Pre-pressure method instead of high-performance injection molding

In recent years, reducing the wall thickness of plastic products has become a clear trend. One such application is in traditional thin-walled packaging products. Traditionally, the required thin walls have been achieved by increasing the injection capacity of the machine, but special designs must be made in advance. At present, a melt pre-compression method can be used as an alternative to high-speed injection.

Reducing wall thickness has different purposes. For packaging products, the main reason is to save money. On the one hand, plastic materials account for a major portion of production costs; on the other hand, the cost is directly proportional to the weight of post-consumer waste. For industrial parts, less emphasis is placed on cost savings, but emphasis is placed on reducing weight and progressive miniaturization. These changes affect process control and thus affect the required machine technology. Netstal's application department studied this basic trend and conducted a series of comparative experiments using the SynErgy machine. The focus is on pre-compression technology. During use, the melt compression and injection molding phases are separate.

Alternatives to high performance injection molding

With high-performance injection molding methods, it is necessary to rely on machines designed specifically for this application. This type of machine is designed for particularly high output and is equipped with a high-performance hydraulic system that operates with a fixed pressure. By means of a hydraulic accumulator, a constant pressure is achieved, which on the one hand provides high productivity without special monitoring and on the other hand enables parallel plate movement. In this way, the high dynamics required in the key injection molding process can be obtained at the same time.

With thin-wall injection molding, the actual molding process dominates the entire injection molding phase. There is only a small effect on the hold pressure phase because cooling takes only a fraction of a second. Equally important is quick and sensitive monitoring to avoid damaging expensive molds.

High injection capacity is a prerequisite for completely filling the part. In addition, there must be sufficient storage capacity to make closed-loop control effective, achieving the required injection molding capacity of 50 to 100 kW in less than 0.1 seconds. Careful observation of the injection molding process will reveal that pressure build-up lags due to material compression. If there is a corresponding high injection capacity, this is not important. However, if the capacity is not enough, the mold can no longer be filled.

Melt preload: Separated from compression and injection procedures

It can be observed in thin-wall molding that most of the injection molding capabilities cannot be fully utilized due to the compressibility of the plastic melt. This is because the material is first compressed and the required rapid pressure build-up will occur later. Fast compression is our goal, but in fact the materials will flow away at the same time. All in all, this can only achieve low compression speeds. In the production of thin-walled products, problems arise because compression occurs when the molding is nearly full. In the product thin wall area, pressure must be established first to fill these areas. Because the material cures quickly, it loses valuable time and cannot be filled up further.

There are different ways to solve this problem. One method is to compensate by further increasing the speed of the injection or screw; the other method is melt pre-compression.

Effect of melt precompression

For industrial plastics such as PC and PC/ABS, the compression rate of the melt is about 10% at a melting temperature from 0 to 2000 bar compressed.

When this compression occurs during the injection molding stage, problems arise. Separating the material compression from the injection molding process is the solution. This method is to make small changes to the machine nozzle and control system, mainly to keep the nozzle closed until the desired compression pressure is reached. This pressure should be the expected fill pressure. At the beginning of the injection molding process, the shutoff nozzle must be opened in the shortest possible time in order to minimize the pressure loss. The hydraulically driven needle closure system proved to be suitable for this purpose. The opening and closing movements of the system ensure the required dynamic characteristics.

Due to the pre-compression of the melt, process control is slightly different from traditional injection molding. Melt precompression occurs at a relatively slow screw speed, creating pressure in a controlled manner. Production of typical thin-walled products uses injection speeds ranging from 300 mm/s to 700 mm/s; for melt pre-compression, speeds of 100 mm/s to 200 mm/s are sufficient. Due to the slow speed, the pressure rise is precisely controlled. Due to its rigid behavior, rapid pressure rises can occur if high compression pressures are used. Once the desired pressure is reached, the needle opens instantly and the injection molding process continues.

When the needle is opened, the pressure stored in the melt drops rapidly and usually does not completely fill the molding. This is why the injection molding process needs to be continued when the needle nozzle is opened. This is the only way to ensure that the mold is completely filled. In addition, with this process control method, there is no need to significantly increase the ration of the feedstock. However, the machine has a capacity reserve to continue the filling process at the same high filling pressure.

Netstal also studied various plastic materials such as different types of PCs and PC/ABS. It is clear that the ease of flow rating is significantly greater than the compression of the low viscosity type. The latter can only achieve a small improvement. The reason is that materials with higher toughness do not flow as fast during injection molding, so the time difference in establishing pressure is not so severe.

Through the study, the following specific application criteria can be defined:
1. Materials that require easy flow 2. High fill pressures greater than 1500 bar are required

In summary, the melt pre-compression can achieve a higher flow length and wall thickness ratio, ie it can safely fill thin walls. To make the best use of this technology, further research on the process is needed. In addition to the adaptability of different materials, mold design must be carefully studied. As far as the injection molding machine is concerned, because the SynErgy machine has all the basic prerequisites, the machine needs only minor changes.

The greatest benefit of melt precompression is that it complements the molding process of each existing mold, as opposed to other known techniques for reducing wall thickness such as casting. The casting pressure of thin-walled objects is still limited to simple geometric shapes and cannot handle more complex shapes.