This blog is translated from German with DeepL.
Thermoforming, colloquially also known as plastic thermoforming, is suitable for the production of shell-shaped parts from a plastic sheet. The plastic sheet is heated in a controlled manner and then drawn onto or into a mold by means of pressure and vacuum. The variety of parts ranges from the well-known biscuit/praline packaging to car dashboards.
Producing two mold halves to form a hollow body in a single operation: this is achieved with twinsheet thermoforming, in which two half-shells are welded together directly in the thermoforming mold before cooling.
The plastic sheets/foils inserted in the first step are pulled into the respective mold half after heating. Directly afterwards, the residual heat in the plastic is used to weld the mold halves into one part.
In addition to the shape, the upper and lower shells can also differ in thickness or color, among other things. The positional accuracy between the half-shells is tool-related and therefore relatively high.
Twinsheet thermoforming can ensure offset-free welding in a tool-bound manner. With machines specially designed for this purpose, the cost factor, process time and process reliability can be advantageous compared to alternatives such as gluing or welding following the manufacture of the individual parts.
Multi-stage thermoforming is not possible on commercially available machines. A prestretch die, as shown in the following figure, cannot be used due to the die arrangement. This can lead to poorer wall thickness distribution in the part at high forming degrees.
Tips for the developers
- Long steep walls result in a high degree of forming, i.e. the material is strongly stretched and thus locally very thin. This has to be considered even more in twinsheet thermoforming than in single-sided thermoforming because, as mentioned, no pre-stretching is possible.
- If the wall thickness distribution is problematic, this can be compensated with a thicker base wall thickness. This must of course be compared with the alternative manufacturing processes.
- Undercuts should be avoided wherever possible.
- On standard machines, the semi-finished product can often only be heated from one side per sheet, which has a considerable effect on the cycle time. In addition, air cooling distances on standardized molds are limited and therefore increase the cooling time.
- The weld seam must be included in the part. This should not be tolerated too precisely, or agreed with the manufacturer, in order to keep manufacturing costs low (depending on the manufacturer, e.g. trimming is done by machine).
- Depending on the requirements for the weld seam, different machine closing forces are necessary.
- Due to the process requirements, the twinsheet process cannot be carried out on a normal thermoforming machine. Therefore, early contact with a suitable producer is recommended.
- Twinsheet thermoforming is often not the most cost-effective way (generally cycle times higher). However, depending on the application, process reliability and offset-free welding may outweigh this. Today, specialized producers with a modern infrastructure also offer other advantages that counteract the cost pressure.