Injection molding is a cyclic process in which a certain measured amount of plastic is melted. This warm liquid mass is then injected into a closed mold. After cooling, the mold can be opened and the products demold.
The injection molding process is understood to mean the entire completion of the injection molding cycle, in which the associated settings are particularly important. For each serial injection molding production, the optimum setting must be chosen for the plastic, the injection molding machine, the injection mold, and the peripheral equipment.
The injection molding cycle
The injection molding cycle is the complete machine run required to make an injection of one or more products. A cycle starts with the open mold, without products, with the ejector cylinder in the rearmost position and the melt plasticized for the next cycle. Then the entire cycle is completed and it ends with an open mold again.
The first step of the cycle is the closing phase. The injection molding machine performs the movement that causes the mold to close. The mold half that moves and the movable clamping plate are both heavy, they have a considerable mass together. Nevertheless, the mold should close as quickly as possible, so that the cycle time is as short as possible. To avoid extremely large mass forces when moving, the closing speed can be set in steps.
The second step is the mold protection. Before the end of the closing movement, a low mold protection pressure is switched over. This low pressure is sufficient to close the mould. The pressure is so low that if there is anything between the two mold halves, the movement stops, so that no mold damage can occur. At the optimum machine setting, the low pressure works until there is only contact between the mold surfaces. Then the mold is closed.
After the mold is closed, the cycle continues with the build-up of the closing force and the start of the injection. First, a switch is made to high closing pressure, which causes the machine to reach closing force. The set closing force must be correct. In order to prevent the nozzle from cooling down, the contact between the nozzle and mold is sometimes broken. The nozzle is then a few centimeters away from the mould. The injection unit only moves forward when there is sufficient closing force. The nozzle of the cylinder builds up a contact force against the aerosol of the mould. This contact force must be sufficient for a good seal between the nozzle and the mould. If the nozzle does not cool down enough and with hot runner molds, the injection unit will remain with the nozzle against the mold for the entire cycle. Then the contact force is built up immediately after the closing force is present. When the application force is large enough, the injection molding machine starts injecting.
Phase 4 is the injection and the beginning of the packing pressure. In order to inject the hot melt into the mold, the screw has to move forward with a certain speed and force. When injecting, we always want to fill the mold cavity quickly and in a controlled manner. The speed is adjustable in the machine program. It may be necessary to control the injection rate in steps. A certain pressure is required to inject the hot melt into the product cavity at the correct speed. This specific injection pressure depends on the speed of injection and the resistance encountered by the melt. If the mold were to be completely filled with the relatively high injection pressure, an extremely high pressure peak would occur. In order to avoid this, as soon as the mold has been filled sufficiently, it is switched to a usually lower packing pressure. During the injection phase, the mold is filled 95% to 98%. When this filling is reached, the injection molding machine switches to the lower packing pressure. This pressure then ensures that the mold cavity is completely filled.
The cycle continues with the packing pressure phase and the beginning of the cooling phase. The packing pressure is needed to refill the injected material, which shrinks during cooling. The packing pressure should remain active until sufficient material is refilled. This gives the product a well-wrapped and sleek appearance. The product therefore does not shrink excessively after it has been removed from the mould. At a certain point, the product has cooled down and solidified to such an extent that packing pressure is no longer useful. This results in extra stresses in the product and loss of energy and time. In fact, the warm plastic immediately starts to cool as soon as the hot melt touches the cold mold wall during injection. This cooling continues during the packing pressure.
When the pressing is over, the machine can immediately start plasticizing. The screw turns from the buffer point until the set dosing point is reached. The speed of the screw must be set high, so that the dosing point is reached before the cooling time has ended. If the thrust is set higher, the screw moves more slowly towards the dosing point due to this resistance. If as a result the dosing time becomes too long to stay within the cooling time, the speed of the screw must be increased.
The next step is decompression and end of cooling time. After plasticization, there is an overpressure in the melt in front of the screw tip. The higher the thrust, the higher the residual pressure in the melt. With an open nozzle, material can flow out of the nozzle. The material can also flow through the open sprue can into the mold, and cool there. Therefore, the screw is retracted a few millimeters after plasticizing. This removal of the pressure is called decompression. After decompression, if the product has cured sufficiently and is thus dimensionally stable, the cooling time can end. The mold can now open.
The final step of the cycle is to open the mold and eject the product. Immediately after decompression, the injection unit can be withdrawn to prevent further cooling of the spray nozzle. After the closing force has been reduced, the mold opens. The mold is designed to carry a product on the back mould. The product should release smoothly from the pre-mold and pull off any sprue. When the mold is open, the product can be ejected. The ejection movement can be started before the mold is fully open, saving time. As soon as the mold is fully opened, the pause time follows. This time can be used to unpack the product between the mold halves with a robot arm. When the pause time has elapsed, the cycle can start again.
The setting values
It is very important to use the correct setting values. During a production run, the process must be undisturbed and without, for example, temperature fluctuations. Only then will products of the same quality be created. These various parameters can relate to temperature, speed, pressure, time, and the waypoints to be followed. One step usually has multiple setting values. For example, the closing movement has three closing speeds and three waypoints. The correct settings for a new production to be set up are usually determined in a test spray. For test spraying, the setpoints should be selected in such a way that they can also be used unchanged for a subsequent production.
Most parameters influence each other and the final injection molded product. For example, if the thrust is increased, the density and homogeneity of the melt will increase and more material is plasticized and injected. Due to the higher friction, the melt will also become warmer. To dissipate this heat after injection, the required cooling time can also be extended. The total plasticizing time can also increase. Changing the decompression path not only changes the residual pressure in the melt, but also changes the amount of melt injected. All selected setting values must be registered. With modern machines, this is executed completely digitally and they are stored in the machine program.
Control of injection molding products
Injection molding usually involves producing large series of products. Sometimes an injection molding machine produces the same product for weeks, or even months. Changes in the process occur during production. Contamination of the mold or small differences in the properties of the raw material cause product deviations. There can also be a malfunction in the injection molding machine, the mold or the peripheral equipment. Therefore the products are checked at predetermined times during production. Before production starts, it has already been determined how often and in which way the products must be checked.
