Thermoforming Process

Thermoforming

• Process used to shape thermoplastic sheet into discrete parts

• Basic principles– Heat a thermoplastic sheet until it softens– Force the hot and pliable material against the

contours of a mold by using either mechanical, air or vacuum pressure

– Held against mold and allow to cool, and plastics retains its shape

• Thermoforming process is differed from other plastic processing because the material is not melted, lower pressures are required to thermoform (the mold materials is less sturdy materials compared to other processings)

• Disadvantage; generates greater amount of scrap, cost of the sheet materials is raised (because of separate sheet-forming step), limited design parts (parts with sharp bends and corners are difficult to be produced), process results in internal stresses

Summary of advantages & disadvantages of thermoforming process

Forming Process

• The techniques of forming process can be grouped into several types;– Fundamental Vacuum forming– Pressure forming– Plug-assist forming– Reverse draw forming– Free forming– Matched-die forming, etc

• The term ‘Fundamental’ is used to emphasize that this is the simplest method of vacuum forming

• Glossary: Sag point- the condition (temperature, time, thickness, etc) at which a sheet begins to sag inside the thermoforming oven

Forming Process: Fundamental Vacuum forming

• The material is clamped into a frame

• Material is heated, begin to sag, the center of materials moves downward and seal against the mold

• Vacuum is applied to the back of the mold (outside pressure of the air pushes the plastic against the mold)

Forming Process: Fundamental Vacuum forming

• Uneven wall thicknesses are observed in this technique

• As the sheet touch the mold, the thickness at the location become fixed, further stretching occur in the area that have not yet touched the mold, typically at he corner.

• Therefore the thinnest area occur at the corner, near the clamp

Forming Process: Pressure forming

• Positive air pressure from the top of the plastic used to force the materials against the mold

Pressure forming

• Methods– The materials is clamped and heated– Softening sheet is transferred to the mold

area and the seal is made– Air pressure is introduced and force the

plastic against the mold– The pressure is applied quickly to prevent the

sheet from cooling

• Advantages;– Mold cycle can be faster than fundamental

vacuum forming– Sheet can be formed at lower temperature,

because the forcing pressure is high– Greater dimensional control

Pressure forming

Plug-Assist Forming

• In this system, a plug is used to force the material into the mold

• Generally the plug will not pushed the material completely into the mold, then a vacuum is applied to draw the materials against the cavity walls

• Instead a vacuum, air pressure could be used to force the parts against the walls

new thermoplastic composite material that can be used to make thermoformingplug assists, it displays tremendous dimensional stability and low thermal conductivity. Designed to replace Syntactic foam, machined aluminum and wood for plug assists.

Plug-Assist Forming

• Better wall thickness uniformity than obtained from vacuum and pressure forming

• Plugs can be made of metal, wood or thermoset materials

• The plug should be 10% to 20% smaller in length and width than the female cavity

Plug-Assist Forming

Reverse draw forming

• Is used when very deep draws are needed• In this technique, the material is heated to the

sag point, and then blown away from the mold• The purpose of blowing is to thin the material in

the center of the sheet • The size of the bubble is controlled by an

electric eye. When the buble reachest the correct size, the pressure is topped and the plug press the material to the mold

• The vacuum is activated through the mold to draw the plastic against the mold wall

(a) Reverse draw step to Pre-stretch the material

(b) plug-assist and vacuumForming step

Reverse draw forming: Plug assist air slip forming

Free forming

• The forming is done without a mold, especially those where very high optical quality is required

• Touching the mold can result in undesirable changes in the surface quality of the part

• Complexity of shapes of parts is limited• Often used for canopies

• The part is expanded with air pressure

• The size of the bubble is monitored by an electrical eye

• When the bubble reach the desired shape, air pressure is reduced to a level that maintains the size while the part cools

Free forming

• Some parts with very complicated shapes, can best be formed if both plug and cavity mold are used together

• These mold must be matched so that it can be fit together

Matched die forming

Matched die forming• Material is heated to the

sag point, and then transferred to the molding station, where the plug and cavity molds are brought together to squeeze the material

• No vacuum or air pressure is applied during the forming process

• Material is mechanically pressed into the shape defined by matched molds, and allow to cool while the mold continue to press against it

Thermoforming Machine

• Not as complicated or costly as other plastic processing machine that melt the plastic materials

• Element of the machine; heating chamber or oven, method for removing the material from the oven, and locating it so that the sheet and the mold can be brought into airtight contact

• Machine can be single-mold batch system or continuous, highly automated machine

Single-station thermoforming machine

• Heating the sheet

• Clamping the sheet

• Moving the sheet and mold into proper relationship for forming, a vacuum or pressure system

Critical operation parameters

• The most important sheet property is thickness (variation in thickness over the sheet should be kept under 5%)

• Uniformity from sheet to sheet is desired, so that the processing parameter can be kept constant, i.e. melt index, if one sheet has a lower melt index than another, the amount of heat to achieve the same formation will be higher than in the sheet with lower melt index

• Other variables that might change from sheet to sheet are density, molecular orientation, etc

• Key parameters during the forming operations;– Speed of vacuum application- vacuum should

be applied as quickly as possible– Temperature of the mold- the mold is normally

at room temperature, or temperature below solidification point of the plastic

– Size of the bubble– Plug size

Critical operation parameters

Male and Female mold• Female mold- a mold in which the part is pressed into a

cavity

Draw ratio

• Is used to estimate the amount of wall thickness variation that might occur

• High draw ratio result in excessive thinning and wall nonuniformities

• Draw ratio = depth of part / width of part

Area ratio• Give some approximation of the amount of thinning that

will be experienced by the plastic sheet when it is transformed

• Area ratio = area of the sheet before forming area of the part after forming

• If the sheet is 200 cm2, and will be thermoformed into a part that has total area of 400cm2, the area ratio is 1:2

• Area ratio x desired thickness of finished part = minimum original thickness (thickness of the blank)

• The overall average thickness of the part will therefore be one-half of original thickness

• The area ratio is often used to calculate the size of the unformed sheet that must be used to make a particular part

• Determine the thickness of the blank for fundamental vacuum thermoforming, assuming that the thickness of the finished part is uniform. The finished dimensions of the part to be made is 60 x 50 x 5 cm prepared by 2mm thick blank. Consider the allowance for clamping and mold clearance = 2 cm on each side

1. Dimension of blank (with allowance)= 64 x 54 cm2. Calculate surface are of the part

50 x 60 x 1 side = 3000 cm250 x 5 x 2 sides = 500 cm260 x 5 x 21 sides = 600 cm2

Total area of finished part = 4100 cm23. Thermoforming area of the blank= 60 x 50

= 3000 cm24. Area ratio = 3000/41005. Thickness of the blank = 2 mm/0.7317 = 2.73 mm