Introduction to Polymer Science and Technology Polymer processing

3.2.4 Thermoforming/ vacuum forming

The processes of thermoforming and/or vacuum forming are often referred to interchangeably. In thermoforming, however, a greater use is made of air pressure and plug assisted forming of the softened sheet. Thermoforming is often automated and thus faster cycle times are achieved than in vacuum forming, it uses plastic sheeting on rolls in a continuous operation rather than sheet blanks that are employed in vacuum forming of a discrete batch process. These processes and the differences between them are explained in detail by Strong 1996.

The materials most generally used in descending order of processing ease are PS, ABS, PVC, acrylics, PC, HDPE, PP and LDPE. The process of vacuum forming (Figure 3.43) is straightforward. A sheet blank is clamped over the vacuum box (thus sealing off the box) beneath a heater, which is often a retractable one. The heater may consist of infra-red elements mounted within an aluminium reflector plate, coiled ni-chrome resistance wire, metal rod heaters, hot air ovens, ceramic elements, or quartz tube. Once the blank becomes sufficiently pliable and begins to sag (the sag point), it is then sucked down and held tight into the mould cavity by the application of vacuum for shaping. After the part has sufficiently cooled, it is removed from the mould. Figure 3.43 shows a blank which is partially sucked into the vacuum box. In this process the mould is a female/cavity mould.

Introduction to Polymer Science and Technology

Polymer processing

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vacuum

Figure 3.43Illustration of vacuum forming

Alternatively, the heated blank is mechanically moved onto a male mould and draped over it and, simultaneously, vacuum is applied through the vent holes in the mould to force the sheet tightly onto the contours of the mould and hold it fast to give it the final shape. See Strong (1996, p378) for detailed description. In all these processes the forming operation is followed by cooling, often with air, and by a secondary operation to trim off any excess material from the moulding to obtain the finished product.

The process of stretching the sheet over the form or suction into a female mould causes thinning of the sheet, especially along the sides of deep drawn parts. The extent of draw depth is described by a parameter known as the draw ratio, which can be expressed in different ways:

draw ratio = (sheet thickness) / (part thickness);

draw ratio = (surface area of the formed moulding or part) / [(surface area of the blank) (or the footprint of the part)].

A real draw ratio is a measure of the biaxial orientation that the heated blank undergoes during forming. A linear draw ratio can be determined by scribing a line of known length onto the blank, the draw ratio then becomes the ratio of the length of the scribed line on the formed moulding to that of the scribed line on the sheet blank used to form the product. It is a measure of the overall uniaxial elongation capacity the softened plastic must have for the forming process.

The depth to which the material is to be drawn is important in determining the best technique to be used: for moderately deep draws (draw ratios less than 2:1), a basic vacuum female forming can be used. For products that require deep draw ratios, greater than 2:1, pre-stretched male forming or plug-assisted female forming is suggested to obtain the most uniform material distribution.

Pre-stretchis used to achieve "even" wall thickness. A small "bubble" is blown and the male mould is then raised into the pre-stretched sheet.

Plug assistis used for a deep draw product: a "plug" is used to push the material into the female mould during the forming process.

Introduction to Polymer Science and Technology Polymer processing

The BPF website (www.bpf.co.uk/bpfindustry/process plastics.cfm) shows animations of these techniques as a stand­alone operation as well as a continuous operation that starts with feeding of the sheet from a roll onto a processing line of heating-forming-trimming-stacking, and finishes with winding of waste off-cuts (skeletal) onto a roll.

Vacuum formed products include: PS - packaging trays, egg boxes, refrigerator liners and food tubs/pots; PVC - packaging and containers; and ABS - boats, caravans, vehicle body parts, shower bases and surfboards. The moulds can be made of wood, but for extended production runs reinforced epoxy resin or aluminium is preferred.

3.2.5 Rotational moulding

The Rotational Moulding process involves the following operations (Figure 3.44):

Charging mould- a pre-determined amount of pre-compounded polymer powder is placed in the mould; the mould is closed, locked and moved into the oven.

Heating and fusion- once inside the oven, the mould is rotated around two axes. The speed of rotation is relatively slow, less than 20 rev/min. The ovens are usually air circulating ovens with gas (propane) burners and the moulds are therefore heated mainly by convection. During the process, the plastic powder/pellets remain in the bottom of the cavity of the rotating mould. As the mould becomes hotter the powder adheres to the passing mould surface of the rotating mould, begins to melt and sinter/fuse together. Mould surfaces can be insulated (see Figure 3.45) and, therefore, left cooler during the heating cycle in order to either achieve a section with lower wall thickness or a section that requires no coverage with plastic powder to leave the product completely open in that section as in most agricultural water containers and feeders, wheel barrows and trolleys but in fact nearly all products require some sort of opening in their structure. Shielding to achieve lower wall thicknesses and the openings, as in a wheel barrow (mould is shown in the figure), are achieved by a layer of insulation such as rock wool placed inside the relevant mould parts (as in the lid of the wheel barrow mould).

When the melt has been consolidated to the desired level, the mould is cooledeither by air, water or a combination of both. The polymer solidifies to the desired shape.

De-moulding and unloading- when the polymer has cooled sufficiently to retain its shape and be easily handled, the mould is opened and the product removed. At this point powder can once again be placed in the mould and the cycle repeated.

Date: 2015-12-11; view: 826