Polymeric trays, containers and disposable tableware made of thermoplastic sheets for food items have become very popular recently. Almost all of those trays, as well as such large-sized items as store signs and displays, are made by means of the thermoforming technology.
The technology of sheet and film polymeric materials thermoforming is based on the adaptation of highly elastic properties of a polymeric half-way product (parison). Such properties can be achieved in the process of heating the parison in the specific temperature range. Under pressure (from 0.01 MPa) on the parison surface heated up to the elastic state, its plastic deformation takes place. If we put a molding surface as a roadblock to the way of the polymeric sheet deformation, then, after the cooling, we get its plastic copy.
There are three different ways of the molding pressure generation distinguished in thermoforming:
Vacuum Thermoforming of Sheet and Film Polymeric Materials
Vacuum thermoforming is accomplished using special vacuum-forming machines (VFM). The forming process includes several stages:
1. Pressure of the polymeric parison to the intermediate layer of the VFM lodgement.
This procedure is essential for reliable pressurization of the polymeric parison that excludes possible air leakage resulting from pulling out of the parison edge in the process of its deformation or temperature shrinkage.
2. Heating the parison up to the highly elastic state.
At this stage, it is required to achieve the uniform thermal field, which ensures the equality of physical characteristics of the polymeric material in every spot of the parison. Unequal resistance of the material in various spots of the polymeric parison leads to unequal degree of deformation in these spots. And thus, some walls of the product will be thin.
3. Preliminary blow forming (dome blowing)
The surface area of any item produced via vacuum forming is bigger than the surface area of the primary parison. Using this characteristic, it is possible to achieve uniform thickness of the item walls, if to blow preliminary the heated parison up to the surface area, corresponding to the final surface area of the formed sheet. That technological operation is carried out by means of creating excessive pressure in the air-proof cabin, the upper wall of which is a ready-for-molding polymeric sheet. The speed of blowing should be high enough to exclude early cooling of the polymeric material.
4. The mold lifting, vacuum forming.
- The worktable with the mold is lifted up to the highest position, where the table edge is pressed to the intermediate layer on the bottom side of the lodgement (on the upper side of the lodgement, the frame presses the polymeric sheet, which is making-up the air-proof forming cabin together with the worktable);
- The air is pumped out of the forming cabin – the vacuum blowing is carried out. As a result, under the influence of difference in pressure between the atmosphere air and the forming cabin, the heated-up parison starts to undergo deformation taking the shape of the mold set on the worktable.
5. The product cooling.
When the process of the parison deformation (forming) is done, the finished item is cooled down by centrifugal ventilators.
6. Preparatory actions for taking the finished product off.
After the cooling and vacuuming are done, a short-term air delivery into the forming cabin is carried out in order to take the finished product off the mold and put the worktable down.
The blow thermoforming principle is similar to the vacuum one. It is also related to the thermoplastic sheet mollification and heating up, however, there is a range of characteristics, which differ from the scheme given above:
For all the types of thermoforming, the final procedure is cutting-off the flash mold material.
The field of thermoforming technology application is huge – disposable tableware and plastic containers (for example, packaging for cakes), shrink film packaging, reusable molds for the manufacturing of cast stones and flagstones. The application of this technology in outdoor advertising – to produce 3D letters and figures - is of much interest.