1.1 What is an injection mold?
An injection mold is a device that produces (usually in large quantities) plastic parts or products by assembling a cavity (one or more) to form the desired shape of the molded product.
The mold cavity is composed of a female mold part called the mold cavity and a male mold part called the mold core. The mold is installed on the injection molding machine and fills the mold cavity in the following time sequence (usually automatically): ●Mold closing; A
●Inject (hot, nearly liquid) plastic into the mold cavity; ●Keep the mold closed until the plastic cools enough to be ejected; ●Open the mold; ●Eject the plastic product;
●If necessary, the injection molding machine can extend the mold opening (MO) time to prepare for the next (injection) cycle.
The cycle (a few seconds) is defined as the length of time between the mold moment of one injection and the mold closing moment of the next injection in fully automatic (FA) operations. But usually, people express the production productivity in terms of the number of injections per minute (or hour), rather than the length of the molding cycle in seconds.
1.2 What is an injection molding machine?
For mold designers, an important thing is to first understand the process and terminology of the injection molding process. An injection molding machine consists of four basic units with different functions.
① Clamping mechanism;
② Plasticizing device; ③ Injection device;
④ All control systems.
1.2.1 Clamping mechanism
The clamping mechanism completes the mold opening and closing actions (as quickly as possible) during the molding cycle, and provides the necessary clamping force to keep the mold closed during plastic injection. Because the injection pressure acting on the inner surface (projection surface) of the mold cavity has a tendency to open the mold seam. The seam here is the mold parting surface, also known as the parting line (P/L).
1.2.2 Plasticizing device
Almost all the plasticizing devices currently used are screw extruder plasticizing devices, which heat the plastic raw materials to the molten state required for injection. Most of the required heat is generated by the mechanical energy conversion of the extrusion screw rotating in the barrel, which is provided by the screw motor. The rotation of the screw also pushes the plastic material to the top of the screw.
Heaters are installed around the barrel, generally divided into 3 or more heating sections, to provide additional heat. This is mainly needed when the injection molding machine is started. On the other hand, the mechanical movement of the screw alone is not enough to heat and plasticize all the plastic materials required for each injection.
1.2.3 Injection device
The injection device injects the plastic melt into the mold by pressure. The required pressure depends mainly on the wall thickness of the product.
The pressure required for thick-walled products is relatively low (49.0~98.1MPa), so that only under the three forces of the extruder is sufficient to fill the (flow molding) mold cavity with the plastic melt. Thin-walled products, especially when L/t (see definition in Section 1.3.4.2) is greater than 200, require a relatively high injection pressure (137.3~196.1MPa) to fill the mold cavity before the plastic melt solidifies.
There are two injection methods: one-stage injection and two-stage injection. The types of injection molding machines used are introduced as follows.
1.2.3.1 RS type injection molding machine
At present, the extrusion mechanism and injection mechanism of most injection molding machines are designed in one device. When the plastic melt for one injection is sufficient, the extrusion screw stops rotating, and then the screw is pushed forward to eject the plastic melt gathered at the front of the screw top. This device has several names: axial piston screw, reciprocating (RS) screw or one-stage injection device.
The manufacturing industry generally evaluates the processing capacity of the screw by the amount of plasticized plastic raw materials processed per hour. However, the size and strength limit of the thrust bearing play a decisive role. The extrusion screw only plays a plasticizing role in the non-injection stage of the molding cycle. Therefore, in fact, the effective plasticizing capacity is lower than the nominal (indicated in the injection molding machine manual). For large injection volume, high-speed injection is adopted, and the injection time accounts for most of the molding cycle, so that the plasticizing capacity that the screw can recover and complete is often only 60% to 80% of its nominal plasticizing capacity.
The amount of plastic that can be injected into the mold depends largely on the efficiency (sealing degree) of the one-way valve at the top of the screw. If this one-way valve is of poor quality or has been worn, plastic will leak through the one-way valve during the high-pressure injection stage. This will affect the amount of plastic entering the mold cavity space, and thus cause under-injection or pressure holding (over-filling), which will affect the production efficiency of the mold. Leakage will also affect the quality (density), size (caused by changes in shrinkage) and internal quality of the product.
In order to avoid the impact of changes in injection volume, molders usually use a nozzle size larger than the actual injection requirement when using a reciprocating injection molding machine. Therefore, the push movement will not reach the end very early, so that a melt storage pool is formed at the top of the screw about 5 to 10 mm.
1.2.3.2 P-type injection molding machine
The pre-plasticizing injection molding machine system separates the function of the extrusion mechanism from the function of the injection device. The extrusion mechanism plasticizes the raw material and fills the injection device's material box, that is, the material tank. This type of injection molding is called pre-plasticizing, two-stage or P-type injection molding machine. The advantages of the two-stage injection molding system are as follows.
The screw can run continuously, so the available time (rated time) can be used 100% for plasticizing the raw material. In this way, for the same injection volume, the P-type injection molding machine can use a smaller extruder than the RS-type injection molding machine.
② Due to continuous operation, the P-type injection molding machine mixes the melt better and the temperature is lower than that of the RS-type injection molding machine, which may be very necessary for some heat-sensitive materials.
③ There is no one-way valve on the top of the screw, and the injection volume in the injection barrel is mechanically metered, and the repeatability and accuracy of the main shot volume are better than using the RS-type injection molding machine. The volume of plastic prepared for injection without a storage tank can be accurately calculated based on the cavity volume.
④ Since the transfer of the plastic melt from the extruder to the injection barrel is carried out at a very low pressure, it is relatively easy to place an effective filter on its path to remove impurities in the plastic. This will not affect (reduce) the injection pressure from the barrel to the mold. However, such filtering effect is not feasible in RS type injection molding machine, because its pressure is too high and will damage the filter.
The disadvantages of the two-stage injection system are:
Because it requires more components and control devices, the cost of the equipment is relatively high. The system is not suitable for highly heat-sensitive materials, such as polyvinyl chloride (PVC) plastics.
1.2.4 Control device
The control system controls the operation process of the injection molding machine. The control system of the injection molding machine can be divided into four basic elements.
① The control component is installed near the mold closing safety door, where the operator can observe the mold dog status and can easily operate the buttons to achieve the required operation. Some injection molding machines are designed with buttons and switches for adjusting the machine status on the control panel near the safety door.
② The logic control controls the machine status and processes signals from position sensors and time relays, etc., so that the injection molding machine can operate as required. At present, the logic operation of injection molding machines is almost entirely controlled by electronic switches or microcomputers. (Note: Mechanically driven switches and time relays have a much shorter service life than electronic switches and time relays, and their reliability and repeatability are much worse. However, they are easier to master and maintain. Electronic switches require better qualified maintenance personnel and better electronic testing equipment for testing and maintenance. On the other hand, electronic components are more sensitive to temperature increases, and in some hot areas, it is necessary to consider the temperature control of the working environment.)
③ Power supply and distribution of motors and heaters. ④ Temperature control of injection molding machines and molds.
For ease of use, injection molding machines usually have some other characteristics, but for the understanding of the injection molding process, the above explanation of the basic elements is sufficient.
1.3 Molding time and technical terms
Process cycle The total time required for the opening and closing of the mold clamping mechanism of the injection molding machine is the sum of the mold opening process time and the closing process time. The idle cycle of the county's current fast injection molding machine is 1 to 3 seconds.
