Vacuum forming

PlasticsThermoforming

Dash board

Blister packaging

Disposable trayBoat hulls

Blister packaging

Thermoforming

• Thermoforming is a process in which a flat thermoplastic sheet is heated

and deformed into the desired shape.

• The process is widely used in packaging of consumer products and to

fabricate large items such as bathtubs, contoured skylights, and internal

door liners for refrigerators.

• Thermoforming consists of two main steps: heating and forming.

• Heating is usually accomplished by radiant electric heaters, located on one

or both sides of the starting plastic sheet at a distance of roughly 125 mm

(5in).

Thermoforming

• Duration of the heating cycle needed to sufficiently soften the sheet

depends on the polymer, its thickness and color.

• The methods by which the forming step is accomplished can be classified

into three basic categories:

1. Vacuum thermoforming

2. Pressure thermoforming

3. Mechanical thermoforming

Vacuum Forming

• Vacuum forming, commonly known as vacuuforming, is a simplified

version of thermoforming, whereby a sheet of plastic is heated to a forming

temperature, stretched onto or into a single-surface mold, and held against

the mold by applying vacuum between the mold surface and the sheet.

Vacuum Forming

• The vacuum forming process can be used to make most product packaging,

speaker casings, and even car dashboards.

Canopy of military aircraft

Vacuum Forming

Vacuum Forming

• The mobile phone seen below has plain packaging.

• The front is manufactured from vacuum formed high density polystyrene.

• The polystyrene is transparent which means that the mobile phone can be

seen through it.

• This type of packaging is called 'blister packaging'.

Vacuum Forming

• The first stage of vacuum forming is to manufacture a precise mould.

• This is a skilled job as any imperfections to the mould will show up every

time it is used to shape plastic such as high density polystyrene.

• The mould can be used hundreds and even thousands of times to produce

the same plastic part.

• Each part will be exactly the same.

Vacuum Forming

• The mould is placed in the vacuum former. It is placed centrally.

Vacuum Forming

• A sheet of high density polystyrene is placed above the mould and clamped

in position.

• Various thicknesses are available.

• Usually material 1mm thick is the most suitable for this type of vacuum

forming.

Vacuum Forming

• The heater is turned on.

• When the machine is being used from ‘cold’ it takes approximately five to

ten minutes to warm up to the temperature needed to heat the polystyrene

sufficiently.

• Once warm, polystyrene of this type takes only four or five minutes to heat

up before vacuum forming can begin.

Vacuum Forming

• The plastic (polystyrene) becomes pliable and flexible after a short time. It

must be very flexible before it can be formed properly.

Vacuum Forming

• When the polystyrene is ready the shelf is then lifted up towards the

polystyrene sheet.

• The air underneath the former is pumped out and the polystyrene takes the

form of the mould.

Vacuum Forming

• Normally, draft angles must be present in the design on the mold (a

recommended minimum of 3°). Otherwise, release of the formed plastic

and the mold is probably difficult.

Shower tray

Vacuum Forming

• Vacuum forming is usually, but not always, restricted to forming plastic

parts that are rather shallow in depth.

• A thin sheet is formed into rigid cavities for unit doses of pharmaceuticals.

Bath tray

Vacuum Forming

• Thick sheet is formed into permanent objects such as turnpike signs and

protective covers.

Yogurt pots

Vacuum Forming

• Relatively deep parts can be formed if the form-able sheet is mechanically

or pneumatically stretched prior to bringing it in contact with the mold

surface and before vacuum is applied.

Ski-boxes

Vacuum Forming

• Suitable materials for use in vacuum forming are

conventionally thermoplastics.

Refrigerator liners

Vacuum Forming

• The most common and easiest to use thermoplastic is High

Impact Polystyrene Sheeting (HIPS).

Sandwich box

Vacuum Forming

• This is molded around a wood, structural foam or cast/machined aluminum

mold and can form to almost any shape.

• Vacuum forming is also appropriate for transparent materials such as

acrylic which are widely used in applications for aerospace such as

passenger cabin window, canopies for military fixed wing aircraft.

Machine safety guards

Vacuum Forming

Common Problems:

• Moisture absorption:

– Absorbed moisture expands forming bubbles within the plastic's inner

layers.

– This will be solved by drying the plastic for an extended period at high

but sub-melting temperature.

Shop signsToy exterior body

Vacuum Forming

Common Problems:

• Webbing:

– Webs form around the mold, which is due to overheating the plastic

and so must be carefully monitored.

– Webbing can also occur when a mold is too large or parts of the mold

are too close together.

Disposable glass

Ice cube tray

Vacuum Forming

Common Problems:

• Parts Sticking:

– Objects that are formed often stick to the mold, so make sure there is

an angle of 3 degrees or more.

Disposable lunch box

Wall clock

Vacuum Forming

• Virtually all thermoplastics can be supplied as sheet and vacuum formed. The

more commonly used materials are listed below. Typical Materials Used:

– Acrylonitrile Butadiene Styrene

– Polyester Copolymer

– Polystyrene

– Polycarbonate

– Polypropylene

– Polyethylene (sheet and foamed sheet)

– Polyvinyl Chloride

– Acrylic

Passenger cabin windows in plane

Pressure Thermoforming

• At this point it is useful to distinguish between negative and positive molds.

• The molds having concave cavities are known as negative molds.

• A positive mold has a convex shape.

• Both types are used in thermoforming.

• In the case of the positive mold, the heated sheet is draped over the convex

form and negative or positive pressure is used to force the plastic against the

mold surface.

• The difference between positive and negative molds may seem unimportant,

since the part shapes are virtually identical.


• However, if the part is drawn into the negative mold, then its exterior

surface will have the exact surface contour of the mold cavity.

• The inside surface will be an approximation of the contour and will

possess a finish corresponding to that of the staring sheet.

• By contrast, if the sheet is draped over a positive mold, then its interior

surface will be identical to that of the convex mold; and its outside surface

will follow approximately.

• Depending upon the requirements of the product, the distinction might be

important.


• Another difference is in the thinning of the plastic sheet, one of the problems in

thermoforming.

• Unless the contour of the mold is very shallow, there will be significant thinning

of the sheet as it is stretched to conform to the mold contour.

• Positive and negative molds produce a different pattern of thinning in a given

part.

• Consider our tub shaped part as sample.

• In the positive mold, as the sheet is draped over the convex form, the portion

making contact with the top surface (corresponding to the base of the tub)

solidifies quickly and experiences virtually no stretching.


• This results in a thick base but with significant thinning in the walls of the

tub.

• By contrast, a negative mold results in a more even distribution of

stretching and thinning in the sheet before contact is made with the cold

surface.

• A way to improve the thinning distribution with a positive mold is to pre

stretch the sheet before draping it over the convex form.

• The heated plastic sheet is stretched uniformly by vacuum pressure into a

spherical shape prior to drawing it over the mold.


• In the process, closely controlled air pressure is applied to inflate the soft

sheet.

• The pressure is maintained until the blown shape has solidified.

Mechanical Thermoforming

• The third method uses matching positive and negative molds that are

brought against the heated plastic sheet, forcing it to assume their shape.

• In the pure mechanical forming method, air pressure (positive or negative)

is not used at all.

• Its advantages are better dimensional control and the opportunity for

surface detailing on both sides of the part.

• The disadvantage is that two mold halves are required; the molds for the

other two methods are therefore less costly.

Vacuum forming

Technology

Transcript of Vacuum forming