Structure and functionality of an injection molding machine

Structure and functionality of an injection molding machine

Most injection molding machines are universal machines. They are intended for the discontinuous production of molded parts. Macromolecular molding compounds (thermoplastics in plastics technology) are generally used for this purpose. The forming of these masses takes place under pressure and usually at high temperatures.

In order to be able to carry out these tasks, in most cases a certain structure is required for an injection molding machine. This has a direct impact on how the injection molding process works .

The structure of an injection molding machine consists of different assemblies, each of which has its own tasks. In the following graphic you can see an injection molding machine (shown from the front) with the most important functional units.

The back of the machine is often referred to as the removal side, as this is where robots usually automatically remove the finished molded parts. The machine’s hydraulic hoses and a water battery are usually located here.

The injection unit in the structure of an injection molding machine

The so-called plasticizing unit (injection unit) is of central importance for the functionality of the injection molding process . The plastic to be processed is melted in this unit. The plastic is also homogenized, conveyed and dosed here. The injection unit also injects the plastic into the injection molding tool. The area of responsibility of the injection unit includes, on the one hand, the plasticization of the plastic and, on the other hand, its injection into the tool.

The screw or screw shaft, which is placed in a cylinder, represents the core of the injection unit. The outer diameter of the screw always corresponds to the inner diameter of the cylinder, which is sometimes referred to as a screw cylinder. In the rear area there is a funnel into which the plastic granules to be processed are poured. The granules trickle into the cylinder through an opening in this funnel, the so-called filling block. The screw, rotated by a drive, rotates in the cylinder and thereby transports the raw material forward.

The way the injection molding process works in plastics processing is that the screw barrel is heated from the outside using heating strips. Due to the special geometry of the screw shaft and the heat supplied from the outside, the raw material is also sheared in addition to the transport of the granules in the screw cylinder. This causes the plastic to melt , whereby it is simultaneously plasticized and homogenized. The tip of the screw barrel is equipped with a nozzle. It forms the transition to the tool.

During the dosing process, the molding compound is transported to the nozzle and stored in front of it. It is usually routed through a non-return valve. The plastic melt is compressed by the dynamic pressure. The pressure exerted by the melt pushes the screw back.

Now the injection process takes place. The screw is pressed in axial alignment towards the nozzle. The non-return valve closes and the mass volume is pressed through the nozzle into the tool.

An important function of the injection molding process is that after the tool has been filled, it switches to holding pressure. The holding pressure is needed to compensate for the loss in volume of the mass during hardening. This is the only way to ensure the dimensional accuracy of the finished injection molded parts and the optimal surface quality.

The locking unit

In most cases, three plates are arranged vertically in a row on the frame of the injection molding machine. They form the so-called locking unit (often also a clasp). It is comparable to a horizontal press. While the nozzle-side clamping plate is fixed, the closing-side clamping plate is movably mounted on four bars.

The fixed platen is the one that is closest to the injection unit. It carries half of the injection molding tool on the nozzle side. It has a hole in the middle. This allows the nozzle to be moved up to the tool and docked.

The second plate is a movable clamping plate that makes the described functionality of the injection molding process possible – the so-called ejector side. The other half of the tool is mounted on it. This plate is always movable. It is pushed towards the fixed clamping plate using mechanics , hydraulics or electricity. The two tool halves are pushed together. This is why we also speak of “closing” in this context.

The third plate is the so-called end plate. It takes on a supporting function because the toggle lever, hydraulic cylinder or alternatively an electric drive that is needed to close the clamping plates is located between the front plate and the movable clamping plate. The front plate is used to build up force when closing.

The so-called ejectors are also mounted on the movable clamping plate. These are hydraulic cylinders. In the open position, you operate small metal pins through which the finished molded part is pushed out of the tool.

Depending on the structure of an injection molding machine, clamping units are available in different designs. These are:

• Electric locking units
• Toggle clamping units (partially hydraulic)
• fully hydraulic locking units

The fully hydraulic system has the advantages that it can be positioned as desired, greater precision compared to the other variants, and the elimination of the risk of unacceptable tool deformation and bar breakage.

In the toggle lever clamping unit, a hydraulic cylinder drives a toggle lever. The advantage of such systems is their self-locking and fast speed and movement. On the other hand, there is a risk of broken bars or deformation of the tool.

The machine bed

The plasticizing and clamping units are accommodated in the machine bed. It also serves as a container for hydraulic oil and houses the drive for the machine hydraulics. In some cases, control and operating devices are also housed in the machine bed.

The control cabinet

The control cabinet houses the instruments, electrical switching elements and controllers for the energy supply system of the injection molding machine. Therefore, it is actually the control and regulation unit of the machine. Modern machines are supplied with the parameters necessary for operation using a touchscreen, buttons or keyboard with a screen. A microcomputer is housed in the control cabinet. This switches the sequence control, monitors the manufacturing process, and saves the production and process data for later use.

The control panel

The modern design of an injection molding machine usually includes a central control panel. In contrast, the machines used to be controlled with regulators, slides and pressure counters, which were sometimes distributed throughout the entire machine. The first machine with a display only used this to display the set values. Over time, more and more functions were linked directly to the display or control panel and made available in a central location at the touch of a button. Since around 2004, injection molding machines have been continuously converted to operation using touchscreen displays.