VACUUM-FORMING PLASTICS Flashcards
When is vacuum forming especially useful?
If you wish to make several identical mouldings.
What is vacuum forming?
The pressing of heat-softened plastic sheet over a mould by means of atmospheric pressure. Margarine tubs are formed this way.
Name one difficulty for vacuum forming?
It can be very difficult to make deep mouldings.
What can all vacuum-forming machines do? 3 points
a) enable a plastic sheet to be sealed temporarily on top of a vacuum chamber
b) heat up the sheet
c) caused a vacuum underneath the softened sheet to draw it down over a mould
Step one of vacuum-forming (4 parts)
a) The plastic sheet (usually polystyrene) is clamped in a frame over the vacuum chamber.
b) It rests on a SILICONE rubber sealing strip which is heat resistant.
c) The mould sits on a platform under the sheet.
d) The platform moves up and down.
Step two of Vacuum-Forming
A heater is pulled along a track from the rear to cover the sheet.
As it warms up, the sheet buckles due to expansion. It then goes flat again, like a drum skin.
Step 3
When the sheet is at the correct temperature, a lever is pulled that raises the platform inside the vacuum chamber.
The mould sitting on this platform is pushed into this hot sheet.
Step 4
A pump is turned on which quickly exhausts the chamber causing the sheet foot form tightly around the mould.
Step 5
The sheet is unclaimed from the frame and the mould tapped out.
Excess material around the mould is trimmed off.
Some vacuum-forming machines do not have the rising platform.
How do you normally make a mould?
From wood, MDF, is an ideal material.
Why are mould usually solid? 2 points
- Because the atmospheric pressure over many square inches can add up to a large crushing force
- So that it can be easily withdrawn from the plastic sheet
What is WEBBING?
If your mould is too deep, the excess plastic will stick together at the corners and this problem is called webbing.
Having a mould that is too deep will also mean that the sheet will become very thin on the vertical surfaces as it is drawn down.
What should you do to the surfaces of your mould?
Smooth them down with fine GLASSPAPER
Why should you smooth your mould with fine GLASSPAPER?
Because a rough surface may show through the thin plastic sheet and it will be more difficult to remove later when formed
Why should the mould have small holes drilled?
If possible, small holes misuse be drilled through the base so that pockets of air are not trapped when the sheet forms.
