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Shrinkage: Moulds must be made oversized to allow for shrinkage. Usually, an
allowance of 0.003-0.005 in./in. (cm/cm) on male moulds and 0.005-0.006 in./in.
(cm./cm.) on female moulds is adequate. The material, its coefficient of thermal
expansion, and part design all affect shrinkage
Radii: The greater the radii the better from the perspective of material distribution
(and therefore cost). Over a male feature, the thickness of the material plus .030-
.060” is desirable. Radii on ribs and fillets should not be less than the minimum part
thickness. The radii should four times the wall thickness in areas of high loading or
where extra stiffness is desirable. Related to radii is the intersection of adjoining
surfaces.
Plug Assists – Plug assists pre-stretch the sheet and assist in forming. Plugs are
designed to conform to the cavity. Compared to the cavity, plugs should be 10
percent to 20 percent smaller in length and width to allow for clearance between the
sheet and the mould. Moreover, the plug should have no sharp corners.
Other considerations:
Corners caution: Avoid intersections of walls that are >90º, as it can be very difficult to
get material to flow into that feature.
Generally distance between two features, like ribs or cooling vents, needs to be no
less than 2x the material thickness.
Avoid multiple tall features too close to each other
As general rule, deeper or taller the part, thicker the starting gauge of sheet required.
Provide wider ribs for thicker walls.
Raw Materials: Common thermoplastics in sheet form are Polyethylene,
Polypropylene, PET, Polystyrene, HIPS, ABS, PC, Cellulose Acetate, ABS, PVC and
Acrylic. Only thermoplastics can be thermoformed not Thermosets.
Applications: Thin film packaging items include blister packs and skin packs for
packing cosmetics, toiletries, small tools and fasteners. Thin-gauge thermoforming
(less than 2 mm thickness) is primarily the manufacture of disposable cups,
containers, lids, trays, blisters, clamshells and other packaging of consumer
products. Thick-gauge thermoforming (greater than 3 mm) includes parts as diverse
as vehicle door and dash panels, bathtubs, contoured skylights, refrigerator liners,
automotive components, aircraft industry for windshields, interior panels, arm rests,
serving trays, etc, toys, electronic equipment, business machines, , spas and shower
enclosures, advertising displays , marine, and plastic pallets. Multi layer sheets are
used with a heat compliant top layer, used to make heat-sealed applications.PVC /
PE for Meat Trays, and CPET / APET for Meat Trays and Ready Meals
Advantages: Thermoforming offers processing advantages over competitive
processes such as blow moulding and injection moulding. Relatively low forming
pressures, large size parts can be economically fabricated; low moulds cost and
minimized lead times, extremely adaptive to design requirement and low production
costs, rapid prototype development, low initial capital setup costs.
Disadvantages: All parts needs trimming, parts with non-uniform wall thickness,
only one side of part is defined by the mould, material limitations to use and very thick
sheets cannot be formed.

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