WHAT IS THERMOFORMING ?

Thermoforming is a manufacturing process in which flat films or sheets are preliminarily heated to the temperature specific to a given type of plastic and then formed into a product with a spatial, desired shape. Relatively cheap and highly effective process results in thermoforming being commonly used for production of both: packaging and large-dimensioned products. The offered solutions take into account many different variants: from production of individual items and prototypes to large-scale, industry mass production.

Thermoforming consists of two basic processes: heating of the semi-finished product and shaping (forming). The plastic material is fixed around the edges between clamping (upper) frame and forming (lower) frame. Electric heating is provided by radiators located on one or both sides of the material. Heating time required to obtain the appropriate plasticization of given material depends on the type of polymer, thickness and colour. The type of polymer and the filler, if any, are related to the different thermal conductivity of the material being subject to heat treatment. White sheets absorb thermal IR radiation with greater difficulty and require longer heating time than dark-tinged elements. The advantage of thermoforming process over the injection moulding technology is clear. Thermoforming uses relatively cheap equipment and moulds in relatively low processing temperature and low pressure.

For the purposes of vacuum forming the following types of moulds are used:

• negative moulds (precise image of the form’s texture and dimensions is on the external side of the product),
• positive moulds (precise image of the form’s texture and dimensions is on the internal side of the product).

Thermoplastic sheets for thermoforming. Depending on the required properties of products (mechanical strength, chemical durability, resistance to weather, transparency, stamping depth, acceptable price), sheets of different thermoplastic materials are used for processing. Some thermoplastics require preliminary drying before processing. They include the following: ABS, ASA, PMMA, PA, PC, PSU, PES, and PET. Temperature and time of drying depend on the type and thickness of the plastic.

The following special, multi-material sheets are used with increasing frequency:

• made of polymer mixtures: PC + ABS, PC + PBT, PC + ASA;
• multi-layer: ABS/PMMA, ABS/ASA, ABS/PVC, ABS/PMMA, PS/PE, filled PP/PP, PP/EVA, PETG/PETA/PETG, PETG/PP;
• high-barrier: PS/EVOH/PE, PE/EVOH/PE, PP/EVOH/PP, PP/EVOH/PA.

For the purposes of thermoforming large-dimensioned products, rigid films 2÷4 mm thick or sheets 3÷15 mm thick are used.

Heating. The heaters are equipped with electric IR radiators: ceramic, quartz or halogen. They allow to heat plastic material from one side or from both: top and bottom simultaneously. Ceramic radiators are simple and most popular, whereas halogens are most effective and the fastest source of heat, allowing to save energy and reduce the heating time.

Forming is done by applying vacuum on the bottom side of the heated and plasticized material. Soft plastic is pulled and it settles on the mould mapping the shape. In case of complicated shapes and/or asymmetric oval parts the properly selected plug assist (stamp) can be used to pre-stretch the material and make the forming process easier.

Cooling. Cooling of the product in a mould occurs as the heat radiates out into the environment, cold air is blown through jet nozzle fans, and heat is absorbed by the mould. Sometimes, water mist is sprayed over the hot product surface to accelerate cooling. Cooling time, similarly to heating time, depends on the type of plastic, specifically on its thermal conductivity. Materials with low thermal conductivity require longer cooling (and heating) time. If thermal conductivity of polystyrene (PS) is assumed to be 1, then PVC and PET would be similar, ABS and PP would have smaller values (0.67), and those of PE-LD and PE-HD would be significantly higher (>2.2). After the formed element is cooled down it becomes stiff and can be demoulded and transported for further processing.