Thermoforming is a post-extrusion process in plastic manufacturing, where the preceding extrusion process is producing sheets according to the thermoforming specification requirements. It is called Thermoforming, because the plastic sheets are produced continuously (like on a conveyor belt), heated in an oven, and then drawn with a draw ratio that matches with the mould size and with the help of a vacuum mechanism are moulded, cut, to finally become articles according to the mould design.
Plug assist sometimes helps the formation of the articles inside the mould. This technique simultaneously form the articles. It means that the thermoforming mould consist of many cavities which are arranged in rows and columns. Moulds with 42 cavities are generally used, in which case, one thermoforming cycle can produce 42 articles and there will be 42 holes in the plastic sheeton every thermoforming cycle. The cycles run continuously according to the sheet length that the extrusion stage produces. This Thermoforming process can be done in-line or off-line. In-line process is when the sheets are fed directly from the extruder into the thermoforming zone. Off-line process is when the raw material for the thermoforming process is in the form of rolls.
Compared with injection moulding, the shape of thermoformed articles are limited because the articles are formed not from molten material, but from plastic sheets which have a low modulus that is changed from one solid form into another solid form. Generally the articles are in an open shape, with top size being bigger than the body size, for example: mineral water cup. The thermoforming machinery is also simpler compared to the injection moulding machinery because of lower processing pressure and temperature. The processing pressure in thermoforming is slightly higher than atmospheric condition.
On the other hand, wall thickness variation is unavoidable. Once the sheets are stretched and orientated to fit the mould, the thickness in that position is fixed.Stretching is only possible on parts of the sheet that is not fit to the mould.See the figure below showing a sheet stretching by vacuum or pressure with plug assist.
Figure of a cup forming:
a) vacuum forming; b) pressure forming with plug assist.
The further the material has to flow in the mould, the thinner that part will be. Due to varying wall thickness, the depth/height of the article is also limited. If the stretched area gets narrower, the stretching depth also becomes more limited. The draw ratio is the ratio of article depth or height to the top part diameter. By using plug assist, the draw ratio can be increased while maintaining the wall thickness. The draw ratio of 1:1 can be increased to 2:1 by using plug assist. Using plug assist means using a female mould. Modern plug assist is made of teflon and to avoid premature cooling, the plug assist should be heated to a temperature near that of the of the sheets.
PS, HIPS, and PP are generally used in thermoforming applications, such as mineral water cups, yogurt cups, snack trays, and automotive spareparts. The degree of crystalinity of PP is typically around 65%, even though its crystalization rate is slow, which is 0.4 - 0.5 times slower than that of HDPE. Because of its slow crystalization rate, a higher cooling rate is required. Like PE sheets, PP sheets also look translucent. The translucent characteristic is influenced byits crystal (spherulite) size, which is evaluated in the visible light range wavelength ( +0.4 micron in the purple zone, and +0.7 micron in the red zone). Because the spherulite size of PP is smaller than that of HDPE, PP is more translucent than HDPE. Mineral water cups have a contact transparency, so that when the cups are filled with water, they look transparent.
Melted homo-PP has a much higher viscosity, but a lower elasticity compared to HDPE. The thermoforming process is a solid state transformation process relying on the sheet's elasticity at a relatively high temperature, approaching its melt temperature. Due to the low elasticity of the melted material, PP sheets tend to sag at high temperatures. If the PP sheet is heated only 3 - 5 oC higher than its melting point (163 oC), it deforms immediately from the rigid rubbery solid state into a viscous molten state. Therefore the temperature control in thermoforming is very critical.
PP sheets are generally produced by extrusion with an L/D ratio of 32, using a flat die, and the extrudate is cooled while being pulled by a chill roll to the required thickness. Usually the process uses a 3 stack chill roll. Usage levels of regrinded/recycled material must be a consideration, especially using recycled PP that already has a much higher MFI compared to virgin PP. Not only does it affect the optical properties, but it also influences the mechanical properties, and even the extrusion process. Monitoring to MFI of regrinded material is a must in order to maintain the quality of the sheet to conform to the required specification.
The speed and temperature of the 3 stack chill roll should be controlled carefully according to the temperature of the extrudate and the die gap. The temperature of the polished chill roll, that comes first in contact with the extrudate, is set at 55 - 65 oC. However, the third chill roll must be set as low as possible. For PP sheet extrusion generally a range of 6 - 8 oCis used. In practice the actual temperature of the sheet is much higher than that of the chill roll, depending also on the residence time of the sheet on the chill roll.
The sheet will pass through an oven, heating the upper and lower parts of the sheet. It is important to ensure that the sheet temperature is uniform when it comes out of the oven. The temperature difference between the side and centre part should not be more than 5 oC. The heating process must be very fast in order to achieve an optimum cycle time. An infrared heater is generally used, as well as a high frequency heater.