According to the report of injection molding processing: Under the premise that the current market is becoming more and more diversified, the injection molding industry is also constantly developing and broadening, and new technologies such as multi-color injection molding, gas assist, in-mold lamination, and co-injection molding have emerged. Similarly, the specifications of injection molding machines are also developing in two directions-large-tonnage injection molding machines and micro injection molding machines are constantly being updated.
The development of micro-injection technology is getting faster
In recent years, the demand for micro-products has increased. Whether in the electronics industry, the watch industry or the military industry, there is a large demand for small injection molded parts. These injection molded products have very high requirements on size and accuracy.
Under such a premise, the micro-injection process is also facing huge challenges. How can injection molded parts meet the micron-level size requirements while also having good appearance and performance? In the following, we will briefly introduce the difference between micro-injection molding and traditional injection molding in terms of molds, equipment, materials, and processes.
Mold processing and key points
In terms of molds, micro-injection requires much higher processing equipment than traditional injection molding.
Micro injection molding usually has two trends in mold processing: the first is to use mirror spark machining. In order to ensure high precision, it is best to use graphite electrodes for EDM, because the loss of graphite electrodes is higher than that of ordinary copper electrodes. Much smaller.
The second more commonly used processing method is to use electroforming. The electroforming process can ensure very high accuracy, but the disadvantage is that the processing cycle is long, each hole must be processed independently, and if there is a slight damage in production, it cannot be repaired. , Can only replace damaged acupuncture points.
In terms of mold, mold temperature is also a very important parameter for micro-injection. In the face of high-end customers, the current common practice is to borrow the concept of high-gloss injection molding and introduce a rapid heating and cooling system.
In theory, high mold temperature is very helpful for micro-injection, for example, it can prevent thin-wall filling difficulties and lack of material, but too high mold temperature will bring new problems, such as cycle lengthening and shrinkage deformation after mold opening. Therefore, it is very important to introduce a new mold temperature control system. During the injection molding process, the mold temperature can be increased (which can exceed the melting point of the plastic used), so that the melt can quickly fill the cavity and prevent the melt temperature from decreasing during the filling process. It is fast and causes incomplete filling; and when demolding, the mold temperature can be quickly reduced, kept at a temperature slightly lower than the thermal deformation temperature of the plastic, and then the mold is opened and ejected.
In addition, because micro-injection molding is a product with a quality of milligrams, if an ordinary gating system is used to inject the product, even after optimization and improvement, the mass ratio of the product and the material in the gating system is still 1: 10. only less than 10% of the materials are injected into micro-products, producing a large amount of gating system aggregates, so micro-injection should use a hot runner gating system.
Material selection points
In terms of material selection, it is recommended that some general engineering plastics with low viscosity and good thermal stability can be selected in the early stage of development.
The choice of low-viscosity materials is because the viscosity of the melt is low during the filling process, the resistance of the entire gating system is relatively small, the filling speed is faster, and the melt can be smoothly filled into the cavity, and the melt temperature will not be significantly reduced. , Otherwise it is easy to form cold joints on the product, and the molecular orientation is less during the filling process, and the performance of the obtained product is relatively uniform.
If you choose a high-viscosity plastic, not only the filling is slower, but also the feeding time is longer. The shear flow caused by the feeding will easily align the chain molecules in the direction of the shear flow. In this case, the orientation state will be when cooled below the softening point. It is frozen, and this frozen orientation to a certain extent is easy to cause internal stress of the product, and even cause stress cracking or warping deformation of the product.
The reason for the good thermal stability of the plastic is that the material stays in the hot runner for a long time or is easily thermally degraded by the shearing action of the screw, especially for heat-sensitive plastics, even in a short cycle time, due to material injection The amount is small, and the residence time in the gating system is relatively long, which causes a considerable degree of degradation of the plastic. Therefore, heat-sensitive plastics are not suitable for micro-injection.
Points for equipment selection
In terms of equipment selection, since the size of the micro-injected parts is micron-level products, it is advisable to use an injection machine with an injection volume of milligrams.
The injection unit of this type of injection machine generally adopts a screw-plunger combination. The screw part plasticizes the material, and the plunger injects the melt into the cavity. The screw plunger injection molding machine can combine the high precision of the screw with the high speed of the plunger equipment to ensure the accuracy of the production process and the filling speed.
In addition, this kind of injection machine is usually composed of a clamping guide mechanism, an injection system, a pneumatic demolding mechanism, a quality inspection mechanism and an automatic packaging system. A good quality inspection system can ensure the yield of micro-precision injection molded products and monitor parameter fluctuations during the entire process.
Key points of injection molding process
Finally, we look at the requirements of micro-injection molding in terms of injection molding process. In the injection molding process, we need to consider the gas mark and stress of the gate, usually multi-stage injection molding is required to ensure that the material can be filled in a stable flow state.
In addition, you also need to consider the holding time. Too small holding pressure will cause the product to shrink, but too large holding pressure will cause stress concentration and larger dimensions.
In addition, the residence time of the material in the material tube also needs to be strictly monitored. If the material stays in the material tube for too long, it will cause the degradation of the material and affect the function of the product. It is recommended to carry out standard parameter control in the process parameter management. It is best to do DOE verification for each product before mass production. All changes in production must be re-tested for size and function.
As a branch of the injection molding field, micro-injection is developing in the direction of high dimensional accuracy, high functional requirements, and high appearance requirements. only through strict control of molds, equipment, materials, processes and other processes and continuous improvement of technology can the market be satisfied. Field development. (This article is original by injection molding, please indicate the source for reprinting!)
