Which computers use an injection molding machine
A Injection molding machine (Completely Plastic injection molding machine, short often SGM called) is a machine that produces directly usable plastic parts from plastic granulate. For this purpose, the required molding compound is prepared in the injection unit and injected into a tool that represents a negative mold (cavity) of the desired plastic part. Depending on the process used (thermoplastic injection molding, thermoset injection molding or elastomer injection molding), various components of the machine are heated or tempered.
Injection molding machines generally consist of two parts: The Injection unitwho have favourited the plastic granulesprocessed and injects into the tool under pressure, and the Clamping unitthat picks up the tool (also form) and opens and closes it.
The centerpiece of the injection unit is a worm shaft, too slug called, which is in a cylinder. The inside diameter of the cylinder is equal to the outside diameter of the screw. The cylinder will mostly Screw barrel called. In the rear area of the screw cylinder there is a funnel into which the plastic granulate is filled. Through an opening (the Filling block) the granulate now trickles into the cylinder. Turned by a drive, the screw rotates in the screw cylinder and transports the granulate forwards.
In thermoplastic injection molding, the screw cylinder is powered by an electric Heating tapes heated from the outside. Due to this heat and the special geometry of the screw, the granulate is not only transported, but also sheared, the plastic melts, plasticized and homogenized. At the tip of the screw cylinder there is a nozzle that forms the transition to the tool.
In the case of thermoset injection molding and elastomer injection molding, on the other hand, the cylinder is tempered in order to prevent an excessively high melt temperature, which is caused by internal friction, as otherwise the molding compound would already react in the cylinder.
In the course of the metering process, the molding compound is now usually transported through a non-return valve to the nozzle and then backed up in front of it. In order to provide enough storage space for the molding compound, the screw is only subjected to a low pressure (back pressure) axially so that it can move in the direction of the feed hopper and the so-called screw antechamber is formed between the non-return valve and the nozzle, in which the volume of the material is located.
The back pressure acts against the melt, so that the melt is compressed and does not pull the screw back. The pressure exerted by the melt moves the screw back.
During the injection process, the screw is pushed axially towards the nozzle, whereby the non-return valve locks and the mass volume is injected through the nozzle into the tool.
After the parts are 90-98 percent full, the system switches to holding pressure. The mass must remain in the cylinder (residual mass cushion), otherwise the pressure cannot act on the mass. The holding pressure is required to compensate for the volume shrinkage.
The three-zone screw is often used in thermoplastic processing. The plastic granulate is drawn in in the so-called feed zone and conveyed to the next zone, the compression zone, where the plastic is plasticized and compressed (degassed). The melt is then homogenized in the metering zone and finally pushed by the non-return valve in front of the screw, which moves axially backwards as a result of the increasing dynamic pressure in the cylinder.
The non-return valve prevents the volume of mass in front of the screw from flowing back into the screw threads during injection and repressing, so that the screw functions as a piston. The non-return valve is located at the end of the screw and usually consists of three parts: the tip, also known as the screw tip, the locking ring and the pressure ring. The locking ring sits on the screw tip and in front of the pressure ring and both together act as a stop. If the dosage is carried out, the melt presses the locking ring against the screw tip, so that a gap is created between the locking ring and the pressure ring (locking ring stroke) through which the melt can flow. When the injection is carried out, the locking ring is pressed against the pressure ring and thus closes the screw threads. The distance between the locking ring and the cylinder is called the screw play.
The clamping unit usually consists of three plates which are arranged vertically on a machine frame in an axis. The fixed platen carries one half of the mold (nozzle side) and is the platen closest to the injection unit. The fixed clamping plate has a hole in the middle through which the nozzle can pass and dock onto the tool. The second platen is the movable platen. The second half of the tool (ejector side) is mounted on it. It is movable and is pushed mechanically or hydraulically towards the fixed platen. Since the two halves of the tool are pushed together, this is referred to as To drive. Finally, the third plate is the face plate. It has a supporting function, because the toggle lever or the hydraulic cylinder for closing the clamping plates is located between the face plate and the moving clamping plate. The face plate is therefore necessary to build up strength. There is also the on the moving clamping plate Ejector, these are hydraulic cylinders which, when the mold is open, actuate small metal pins in the mold, which then push the molded part out of the mold. There are also connections for Core pulls. So can Slider be actuated in the tool, these are movable areas in the mold in order to be able to produce undercuts (see also mold making).
The force values acting on the clamping unit are defined (according to DIN 24450) as follows:
• Closing force
• Locking force
• (maximum) mold opening force
• Ejector force
The ejector (often also called ejector) has the task of ejecting a finished plastic part from the mold after the mold plates have been opened. The ejector usually consists of a hydraulic cylinder attached behind the movable mold plate, the piston rod of which extends through the movable mold plate. Ejector pins in the mold can be coupled to this piston rod so that when the ejector moves forward, the ejector pins in the tool also move forward and eject the part. Often one encounters the central ejector or the ejector plate. A central ejector is mounted in the middle of the movable mold plate and its piston rod extends through the plate. There are also ejectors that move a plate behind the movable mold plate. Several more rods can be attached to this plate, which can also be moved through the movable mold plate. In this way, parts of molds can also be ejected that are not arranged centrally or that several ejector pins have to move forward separately. With the advent of the fully electric injection molding machine (i.e. no more hydraulics), the ejectors are no longer operated by a hydraulic cylinder, but by an electric motor. Often, ejectors are also installed on the fixed mold plate or the cylinder is built into the mold and not on the injection molding machine. The position of the ejector is usually monitored, for example by limit switches or distance measurement. This means, inter alia. prevents the mold from being closed with the ejector advanced. Monitoring is also required when the ejector and core pull interact.
The process depends on many things. However, there is always a basic scheme in which further steps can be integrated as required.
At the beginning of the cycle, the mass volume is added and the mold is opened. First, the mold is closed and the injection unit is brought up to the mold. The molding compound is then injected. After a certain waiting time for the molding compound to cool down or react, metering is carried out and the injection unit is lifted off again. If the molded part has now cooled down sufficiently or has reacted completely to be removed from the mold, the tool is opened and the molded part is usually ejected and a new cycle can begin.
The arrangement of the clamping unit in combination with the injection unit makes different working positions of injection molding machines possible.
Orders of magnitude
Injection molding machines come in a wide variety of sizes. They differ not only in the amount of molding compound processed, but also in the pressure with which the plastic is injected, in the area of closing, and consequently in the force with which the tool is compressed.
Another difference is the way in which the force in the clamping unit (Closing force) is built up. A distinction is made between hydraulic machines, in which the mold is built up and held by large hydraulic cylinders, and toggle-lever machines, in which a large, double-acting toggle lever compresses the tool. The toggle lever itself is in turn moved either by a hydraulic cylinder or, in the case of an electric machine, by an electric motor, but the mechanics of the lever require significantly less force to hold the mold together than with exclusively hydraulic systems.
Knee lever systems usually work faster and more energy-efficiently, but become inefficient in larger machines due to the huge toggle mechanism. That is why one works more hydraulically in larger systems.
There are also some special forms, such as the two-platen clamping unit. Here the tool is not compressed from behind, but rather pulled together hydraulically. Due to the special structure, special, large tools can be used.
There are also electrical series in which the clamping force is generated by electric motors. However, this is only possible with machines with very low clamping forces, since electric motors cannot provide a good compromise between force and speed.
Multi-component injection molding machines offer the possibility of producing the often required complex molded parts from different colors or types of plastic in one process. In addition to the classic multi-color and hard-soft combinations, various special processes can also be implemented with the multi-component machines.
Another design are the electric injection molding machines, in which all axes are driven electrically. The clasps are built as toggle levers.
The information with which one can evaluate and assess an injection molding machine in terms of its size is specified in the Euromap standard (Euromap website). Since an injection molding machine consists of two parts, the injection unit and the clamping unit, clear parameters are given there. Then an injection molding machine is characterized by
- the clamping force with which the tool is compressed, given in kN (kilo-newtons)
- the calculated stroke volume of the injection unit in cm³, based on an injection pressure of 1000 bar. In other words, the amount of molding compound that the injection unit can inject into the clamping unit at a pressure of 1000 bar (see below).
Example: 2100/1330 is an injection molding machine with a clamping force of 2100 kN and a stroke volume of 1330 cm³.
Category: injection molding
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