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Manufacturing Processes > Plastic processing > Vacuum Forming

 

Vacuum Forming

 

Process description

A plastic sheet is softened by heating elements and pulled under vacuum on to the surface form of a cold mold and allowed to cool. The part is then removed.

Materials

  • Several thermoplastics that can be produced in sheet form.
  • The material to be processed should exhibit high uniform elongation.
  • Can also introduce some fiber reinforcing material to improve strength and rigidity.
Vacuum Forming

Process variations

  • Molds are usually made of cast aluminum or aluminum filled epoxy.
  • Sheets can be heated by infrared heaters or in ovens.
  • Can have top and bottom heating elements, or top heating element only.
  • For thick sheets, a top enclosure and compressed air is used.
  • Sheet is drawn over mold with additional force, other than provided by the vacuum, until cooled.
  • Thermoforming: for thin-walled parts such as packaging.

Economic considerations

  • Production rates from 60 to 360/h commonly. Cups can be produced at 3600/h.
  • Lead times of a few days typically.
  • Material utilization moderate to low. Unformed parts of the sheet are lost and cannot be directly recycled.
  • Full automation achievable.
  • Multiple molds may be used.
  • Setup times and changeover times low.
  • Sheet material much more expensive than raw pellet material.
  • Production volumes economical in small batches of 10– 1000.
  • Tooling costs low to moderate, depending on complexity.
  • Equipment costs low to moderate, but can be high if automated.
  • Labor costs low to moderate.
  • Finishing costs low. Some trimming of unformed material after molding.

Typical applications

  • Open plastic containers and panels
  • Pages of Braille text
  • Vending cups
  • Food packaging and containers
  • Automotive parts
  • Electrical cabinets and enclosures
  • Bath tubs, sink units and shower panels
  • Dinghy hulls
  • Signs

Design aspects

  • Shape complexity limited to moldings in one plane.
  • Open forms of constant thickness.
  • Undercuts possible with a split mold.
  • Cannot produce parts with large surface areas.
  • Bosses, ribs and lettering possible, but at large added cost.
  • Parts with molded-in holes not possible.
  • Corner radii should be large compared to thickness of material.
  • Sharp corners should be avoided.
  • No parting lines.
  • Draft angles of 1° or greater recommended.
  • Maximum section =3 mm.
  • Minimum section =0.05–0.5 mm, depending on material used.
  • Sizes ranging from 25mm² to 2.5 × 7.5m in area.

Quality issues

  • Control of temperature, clamping force and vacuum pressure important if variability is to be minimized.
  • Thermoplastic material must possess a high uniform elongation otherwise tearing at critical points in the mold may occur.
  • Sheet material will have a plastic memory and so at high temperatures the formed part will revert back to original sheet profile. Operating temperature therefore important.
  • Uniform temperature control of sheet important.
  • If multiple molds used it is necessary that there is sufficient distance between cavities to avoid flow interference.
  • Excessive thinning can occur, particularly at sharp corners.
  • Surface detail fair.
  • Surface finish good and related to the condition of mold surface.
  • Achievable tolerances ranging ±0.25–±2 mm, and largely mold dependent. Wall thickness tolerances typically ±20 per cent of the nominal.

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