Plastic Injection Molding

In this article on Machine Education mold making and mold design, we’ll give information about the plastic injection molding method. We’ll cover how a plastic injection machine works. We’ll examine the parts of a plastic injection machine.

Plastic Injection Molding

Injection molding isn’t a manufacturing method used continuously on its own; this method is mainly used only in cases where mass production is involved. Additionally, this method uses special machines and special molds attached to these machines. The machines used for this purpose have aspects resembling the machines used in metal die casting. In injection machines, spraying the liquid material into the mold is generally provided by a piston. However, in many cases, presses operating with a screw (auger) system are used, and these machines are gradually replacing piston-type presses.

Plastic Injection Molding Method

Special injection machines have been developed to prepare large volumes of liquid plastic and to inject it directly into the mold at any desired moment. This method closely resembles the long-known die casting of metals. Injection machines basically consist of a compression cylinder containing a piston, along with heating equipment used to melt the injected molding material and keep it at the desired temperature.

1- Outer mold half 4- Inner mold half 6- Injection piston 7- Hopper 8- Heater 9- Nozzle

The molding material is loaded into the funnel-shaped hopper (7) mounted on the machine’s injection cylinder, shown schematically in the figure. A nozzle (9), surrounded by heating elements (8), is fitted to the front of the cylinder. In front of the nozzle are the mold halves (1, 4). When the piston (6) is pulled back, the mold opens on its own; when the piston moves forward, pressure builds and the mold closes immediately.

As the piston moves forward, a certain amount of the molten material is pressed into the mold. Since the mold is cooled, the pressed material solidifies immediately. As the piston retracts, the solidified part is automatically ejected from the mold. While this operating method gives good results, especially for thermoplastic materials, it’s a bit complex.

Plastic Injection Molding Machines

The figure above shows the exterior view of a plastic injection machine and some of its parts.

How does a plastic injection machine work?

The figure below shows the parts of a plastic injection machine. The plastic injection machine’s operating system is explained in the section below.

Since these machines resemble both a machine and a tool, they’re hard to tell apart. For this reason, it’s best to examine these machines together with their molds. The figure above shows the schematic of such an assembly. In the figure, the injection press and cylinder are explained schematically. The moving plate is actuated via a double-jointed lever and cylinder.

Both cylinders are hydraulic; the required hydraulic oil is pumped upward from the reservoir (12) by the oil pump (9,10), and is directed through the injection cylinder (2), shut-off valves (8), the throttle valve (11), and the control valve (6). The control valve is a four-way valve, and this valve can actuate the injection cylinder from either side. Four-way valves are often remotely operated, either electrically or hydraulically. The clamping cylinder (1) is also generally operated the same way. In the system whose schematic is given, a camshaft is fitted to stop the press’s main motor once operation is complete, and to relieve the load on the entire hydraulic system.

Video of a plastic injection machine’s operating system

How does the plastic injection method work?

In the cross-section image above, you can see the approximate construction of the individual parts. This image shows the final state of the machine’s injection stage. Essentially, the mold, made up of a stationary lower part (21) and a moving upper part (23), is held closed by the press’s stationary work table and the moving pressure plate. The molten plastic material is injected from the injection nozzle (17) into the mold cavity through the runner channels (19). Before the pressing operation begins, the plastic material is loaded from the hopper (9) into the injection cylinder (11), and the pressing piston (12) is moved forward, while current is simultaneously supplied to the electric heating elements (15).

Heating is generally increased gradually, in proportion to the rising pressure. The molding mass is evenly mixed and distributed with the help of a torpedo (14) located in the middle of the machine. The torpedo is mounted stationary inside the cylinder (10). The liquefied material reaches the nozzle (5), gaining an even flow rate as it passes through the torpedo. After the injection process is done, the mold is cooled with the help of cooling channels (2).

After this, the moving plate (1) is returned to its starting position and the mold opens; the bridge (25) carrying the ejector pins rests against the stationary ejector. The ejector pins (27) on the bridge extend all the way into the mold cavity. With the help of these pins and the thick central pin (25), the injected workpiece is ejected outward.

Videos Related to Plastic Injection Molding

---

In this article, we tried to give information about the plastic injection molding method. We covered how a plastic injection machine works. We examined the parts of a plastic injection machine. See you in upcoming articles.

Related Questions