Sheet plastic can be heated in an oven for a short time and made soft. The soft plastic sheet can
then be placed over a suitable shape, called a pattern, and when the air is removed from under
the plastic sheet, atmospheric pressure forces it over the pattern. This process is called vacuum
forming.
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The packaging industry uses vacuum
forming extensively, biscuit trays,
margarine containers, hardware
blister packs to name a few.
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Refrigerator linings are commonly
vacuum formed. The egg shelf here
has no moulding marks or join lines.
Atmospheric pressure has forced the
softened sheet into the mould and
left no trace.
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What You Need
Plastic forming equipment is usually very expensive, but Plasform have developed a way to do it
using an oven for heating the plastic sheet, a small wooden box with holes in the top, and a
vacuum cleaner for forming it over a pattern. The plastic can then be used as a mould or joined
with other sections to make a project. Sounds complicated, but really it is quite simple.
What you need is an oven with a clear
door; a forming box with a pattern (in this
case your clay shape); a vacuum cleaner
and hose into the forming box; a hardboard
frame; plastic sheet cut to size - 150mm x
200mm; masking tape; and oven mittens.
Also you will need two empty jam tins with
the labels removed to hold the forming
frame above the wire rack in the oven.
Step by Step
1. Position the jam tins to support the frame
above the oven shelf. Switch the oven on
and set the temperature at 225 C (450 F)
and leave to heat.
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2. Cut a piece of plastic sheet 150mm long
from the 200mm wide strip of plastic sheet.
Take care not to scratch the sheet and avoid
excessive handling of the sheet as it
becomes like a magnet (static electricity) for
all kinds of particles off the bench and
elsewhere.
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3. Tape the sheet to the smooth surface of
the forming frame with the masking tape.
Make sure the centre of the tape follows the
edge of the plastic sheet, and press the tape
down firmly to both the sheet and the frame.
This prevents the plastic from curling up in
the oven.
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4. Position the forming box with the mould in place, close to the oven door. The heated sheet
cools quickly in the air so the oven to mould time must be kept to a minimum. Connect the
vacuum hose to the box.
5. When the oven is at temperature, put the oven mittens on your hands, place the hardboard
frame (smooth side up) in the oven, and shut the oven door. If you watch closely, you will see
ripples slowly pass over the plastic sheet (called stress relief), and then the sheet goes flat and
taut. After about 30-40 seconds the sheet softens and gradually starts to sag.
6. When the plastic sheet starts to sag, open the door, remove the frame, turn it over so the sag is
upwards, and place it firmly on the box. It is a good idea to practice this routine, because the
quicker you get the frame on the box, the better the product. The frame must be placed firmly on
the forming box to create a vacuum, and atmospheric pressure will form the soft plastic sheet
over the pattern. If it doesn't work out real good, you can place it back into the oven (step 5) and
try again. Make sure the tape is still pressed down firmly though.
7. When the sheet has had a few minutes to cool you can remove the pattern, gently tap on the
plastic but avoid too much pressure or the plastic may crease. With patterns like a deodorant
cap, you may need to make a small split in the plastic near the base, and use pliers to gently
rotate the cap until it comes out. Remove the masking tape without tearing as it can be used
again and again.
8. If you have formed a mould, it is not necessary to cut the plastic after the frame has been
removed. Use scissors to cut around the edge of your plastic shape. It is a good idea to cut the
product out to within 5 mm of the finished cut, and then go around again carefully.
9. Place all the off-cuts of plastic in a carton. These will be collected for recycling the next time
Plasform makes a delivery to your school.
10. There are only nine steps, but ten looks better. Maybe you could practice describing how you
made it, because you will be asked for sure. Rattle off terms like patternmaking, thermoforming,
atmospheric pressure, and make it sound like you do it every day.
If you REALLY want to sound as if you know
all about plastic forming, try this test:
Plastic
Forming Test
