VACUUM FORMING

 Process

The vacuum forming process involves the heating of a sheet of plastic until soft and then draping it over a mould.  A vacuum is then applied which sucks the sheet over the mould so forming the desired shape.  Where the shape is more complex or has deep recesses a plug may be used to assist the pressing of the plastic sheet into the mould.

 

Once sufficiently cooled the part is then ejected and trimmed.

 

Materials

A wide variety of thermoplastics are supplied as sheet and vacuum formed.  The most widely used include ABS, PET, Styrene, Polycarbonate, Polypropylene, Polyethylene, PVC and Acrylic.

Products

Products manufactured using this method include:

Baths and shower trays

Yoghurt pots

Boat hulls

Machinery guards

Refrigerator linings

Parts of vehicle cabs

Exterior signs

Advantages

The low forming pressures means comparatively low cost tooling

As moulds can be manufactured from inexpensive materials, tool manufacturing lead times can be relatively short.

Disadvantages

Secondary operations are usually required to trim the formed sheet into the finished part

Uneven sections may result from the stretching process resulting in potential weak spots

Inability to hold tolerances

 

THERMOFORMING

 Process

Thermoforming is very similar to vacuum forming but greater use is made of air pressure and plug assisted forming.  Generally, this process has faster cycle times.  The majority of thermoforming applications are high volume so roll fed machines are used.  However, sheet fed machines may be used for lower volume products.

For very large volumes a fully integrated production line may be used with raw material being fed into a sheet extruder which in turn feeds the thermoformer.

The sheet is heated to soften and then moved to the mould tool.  Air pressure and plugs form the sheet to the required shape.  Another difference from vacuum forming is that many of the thermoforming tools also crop the part rather than using a secondary operation.

Materials

The most commonly used materials include Styrene, Polypropylene, PET and PVC.  Thermoforming is widely used for food packaging which requires barrier properties from the material to resist the passage of odours, moisture and gases.  Therefore, a co-extruded material, i.e. a combination of two materials with one of them usually being Ethylene Vinyl Alcohol (EVOH), is used.

Food packaging is frequently decorated.  This is achieved either by post mould printing or, more usually, placing pre-printed sheet into the mould cavities.

Products

Food packaging including meat trays, ice cream and margarine tubs, sandwich packs

Toiletries containers

Pharmaceutical containers

Blister packaging

Point of sales display trays

Advantages

Short lead times for tooling

Ability to produce thin-walled parts

Ability to produce large parts

Faster cycle times than vacuum forming

Disadvantages

Limited shapes possible

High scrap rates

Only one side of the part can be defined by the mould

As with vacuum forming there are variances in wall thickness

Inability to hold tolerances

 

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