Formech offer technical advice on problems that may occur during the vacuum forming process

Technical Advice

Below we have listed the most common difficulties with vacuum forming which can be rectified in a simple way. If you still need assistance please contact us on +44 (0) 1582 469 797 or fill in our online form.

1 The plastic doesn't pull down properly over the tool! - solution

2 The plastic is webbing or creasing on the mould! - solution

3 The plastic is thinning over the mould - solution

4 The plastic is blistered after forming! - solution

5 The formed part is difficult to release from the mould! - solution

6 There are imperfections on the surface of the formed part! - solution

7 The formed part is smaller than the mould! - solution

Solution 1

The plastic doesn't pull down properly over the tool!

Checklist
1 - Is the tool properly vented?

2 - Is the plastic sheet cast or extruded?
• Only extruded sheet is vacuum formable. Check with your plastic supplier for details.

3 - Has the sheet of material been heated sufficiently?
• Turn up the heater zones or increase the cycle times.

4 - Is there an airtight seal?
• Make sure the clamp frame is tight enough.
• Make sure the white silicone seal is in good condition and not allowing air to escape. Replace with new seal if necessary.

Solution 2

The plastic is webbing or creasing on the mould!

Checklist
1 - Is the plastic being overheated?
• Turn down zone controls slightly, where present, to prevent extensive sheet droop.
• Reduce the heating cycle time.

2 - Is the sheet of plastic too big for the mould?
• The plastic will web on the corners due to an excess of heated plastic with nowhere to go.
• Use a reducing window on the machine to permit the use of a smaller sheet of plastic. This will also make use of the machine more economical.

3 - Is there sufficient draft angle on the mould?
• A minimum draft angle of 5° should be incorporated into the mould design. This will give the mould very slightly sloping sides and will make mould release much easier, as well as helping to prevent webbing.

Solution 3

The plastic is thinning over the mould

Checklist
1 - Have the rules of mould design been followed?
• The mould should be of solid construction.
• Mould should be mounted on a 3mm thick baseboard wherever possible.
• Mould should be vented including base, on edges and internal corners.
• Mould surface should be dry.
• All vertical sides should have draft angles of a minimum of 5°.
• Ensure that any cavities in the mould are no deeper than 75% of the width of the cavity into which the plastic will be sucked. Failure to do so will cause material thinning.

2 - Is sufficient plastic being used for the size of mould?
• Always ensure there is enough plastic, particularly when forming deep moulds. It is possible to stretch the plastic prior to forming, by use of a special facility on some of our machines called pre-stretch. This can help to alleviate the problems associated with thinning.

3 - Is the mould too tall or narrow?
• Any tall and narrow mould with minimal draft angle is likely to thin and web.
• It will also be very difficult to blow the moulding off the tool.

Solution 4

The plastic is blistered after forming!

Checklist
1 - Is the sheet being overheated?
• Reduce heating cycle.

2 - Is the sheet being heated too quickly?
• Turn down the heating zones where applicable.

3 - Is the material hygroscopic?
• Some materials, such as ABS and Acrylic absorb moisture and need to be dried prior to use. These are called hygroscopic materials. Blistering or pitting appears on the sheet of plastic due to the moisture being drawn to the surface upon heating. Dry the plastic thoroughly prior to use in a drying oven.

Solution 5

The formed part is difficult to release from the mould!

Checklist
1 - Is there an undercut on the mould?
• This would make it possible for plastic to be sucked underneath part of the mould. It will therefore not be possible to blow it off the mould at the end of the cycle. Pack out any undercuts with clay, or re-design mould.

2 - Have draft angles been included in the mould design?
• See Rules of Mould Design.

3 - Poor mould surface.
• Make sure the mould has a smooth surface. Sand or smooth where appropriate.

4 - Is the mould too hot?
• This can cause the plastic to stick to the surface of the mould. Allow the mould to cool down before using again.

Solution 6

There are imperfections on the surface of the formed part!

Checklist
1 - Is the mould dirty or damaged?
• Make sure it is free from dust, debris and marks.

2 - Is the mould too hot or too cold?
• If too hot, allow to cool. If too cold, keep forming with it until warmed sufficiently - it won't take long.

3 - Poor mould surface.
• Make sure the mould has a smooth surface. Sand or shape where appropriate.

4 - Was the sheet of plastic already marked prior to forming?
• Use new sheet. Damaged sheet could be cut down and used with reducing windows.

5 - Is air becoming trapped between the plastic and the surface of the mould?
• This can occur on flat moulds. Lightly sand the area, or put a vent hole in the centre of the problem area.

Solution 7

The formed part is smaller than the mould!

Checklist
Due to the nature of thermo-formable material, a certain amount of shrinkage will occur. The following table will act as a guideline: