Forming Thermoplastics

To create this I placed a plaster face mould on the platen of a vacuum former with abstract wire covering it. The plastic sheet then formed itself around to create this gory looking result.

Forming Thermoplastics

The range of thermoplastic materials is vast, though only a few are generally applicable to school workshop applications. Acrylic (ICI trade name'Perspex') is the most popular thermoplastic material for hand working. It is available in sheet, tube, rod and block form, and may be transparent, translucent or coloured.

Colours range right across the spectrum, offering exciting design possibilities. The degree of translucence varies from smoked pinks and greys to near solid colour. Projects related to light and light sources gain a great deal from researching the effects of different translucent acrylics, some of which have a fibre optic effect when viewed on edge.

Strip heaters are used to provide local heating for line bending. The principal is quite straightforward. An electric element, similar to that of an electric fire, is enclosed in a channel with an opening at the top. Acrylic sheet is placed across supports above the opening. The width of the strip to be heated is determined by the height of the supports.

It is difficult to be precise regarding heating times with any radiant heating processes, because it is dependant upon the colour of the thermoplastic. Dark colours absorb heat faster than light colours.

The wider the heated strip, the more gradual is the resultant bend. Thick acrylic should be turned during heating before the strip has become flexible to avoid blistering. Under and over strip heaters do not require the acrylic to be turned.

Ovens enable sheets of acrylics to be heater all over. This is necessary for forming and moulding. Ordinary domestic electric ovens are fine for this operation, though air circulating ovens are available to ensure uniform distribution of heat. The oven thermostat should be set to 170℃. Blistering is caused by overheating and the presence of absorbed moisture. Surprisingly, plastics do absorb moisture, but it can be dried off by subjecting it to heat, approximately 80℃ for 24 hours prior to working.

Single Curvature Forming

Bending Jigs should be used to hold the acrylic at the correct angle until it has set. Jigs are not formers; the shape of the bend is determined by the width of the heated strip and not the jig. Avoid sharp bends, as these will result in thinning and weakness.

Former must be well prepared and ready to accept the hot, flexible sheet that emerges from the oven. You must work quickly and have everything to hand, including gloves or tongs for handling the sheet. Good surface finish on formers is essential. Coarse woodgrain, joint lines and countersunk screw heads will all leave permanent impressions in the surface of hot acrylic. Formers need not always be custom made in wood, make use of piping, metalworking stakes or cooking utensils for example.

Drape forming for a single curve requires stretcher to hold the form until it cools. (Do not use a cloth with a coarse weave.)

Vacuum Forming

Acrylic can be formed by vacuum forming, but it is not an ideal material, because of its short plastic range. Acrylic begins to become flexible at 120℃ and is ideal for most forming processes between 150℃ and 170℃. Plasticity, the desirable quality for vacuum forming, is reached at 180℃, but unfortunately acrylic degenerates at 185℃. It is impossible to maintain this narrow temperature band over the whole are of sheet that is in free air and is also clamped around the edges. Therefore, vacuum formed acrylic lacks the definition possible with more suitable thermoplastics.

Perspex TX (from ICI) is an extruded rather than cast acrylic that goes plastic at 150℃ and is, therefore, suited to vacuum forming.

High density polystyrene, ABS (Acrylonitrile-butadiene-styrene), low density forming grade polythene, and flexible PVC (polyvinyl chloride) are also materials suited to vacuum forming.

Complicated deep shapes can be formed by this process. A high quality mould or patterned is required. the sides should taper slightly for ease of removal and there must be no undercuts. Large moulds and moulds with deep internal draws require small vent holes to avoid trapping pockets of air. The vacuum does not suck, the absence of air allows atmospheric pressure to push.

The Vacuum Forming Process

The thermoplastic sheet to be formed is clamped around its edges by an air tight clamp plate situated above the mould. Heat is applied by radiant heating elements located in a hinged hood just above the thermoplastic. When plasticity is achieved, air is drawn out and the thermoplastic is forced, by air pressure, onto the mould.

The problem with this basic process is the thinning that takes place when the form is deep. Some vacuum forming machines have two major refinements in order to improve upon the basic process. First the platen, the base upon which the mould stands, can be raised, and second there is a low facility.

1. The thermoplastic is heated by the radiant heater in the hood. The blow facility is then used to stretch the thermoplastic over the entire surface area, the thinning effect being more or less uniform.

2. The platen is now raised into the dome created by the blow. This overcomes the need for a deep draw down the sides of the former.

3. Finally the air is evacuated and atmospheric pressure forces the still plastic thermoplastic tightly over the surface of the mould.