
mold and then the process moves to the next phase: holding
pressure. The point
at which we move from the injection phase to the holding
pressure phase is
called the Switch Over Point, abbreviated SOP.
In order to be able to still apply pressure to the melt in the
tool it is required
that there remains a certain amount of material in front of the
screw tip; this
is called the cushion.
The purpose of the holding pressure phase is to replace the
volume lost during
the cooling down and solidifying of the melt. The holding
pressure phase can last
as long as the gate remains open and molten material can still
enter the cavity.
In the cavity the material will cool down and exhibit thermal
shrinkage. In case
of semi crystalline materials shrinkage due to crystallization,
will be additional to
the thermal shrinkage, more detailed information can be found
here.
From the holding pressure phase we move into the cooling phase.
Apart from
cooling the part down sufficiently to be able to eject it from
the tool
undamaged, during the cooling the machine is prepared to deliver
molten
material for the next cycle.
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During the cooling time the material for the next shot is
prepared called
plasticization. The screw is rotated at a pre-set speed and
pushes the material
forward. As the nozzle of the injection unit is in contact with
the tool
containing the solidified part flow of the material is
prevented, pressure will
build up and the screw will be pushed backwards. The check valve
will open,
allowing material to flow in front of the screw tip. A back
pressure is applied
and acts as a counterforce to the screw that is pushed back to
improve the
melting behavior by increasing friction and improves the
degassing. More
detailed information on screw speed, back pressure and
plasticization can be
found here.
The cooling and plasticization phase ends when the screw reaches
its pre-set
position and the cooling time has lapsed and the cycle moves
into the ejection
phase.
The clamping force can be lifted and the tool can be opened to
allow the part to
be released from the tool. Ejector pins etc. usually aid with
the release of the
part from the tool. When the tool is closed and the clamping
force is reinstated
the machine is ready for the new cycle.
As you may gather from the description of the basic process,
there are many
parameters an operator has at his disposal to influence the
process. Please
review this list that mentions and explains the most common
parameters used
in setting the process.
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