Types of plastic injection molding machines used in plastic parts manufacturing industry
Piston Injection Machines
Plastic injection molding with a single stage piston was the predominant system until 1955. This system consists of a barrel that is filled with plastic material, which is melted by heating bands with resistances located around the barrel. Subsequently the molten material is forced through a distributor or torpedo by the axial movement of a piston, thus injecting said material into the mold. In this type of machine, the barrel flow is predominantly laminar, causing poor mixing and very heterogeneous melting.
Machines with Preplasticization System
In the injection system with preplasticization or two-stage, the heating of the material and the development of the pressure necessary to fill the mold are isolated from each other, that is, they are independent, unlike the single-phase injection system in the which both operations are carried out in the same phase. In preplasticization systems, the material is heated to the molding temperature during the first stage of the process, then passes to a receptacle from which it is forced into the mold in a second stage. The first stage is heating or fusion and the second is pressure or injection. Within preplasticization systems, the most common types of machines are those with a piston and screw base or combinations of both.
Alternative Screw Injection Machine
This type of machine is characterized by melting and injecting the material using an alternative screw, which alternates its function of plasticizing and injecting the molten material. This arrangement represents the most significant advance in plastic injection molding and is the most widely used system today.
Multicolor Injection Machines
Initially, multi-color injection molding machines were used to produce keys for typewriters and cash registers. Since the appearance of this type of special machines, an important market has developed, stimulated by the demand for multicolor taillights for the automotive industry. These machines can be classified into two categories:
– Horizontal design with several injection units in parallel with each other.
– Vertical design with vertical connection unit and lateral injection units.
Rotating Machines
Despite the relatively short cool-down time in injection molding, methods are always sought to reduce the total cycle time, i.e. increase production. On some types of machines, the remaining movements of the machine, necessary to complete the cycle, cannot be carried out until the cool-down time has elapsed, unless it is the type of machine called “overlapping movements”. A good reduction in cycle time can be achieved by using multiple molds, placed on a rotating unit (horizontal or vertical). Each of these molds is placed in front of the injection unit to fill the mold and immediately rotate the table to fill the next one. Meanwhile, the former is cooling down and at the right moment the part will be opened and removed, without disturbing the subsequent injection processes.
Rigid Foam Injection Machines
These types of machines are used for manufacturing that require high rigidity, such as housings for electronic equipment (computers, controllers, televisions, etc.), food containers, accessories for washing machines, etc. The easiest way to increase the stiffness of a product is by increasing its thickness. The rigid foam injection technique involves the expansion of the molten material, either directly through the use of a dissolved gas or a gas produced by the decomposition of a chemical reagent at the temperature of the melt. The molten material expands through the gas, producing an increase in volume upon undergoing a pressure change as it leaves the injection unit and enters the mold. Care must be taken to inject a certain amount of material, which leaves enough space to expand and fill the mold.
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