Today, the polymers (plastics) and materials based on them are widely used in all spheres of human life. At the same time, to ensure a wide variety of plastic products, many methods for processing polymer and composite materials have been developed. In general, polymeric materials are divided into two large groups, which, accordingly, determines the method of their processing: thermoplastics and thermosets.
Thermoplastic polymer materials are solid materials that under the effect of physical influence – temperatures (melting) become viscous-flowing (polymer melt) and, as a result, their subsequent processing into products is associated with manipulations with the melt. The most common used thermoplastics are polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC), polyamides (PA), polystyrene (PS).
Thermoset polymer materials are materials that, under the influence of external factors (temperature, radiation, pressure) or additional of reagents, turn into a non-melting and insoluble state as a result of physicochemical transformations (crosslinking and curing reactions). Thermoset plastic include resins of various nature: phenol-formaldehyde, polyester, epoxy, etc.
Below we consider the methods of manufacturing products from plastics of thermoplastic nature.

Injection molding

The most common method for manufacturing plastic products is injection molding. This is a high-performance processing method that allows to get products of the most complex configuration with a mass from a part of a gram to tens of kilograms. The method provides high dimensional accuracy of the products, the necessary performance characteristics and the receipt of products of any color. Almost all thermoplastic materials and some thermosets materials are processed by injection molding.
Injection molding is carried out on special equipment – injection molding machines. (Figure 1) In the classical version, injection molding takes place according to the following scheme: the loading hopper is polymer in the material cylinder of the injection molding machine, in the material cylinder there is a screw that rotates and moves the material further along the material cylinder, while the material melts.
The melt is homogenized and collected in the required dose, then as a result of the transfer of pressure to it due to the translational movements of the screw, the melt flows into the cavity of the mold where the product is formed. An important stage in the injection molding process is the design and manufacture of molds for the corresponding parts, since the quality of the mold depends on the quality of the product and the productivity of the equipment.

Extrusion

Extrusion is another common method of plastics processing. Extrusion is a continuous method of forming molded plastic products by passing the polymer melt through a forming head (die) with subsequent calibration, cooling and fixing the geometric dimensions of the product. Extrusion takes place on special equipment – extruders, which in turn, depending on the configuration and the number of screws, are of different purposes and operating principles. (Figure 2)
In general, the extrusion process consists of the following stages: loading material from a hopper into a material cylinder, which is equipped with heating.
Melting and transport of material due to the rotation of the screw to the forming head (die), the transition of the polymer through the die, sizing of the product (extrudate) on the calibrator, cooling the product and cutting it. Extrusion is the most productive way to process polymer and composite materials. This method is used to make such plastic products as: pipes, sheets, films, profiles of various configurations, etc.

Thermoforming

The thermoforming method has found wide application in the technology of packaging materials. Thermoforming takes place on a special type of machine according to the following scheme: a polymer sheet made of thermoplastic material is heated to a softening temperature and then stretched in an appropriate shape under the action of air pressure, vacuum, press action, etc., with subsequent cooling of the product. The method is most widely used in the manufacture of blister packaging.

Blow molding

To create containers of various types, the blow molding method is used. The polymer blank (which can be obtained by injection molding or extrusion) heated to the required temperature is blown into the appropriate shape. This method is used to make various bottles, cans, containers, etc. (Figure 3).

Rotational molding

A highly specialized method of forming large polymeric hollow products is rotational molding technology. This method produces hollow asymmetric products with a volume of hundreds of cm3 to tens of m3. The method is based on the formation of a product under the action of centrifugal force in the horizontal and (or) vertical planes. During the manufacturing process, a light sheet metal mold with a polymer inside is placed in a heating furnace. Under the influence of temperature and centrifugal force, the polymer material adheres to the walls of the mold with subsequent compaction. (Figure 4).

3D Printing

The method of processing of thermoplastic polymers – 3D printing – is gaining more popularity. The product is formed on 3D printers as a result of layer-by-layer application of polymer material, which is supplied from the extrusion head (FDM printing) (Figure 5). Usually the die head can move freely along all three coordinate axes. By 3D printing, you can get products of almost any configuration, including hollow, with layers of various polymers. The disadvantage of 3D printing is its relative high cost and low speed of forming products, but it is ideal for small-scale production and modeling.

The Replast company manufactures custom-made plastic products using injection molding and extrusion methods, as well as provides services for 3D modeling and 3D printing.

Author – Chief Process Engineer of Replast LLC, Ph.D. Masyuk Andrii