Vacuum Forming or Thermoforming is a plastic moulding method where sheets of plastic are heated to make them pliable, and then placed over or in a single-sided mould. The sheets are then forced to take the shape of the mould by removing all air from under the sheet through the application and holding of a vacuum between the plastic and the mould or tool.

The advantages of this method of plastic moulding for the point-of-sale and display industry are:		Click to view larger image
Required investment in tooling relatively small Low cost of tooling modifications Short turnaround time on production of tools Availability of a large range of suitable materials and colours Short production set-up time Low "Total Cost" of parts and tooling for short to medium runs Ability to pre-print onto materials prior to moulding
Disadvantages:
Some restrictions on part design to allow easy removal from tool Limited "Economies of scale" on longer production runs Limited tool life Broader tolerances than injection moulded parts Slower production rate than injection moulding

Vacuum forming tools are made from either MDF Board, epoxy resin or cast aluminium depending upon the life required from the tool.

Tools are drilled with tiny vacuum holes to assist with the removal of air from under the plastic sheet during moulding, and then fixed to a large vacuum table for placement in the vacuum forming machine. Generally a number of identical tools are placed on the same sheet to allow the production of multiple components from each sheet.

Step 1 - click to view larger image

The main restriction on tooling and therefore product design is that the part must easily lift from the tool once the plastic has cooled and again become rigid. This means that it is not possible to vacuum-form a completely closed shape (eg. a ball) in a single piece. This would normally be achieved by forming the piece in two or more parts and then joining them afterwards to form the closed shape.

	The completed tools are fixed to a "table" within the vacuum-forming machine. This table is mounted on a pneumatic ram inside a sealed "vacuum box" and can be raised to push into a sheet of plastic laid across the top of the vacuum box, at the same time forming a seal against the underside of the plastic sheet.
Step 2 - click to view larger image

Step 3

The sheet of plastic to be moulded is placed on the top of the vacuum box and is held in place by a "clamp-frame" which is lowered from above and presses the four edges of the sheet against the top edge of the vacuum box to form a seal.

Step 3 - click to view larger image

Sheet thicknesses vary from 1 mm up to 8 mm and generally vary depending upon the strength required from the finished part, as well as the vertical "draw" required from the sheet. This draw is the amount that the sheet will be required to stretch to cover the depth of the tool and can be up to 750 mm.

Step 4

A heater panel consisting of either wire elements or, in more modern machines, ceramic elements moves forward over the clamped sheet and heats it evenly across its entire surface.

Step 4 - click to view larger image

In some cases it is necessary to create hotter or colder areas on the surface of the sheet to encourage or discourage stretch in the material to achieve the desired material thickness over the surface of the part.

Once the sheet reaches its desired forming temperature (usually between 150 and 170 degrees C) the heater bank retracts to allow the tool, to be lifted into position from below the now pliable sheet of plastic. Heating times range from 1 minute to several minutes depending upon the thickness of the sheet being moulded.

Step 5

The "table" of tools is then raised pneumatically into the pliable sheet from below forming a seal between the edge of the table and the edge of the sheet.

Once this seal is made strong vacuum is applied to the gap between the tool and the sheet, which causes the sheet to pull hard against the tool and take its exact shape. The tiny vacuum holes in the tool are placed to allow the air to escape from any recesses in the surface of the tool and will then hold the sheet hard against the surface of the tool.

Step 5 - click to view larger image

Whilst maintaining the vacuum under the sheet, powerful cooling fans above the sheet blow chilled, and sometimes dampened, air over the surface of the still-hot sheet to cool it to a temperature where it will regain its rigidity and maintain its new shape once removed from the tool.

	Once the cooling cycle is complete a short blast of compressed air is applied under the sheet to release it from the tool, the tool is lowered into the bottom of the vacuum-box, and the clamp holding the sheet raised to allow the formed sheet to be removed from the machine.
Step 6 - click to view larger image