Vacuum Forming

Mr BILLINGTON

Vacuum Forming (thermoforming) is a manufacturing process which uses heat to soften a thermoplastic over a mould block and use suction to stretch the material around the mould. In schools this is typically done using High Impact Polystyrene (HIPS), but a range of thermoplastics such as ABS, Polypropylene and Polyethylene can be used

A array of heating elements are set to distribute an even heat across the sheet of material. Once a temperature of around 120 - 190°C is reached they are moved over the thermoplastic
The thermoplastic sheet must be securely clamped to form an air tight seal with the machine and placed over the mould block
As the plastic heats it begins to soften and sag away from the clamp into the cavity below. This process is carefully timed depending on the material and its thickness
Once heating is complete the heating elements are moved away and the table is raised up into the sheet with the mould
When secured the vacuum pump is switched on, removing all the air from the cavity and pulling the plastic tight around the mould
As the plastic cools the pump is briefly reversed (blow) forcing air back into the cavity and aiding the release of the mould
The formed sheet is removed and the mould block released from the cavity. Finally the sheet is trimmed to size

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Vacuum forming is ideal for Batch Production (250 - 3000 per year) and can be used for a range of applications. Sheets up to 1.5M x 2.5M can be formed to create packaging inserts, baths, hot tubs and even small pools up to a depth of up to 1M

Importantly the mould block must be precision engineered and designed to allow easy release from the forming. A draft angle of at least 3° must be designed onto all vertical sides to prevent the mould block from being trapped and carefully placed air holes ensure effective suction and an accurate moulding

Advantages of Vacuum Forming are -

Very low production cost compared to other methods
Speed of production - Once setup each form takes minutes
Tooling cost are much lower in comparison to other methods
Ideal for prototyping and testing during design process
Uses extruded sheets of material over granules used in injection

Disadvantages of Vacuum Forming are -

Hollow mould - No base is created by the process and matching the form is difficult if internal fit is required
All side must be less that 87° (External) greater than 93° (Internal)
Forming can cause 'webbing' between surfaces
High material waste due to process
Thinuneven wall thicknesses which are difficult to control

Manufacturing - Vacuum Forming