Munim Shaukat (Lecturer)

Mechanical Engineering Department

University of Lahore

Introdution 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

Thermoforming ProcessThe 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

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

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

Vacuum forming 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.

• 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.

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

Thin corner

Plastics sheet

Pressure Forming 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.

Pressure forming Positive air

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

Mechanical Thermoforming 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.

Mechanical thermoforming: (1) heated sheet is placed above a negative mold, and (2) mold is closed to shape the sheet.

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

Important Note 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

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.

 

Male and Female mold Female mold- a mold in which the part is

pressed into a cavity

PLASTICS AND POLYMERS

General properties: more durable, harder, tough, light.

Typical uses: automobile parts, construction materials.

Plastic types: Thermosets

Examples:

Unsaturated Polyesters: varnishes, boat hulls, furniture

Epoxies and Resins:… glues, coating of electrical circuits,composites: fiberglass in helicopter blades, boats,

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 applicationsjoint seals

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

Plastic types: Thermoplastics

Plastics and Polymers

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

Plastics and Polymerswe 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

Summary of advantages & disadvantages of thermoforming

process

AdvantagesAdvantages 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

DisadvantagesDisadvantages 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

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 ratioIt 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

Applications 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).

Applications 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).

Blister Pack and Skin Pack

Blister Packs

Skin Packs

Formed blister pack material