Methods for Manufacturing Plastic Products. Part 1.

Today, polymers (plastics) and materials based on them are widely used in all areas 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, polymer materials are divided into two major groups, which largely determine how they are processed: thermoplastics and thermosets.

Thermoplastic polymer materials are solid materials that, under the influence of physical factors—specifically temperature (melting)—transition into a viscous-flowing state (polymer melt); consequently, their subsequent processing into products involves manipulating the melt. The most commonly used thermoplastics are polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC), polyamides (PA), and polystyrene (PS).

Reaction-curing polymeric materials are materials that, under the influence of external factors (temperature, radiation, pressure) or additional reagents, transition into a non-meltable and insoluble state as a result of physicochemical transformations (cross-linking and curing reactions). Thermosetting plastics include resins of various types: phenol-formaldehyde, polyester, epoxy, and others.

Below, we will examine methods for manufacturing products from thermoplastic materials.

Die-casting

The most common method for manufacturing plastic products is injection molding. This is a highly productive manufacturing method that allows for the production of products with extremely complex configurations, ranging in weight from fractions of a gram to tens of kilograms. At the same time, the method ensures high dimensional accuracy, the required performance characteristics, and the ability to produce products in any color. Almost all thermoplastic materials and some thermosetting plastics can be processed using injection molding.

Injection molding is performed using specialized equipment—injection molding machines (IMMs) (Figure 1). In the classic process, injection molding proceeds as follows: the polymer is fed from the feed hopper into the barrel of the injection molding machine; inside the barrel is a screw that rotates and moves the material further along the barrel, while simultaneously melting the material. The melt is homogenized and metered in the required dose; then, as pressure is applied to it through the screw’s forward motion, the melt flows into the mold cavity, where the product is formed. An important stage in the injection molding process is the design and manufacture of molds for the respective parts, since the quality of the product and the productivity of the equipment depend on the quality of the mold.

Extrusion

Another widely used method of plastic processing is extrusion. Extrusion is a continuous method of forming linear plastic products by forcing molten polymer through a die, followed by calibration, cooling, and setting the product’s geometric dimensions. Extrusion takes place on specialized equipment—extruders—which, depending on their configuration and the number of screws, vary in purpose and operating principle (Figure 2).

In general, the extrusion process consists of the following stages: feeding material from the hopper into a heated barrel; melting and conveying the material by the rotation of the screw to the die; forcing the polymer melt through the die; calibrating the dimensions of the product (extrudate) on a calibrator; cooling the product and cutting it. Extrusion is the most productive method for processing polymer and composite materials. This method is used to manufacture the following plastic products: pipes, sheets, films, profiles of various configurations, etc.

Thermoforming

Thermoforming has found widespread application in packaging materials technology. Thermoforming takes place in specialized machines according to the following process: a polymer sheet made of thermoplastic material is heated to its softening temperature and then molded into the desired shape under the action of air pressure, vacuum, a press, etc., followed by cooling of the product. This method is most widely used in the manufacture of blister packaging.

Blow Molding

Blow molding is used to produce various types of containers. In this process, a polymer preform (which can be produced by either injection molding or extrusion), heated to the required temperature, is blown into a suitable mold (Figure 3). This method is used to manufacture a wide variety of bottles, canisters, containers, and other items.

Rotational Molding

Rotational molding is a highly specialized method for forming large hollow polymer products. This method is used to produce hollow, asymmetrical products with volumes ranging from hundreds of cm³ to tens of m³. The method is based on shaping the product under the action of centrifugal force in horizontal and/or vertical planes. A mold made of lightweight sheet metal, with the polymer inside, is placed in a heating furnace. Under the influence of temperature and centrifugal force, the polymer material adheres to the mold walls and subsequently compacts (Figure 4).

3D printing

3D printing, a method for processing thermoplastic polymers, is becoming increasingly popular. The product is formed on 3D printers by depositing layers of polymer material fed from an extrusion head (FDM printing) (Figure 5). Typically, the extrusion head can move freely along all three coordinate axes. Using 3D printing, it is possible to produce parts of almost any configuration, including hollow ones, with layers made of different polymers. A drawback of 3D printing is its relatively high cost and slow production speed; however, it is ideal for small-batch production and prototyping.

Replast manufactures custom plastic products using injection molding and extrusion, and also provides 3D modeling and 3D printing services.

Author: Andriy Masyuk, Chief Technologist at Replast LLC, Ph.D. in Engineering