Plastics Extrusion - A Brief Explanation
PVC Plastic has become such a complete part of modern life that we take it
for granted, but on further examination, the process followed by plastic
manufacturers to get a quality, consistent result, is actually quite
complicated.
Plastics extrusion is highly automated, and operates continuously without much
in the way of human intervention. This has driven the cost of production down to
the levels we see today, making plastic a very inexpensive material.
The process can best be described as forcing molten plastic under pressure
through a die to produce a specific product. The range of products varies from
short items cut to length such as piping or tubing, through to continuous
plastic sheeting.
Plastic granules or resins are gravity-fed into a hopper connected to a screw
drive with a dual purpose - to mix the plastic if more than one type is being
used, and to melt the plastic as it makes its way through the screw drive.
The following are just a few of the many different types of plastics extrusion,
with a brief explanation of the manufacturing process for each one:
Standard Plastic Extrusion
The liquid plastic is screened through a filter to remove any particles, then
forced through the die and into a cooling bath before being cut into lengths or
rolled into sheets, depending on the product requirement.
Sheet/Film Extrusion
For plastic sheets or film, the product is pulled through a series of cooling
rollers which both cool the plastic and determine the sheet thickness and
texture. The liquid packaging industry relies on plastic extrusion to meld
plastic to paper and also aluminium foil to create such packaging staples as
juice cartons and wine casks.
Blown Film Extrusion
Blown film extrusion is used to make plastic shopping bags. When the product
reaches the die, in this case an upright cylinder of varying dimensions, it is
pulled upwards to a height ranging from a few metres up to 20 metres, depending
on the amount of cooling it requires. Compressed air is forced into the centre
of the extruded material, creating a bubble which is flattened by rollers into a
double layer of film.
Tubing Extrusion
The process is the same until the product reaches the die, which has a pin in
the centre to create a hollow cross-section which becomes tubing. This pin is
connected to a pressure source to stop the tubing from collapsing when it enters
the cooling bath.
Plastic Manufacturers use a range of materials in extrusion including
polyethylene, polypropylene, acrylic, nylon, polystyrene and polycarbonate,
among others. The versatility and cost effectives of PVC plastic makes it an
essential product material which will continue to evolve as technological
advances bring new processes on line.