Thermoforming

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Mug thermoformed from a striped polystyrene band and punched out after filling and sealing

The thermoforming or hot forming is a process for forming thermoplastic plastics under heat and with the aid of compressed air or vacuum . Because of its superficial similarity to deep drawing of sheet metal, it is also referred to as “vacuum deep drawing” or simply “deep drawing”, but it is a different process in essential points.

The thermoforming process is differentiated according to the semi-finished product used : thinner semi-finished products are called foils , thicker (from approx. 1.5 mm) plates . Semi-finished film products can be fed to the thermoforming machine on large (up to Ø 1.8 m) rolls.

Molded tool part made of open-pore cast aluminum and the thermoformed plastic film.

In addition are the thermoforming tools for the thermoforming.

Thermoforming "from the roll"

The animated gif shows a roll on the left, which, turning clockwise, delivers the film web to the symbolized machine at the top. The film goes through a top and bottom heater. The heating rays are symbolized flashing. The tool station shows a pre-stretcher at the top and the forming tool at the bottom, the parts move to the film web and the film is permanently deformed accordingly. In the next but one feed cycle, a cut-out punching plate moves from below under the downwardly deformed sheet of film and the steel strip is punched out from above. In the last part of the picture, the loose part falls out of the film web.

Articles made of thin film (0.2 to 1 mm) such as chocolate box inserts, game carton inserts, folding blisters for small items on perforated grid walls in retail stores, and yoghurt pots are widespread. Schiebekartenblister, sealing caps and the hard-to-open clamshell, which often small parts are sold and means of radio frequency - RF electrodes are welded, are also included.

The procedure

The film is fed from large rolls on the edges of the film web in so-called spiked chains. The cold film, which is still hard, is first fed to the automatic molding machine and then passed through in cycles. In a heating station , radiant heaters are controlled by industrial heating controls and the film is heated on one or both sides. The warm, now softer film is pulled apart slightly in the spike guides so that it does not sag so much. In the tool station, the film is held in place by means of a clamping frame, the pre-stretchers and the thermoforming tool move through the film level and roughly define the finished contour. Then compressed air comes from one side and a vacuum from the other in order to bring the film quickly and strongly to the water-cooled wall (contour) of the aluminum mold. The air between the foil and the aluminum tool escapes through holes 0.5 to 0.8 mm and / or slots 0.2 to 0.3 mm wide. The cooled, now solid film is separated from the molding tool and fed to the punching station in the next work cycle. There, the external dimensions or holes are punched into the finished workpiece using a steel strip .

In the next work cycle, the article is manually removed from the thermoforming machine or automatically stacked using a stacking shaft, clipboard, lifter and squeegee . The finished film articles are then removed manually in stacks, often packed in poly bags and placed in cardboard boxes.

There are also other types of thermoforming, which are only briefly listed here:

  • High-performance machines work with hardened steel tools and punch out the foil items at the same time as they are shaped.
  • Medium series machines are z. T. also only operated with vacuum or compressed air.
  • Small series are occasionally produced from a small roll or from foil sheets in semi-automatic machines and the parts are then separated using a roll punch and steel strip cut.

Complex designs can also be realized with slides, hinged and rotating cores. It is also possible to punch in more than one level. Packaging used to be cuboid; today they are almost always adapted to the often complex geometries of the articles to be packaged, or are only provided with curved surfaces and edges for this reason in order to obtain individual, "modern" packaging.

Thermoforming of sheet goods

Animated gif shows, in a simplified manner, the insertion of a foil plate into the heating station using a vacuum suction device. At the same time, the plate lies on the tool contour. The radiant heater flashes to indicate that the film is heating. The film then adapts to the contour of the tool

Sheets between 1 and 2 mm thick are processed for sorting trays, transport trays, simple technical parts, etc. v. a. m. Material thicknesses of 2 to 10 mm are used for refrigerator walls , bathtubs , dashboards , lights, prefabricated ponds , etc. This process works in a similar way to the one described above, with the restriction that sheet goods (waste) cannot be produced as efficiently. Plastic panels can be processed in a size of 2,200 × 3,300 mm. The height of the parts can be up to 1,500 mm. The mold size can be even larger for special applications. However, there are only a few semi-finished products (panels) in larger dimensions. When manufacturing thermoformed parts, it is always important to consider the stretching of the material, i.e. how much the material stretches and by how much the wall thickness of the deformed part is reduced. In addition, the undercut must also be taken into account in the geometries. This is limited depending on the material or requires complex mechanisms.

The materials

Monofilms used to be made of PVC , but today they are mainly made of PET , PS and PP . Multi-layer films (PSEVOHPE / PPEVOHPE…) with better heat sealing properties or vapor barriers for better shelf life of food etc. are also often used. Flocked foils: compass box inserts, cutlery box inserts, etc. a.

In principle, however, all thermoplastics can be processed in this process. There is a suitable plastic for every application. Large fenders can be made from talc-reinforced PE. ABS with an acrylic top layer is used for cladding that is exposed to strong weather conditions or comes into contact with chemicals. A certain electrical conductivity of small charge carriers (due to "deadly" static charge in the electronics processing area) is brought about by the addition of soot particles. The deformation of high-performance plastics such as PEEK is now also possible.

Applications

In addition to the classic application area of packaging , thermoforming has increasingly found applications in the industrial sector. As an alternative to injection molding, it is characterized by low tool costs and is particularly economical for small and medium-sized series. Examples of thermoformed parts are hoods and cladding of all kinds for machine and plant construction, for agricultural and construction machinery, industrial trucks and commercial vehicle construction. Furthermore, fenders, fittings and much more are manufactured. In the consumer goods sector, the application is u. a. to be mentioned in the manufacture of refrigerators and freezers. Here the inner containers and the inner lining of the doors are manufactured by thermoforming. Nowadays, this process (also called deep-drawing process) is used to manufacture accessories in the automotive industry, such as B. in the production of trunk tubs. Made of materials such as B. polyurethane foam or fiberglass mats, acoustically and thermally insulating components z. B. manufactured for the automotive industry.

In particular, the combination of shape, color and function allows industrial designers to increasingly resort to this process.

Web links

Commons : Thermoforming  - collection of images, videos and audio files

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