Compression molding Explained
The words compression and tension go hand in hand for any mechanical system. But
fortunately or unfortunately we have only compression molding. Compression molding is
a process of molding plastics in to desired form. This is actually a type of forging only.
The apparatus essentially usually consists of piston and a block which applies pressure
on the material and acts as a moving component and the mould in which the polymer is
kept it acts a lower part. It is generally kept fixed.
The raw material used is generally thermosetting plastics like polyesters and epoxys the
raw material used is in the form of granules or preform. The material is used in the form
of granules and preform because it is easier to do so. If we use the plastic in the form of
a big dough it will naturally take larger time to melt.
The process consists of following three simple steps The charge is placed in the mould
cavity. Later on, the mould is pressurized so that the charge takes the shape of cavity.
Only that much pressure is applied so as to ensure uniform density of the product. The
material has to be cured and solidified before removing.Lastly; the mould is removed to
get the finished component.
The process is generally performed ‘hot’ with molding temperature reaching the range
of about 1100 to 2200 degree Celsius. The pressure applied varies from about 3.5MPa
for polyesters and epoxys. But general pressured exerted on thermoplastics is about 14
to 41.5MPa.
The process time required is 12-15 Sec for a small object of 3 mm thick. It may vary
from 7 to 10 min for larger objects.
The process is also done ‘cold’ but preheating the material reduces the process time
drastically to half. The cold molding cannot maintain high tolerances and has to be
supplemented by binders.
There can be following three divisions of compression molding
1. Positive type 2.Semi positive 3.Flash type. The flash type is the cheapest and most
accurate tolerances can be achieved.
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Compression molding products consist of electrical parts, flatware, gears,
buttons, buckles, knobs, handles, electronic device cases, appliance housing,
and large container.
It is used to make moderately curved or flat parts like lift gates, spoilers,
scoops etc.
The advantages include low initial costs. Secondly, there is practically no
wastage and accurate dimensional tolerances can be maintained.
We can apply this technique to composite thermoplastics with unidirectional
tapes, woven fabrics, randomly orientated fiber mat or chopped strand.
Also one of the greatest advantages is its ability to mold large, fairly
intricate parts. Compression molding produces fewer knit lines and less
fiber-length degradation than injection molding. Excellent surface finish is of
course achieved.
However, if you have ever experienced injection molding then you will
come to know that Production by injection molding is much faster. Also this
process is mostly limited to large parts with less or no curvature at all.
