1. Munim Shaukat (Lecturer)
Mechanical Engineering Department
University of Lahore

2.  Thermoforming is a process in which a 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

3. The process involves
 Heating the plastic sheet to a temperature range
where it softens
 Then stretching the softened plastic against a cold
surface mold
 When the sheet has cooled, it is removed from the
mold and excess plastic is trimmed

4. Thermoforming Process consists of two main steps:
 Heating
 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).
 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

5.  Heating the sheet
 Clamping the
sheet
 Moving the sheet
and mold into
proper
relationship for
forming, a
vacuum or
pressure system

6.  It is the the earliest method used and also called simply vacuum
forming.It is the proces in which negative pressure is used to draw
a preheated sheet into a mold cavity. The process is explained in
next slide in its most basic form. The holes for drawing the
vacuum in the mold are on the order of 0.8 mm (0.031 in) in
diameter, so their effect on the plastic surface is minor.

7. • Vacuum thermoforming: (1) a flat plastic sheet is softened by
heating; (2) the softened sheet is placed over a concave mold cavity; (3)
a vacuum draws the sheet into the cavity; and (4) the plastic hardens
on contact with the cold mold surface, and the part is removed and
subsequently trimmed from the web.

8.  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 the corner.
 Therefore the thinnest area occur at the corner, near the
clamp
Plastics sheet
Thin corner

9.  Pressure forming is an alternative to vacuum forming involves
positive pressure to force the heated plastic into the mold cavity.
 its advantage over vacuum forming is that higher pressures can be
developed .. The process sequence is similar to the previous, the
difference being that the sheet is pressurized form above into the
mold cavity. Vent holes are provided in the mold to exhaust the
trapped air.

10.  Positive air
pressure
from the
top of the
plastic used
to force the
materials
against the
mold

11.  The third method, called mechanical thermoforming, 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.

13.  Heating the sheet
 Clamping the
sheet
 Moving the sheet
and mold into
proper
relationship for
forming, a
vacuum or
pressure system

14.  this point it is useful to distinguish between negative
and positive molds. Both types are used in
thermoforming
 A positive mold has a convex shape 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
positive mold is shown in the case of vacuum forming.
 Negative molds have concave cavities

15.  The difference between positive and negative molds may
seem unimportant, since the part shapes are virtually
identical, as shown in our diagrams.
 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.

16.  Female mold- a mold in which the part is pressed into
a cavity

18. Plastic types: Thermosets
General properties: more durable, harder, tough, light.
Typical uses: automobile parts, construction materials.
Examples:
Unsaturated Polyesters: varnishes, boat hulls, furniture
Epoxies and Resins:… glues, coating of electrical circuits,
composites: fiberglass in helicopter blades, boats,

19. Plastic types: Elastomers
General properties: these are thermosets, and have rubber-like properties.
Typical uses: medical masks, gloves, rubber-substitutes
Examples:
Polyurethanes: mattress, cushion, insulation, toys
Silicones: surgical gloves, oxygen masks in medical applications
joint seals

20. Plastic types: Thermoplastics
General properties: low melting point, softer, flexible.
Typical uses: bottles, food wrappers, toys, …
Examples:
Polyethylene: packaging, electrical insulation, milk and water bottles, packaging film
Polypropylene: carpet fibers, automotive bumpers, microwave containers, prosthetics
Polyvinyl chloride (PVC): electrical cables cover, credit cards, car instrument panels
Polystyrene: disposable spoons, forks, Styrofoam™
Acrylics (PMMA: polymethyl methacrylate): paints, fake fur, plexiglass
Polyamide (nylon): textiles and fabrics, gears, bushing and washers, bearings
PET (polyethylene terephthalate): bottles for acidic foods like juices, food trays
PTFE (polytetrafluoroethylene): non-stick coating, Gore-Tex™ (raincoats), dental floss

21. Thermoforming uses plastic sheet, which is heated, stretched,
cooled and mechanically cut
 The plastic sheet is manipulated as a rubbery solid or
elastic liquid
 The solid or elastic liquid properties are more important
than the viscous properties when thermoforming

22. we can thermoform both amorphous and
crystalline polymers
 Amorphous
 No organization, glass transition
 PS, ABS, PVC, PC
 Crystalline
 Organized region called crystals, glass transition and
melting
 PE, PP, Nylon, Acetal
Important thermal properties
 Enthalpy or heat capacity
 Thermal conductivity
 Temperature dependent density

25. Advantages of thermoforming are
 Low temperature, low pressure required
 Only a single surface mold is required
 Molds are easy to fabricate and use inexpensive materials
 No need for the plastic to flow
 Can make very large surface area to thickness ratios

26. Disadvantages of thermoforming
 Plastic material is more expensive because the pellets have
to be made into sheets
 Generally more waste to reprocess
 Can get a great deal of wall thickness variation

27.  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

28. It gives us 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

29.  Mass production thermoforming operations are performed in the
packaging industry
 Thin film packaging items that are mass produced by thermoforming
include blister packs and skin packs.
 Thermoforming applications include large parts that can be produced
from thicker sheet stock. Examples include covers for business
machines, boat hulls, shower stalls, diffusers for lights, advertising
displays and sins, bathtubs, and certain toys.
 We had previously mentioned contoured skylights and
internal door liners for refrigerators.
 These would be made, respectively, out of acrylic (because of
its transparency) and ABS (because of its ease in forming and
resistance to oil and fats found in refrigerators).

31.  Mass production thermoforming operations are performed in the
packaging industry
 Thin film packaging items that are mass produced by thermoforming
include blister packs and skin packs.
 Thermoforming applications include large parts that can be produced
from thicker sheet stock. Examples include covers for business
machines, boat hulls, shower stalls, diffusers for lights, advertising
displays and sins, bathtubs, and certain toys.
 We had previously mentioned contoured skylights and
internal door liners for refrigerators.
 These would be made, respectively, out of acrylic (because of
its transparency) and ABS (because of its ease in forming and
resistance to oil and fats found in refrigerators).

32. Blister Packs
Formed blister pack material
Skin Packs