Interpreting the Key Points of Micro Injection Molding Technology

Under the premise of increasingly diversified market, the injection molding industry is also constantly developing and expanding, with the emergence of new processes such as multi-color injection molding, gas assisted injection molding, in mold film application, and co injection molding. In terms of the specifications of injection molding machines, they are also developing in two directions - large tonnage injection molding machines and micro injection molding machines are constantly being updated. This article focuses on introducing the author's views on micro injection molding processes.
In recent years, there has been an increasing demand for micro products. Whether in the electronics industry, watch industry, or military industry, there is a large demand for micro injection molded parts, which have high requirements for size and accuracy.
Under such a premise, the micro injection molding process also faces enormous challenges. How can we make the injection molded parts meet the size requirements of the micrometer level, while also having good appearance and performance? Below, we will briefly introduce the differences between micro injection molding and traditional injection molding in terms of molds, equipment, materials, processes, and other aspects.
The demand for micro products is increasing, and the picture shows the micro medical surgical clip products displayed by Weimeng Bartenfield.
Mold processing and key points
In terms of molds, micro injection molding requires much higher processing equipment compared to traditional injection molding.
There are usually two trends in micro injection molding in mold processing: the first is to use mirror surface spark machining. To ensure high precision, it is best to use graphite electrodes for electric spark machining, as the loss of graphite electrodes is much smaller than that of ordinary copper electrodes.
The second commonly used processing method is the use of electroforming molds, which can ensure very high accuracy. However, the disadvantage is that the processing cycle is long, and each hole needs to be processed independently. In addition, if there is slight damage that cannot be repaired during production, the damaged hole can only be replaced.
In terms of molds, mold temperature is also a very important parameter for micro injection molding. The commonly used approach for high-end customers is to borrow the concept of high gloss injection molding and introduce rapid heating and cooling systems.
In theory, high mold temperature is very helpful for micro injection molding, such as preventing difficulties in thin-walled filling and material shortage. However, excessive mold temperature can bring new problems, such as prolonged cycle time and shrinkage deformation after mold opening. Therefore, introducing a new mold temperature control system is quite important. During the injection molding process, the mold temperature can be increased (which can exceed the melting point of the plastic used), allowing the melt to quickly fill the mold cavity and preventing the melt from decreasing too quickly during the filling process, resulting in incomplete filling; When demolding, the mold temperature can quickly decrease and be maintained at a slightly lower temperature than the plastic thermal deformation temperature, and then the mold can be opened and ejected.
In addition, due to the fact that micro injection molding involves products with a mass of milligrams, if a conventional gating system is used for injection molding, even after optimization improvements, the material mass ratio between the product and the gating system is still 1:10. Less than 10% of the materials are injected into micro products, resulting in a large amount of solidification in the pouring system. Therefore, micro injection molding should use a hot runner pouring system.
Key points for material selection
In terms of material selection, it is recommended to choose some general engineering plastics with low viscosity and good thermal stability in the early stages of development.
Choosing low viscosity materials is because during the filling process, the viscosity of the melt is low, resulting in relatively small resistance in the entire pouring system. The filling speed is fast, ensuring that the melt smoothly fills the mold cavity and there is no significant decrease in melt temperature. Otherwise, cold joints are easily formed on the product, and the molecular orientation is less during the filling process, resulting in relatively uniform performance of the product.
If high viscosity plastics are selected, not only will the filling process be slower, but also the feeding time will be longer. Due to the shear flow caused by the feeding, the chain like molecules can easily orient along the shear flow direction. In this case, when cooled below the softening point, the orientation state will be frozen. This freezing orientation to a certain extent can easily cause internal stress in the product, and even cause stress cracking or warping deformation of the product.
The reason for requiring good thermal stability of plastics is that materials that stay in the hot runner for a long time or are prone to thermal degradation due to screw shear. Especially for thermosensitive plastics, even in a short cycle time, due to the small injection volume of the material, the residence time in the pouring system is relatively long, causing considerable degradation of the plastic. Therefore, thermosensitive plastics are not suitable for micro injection molding.
Key points for equipment selection
In terms of equipment selection, as the size of micro injection molded parts is in the micrometer level, it is advisable to choose an injection machine with an injection volume of milligrams.
The injection unit of this type of injection machine generally adopts a screw plunger combination type, where the screw part completes the plasticization of the material and the plunger injects the melt into the mold cavity. The screw plunger injection molding machine can combine the high precision of the screw with the high speed of the plunger equipment, ensuring the accuracy and filling speed of the production process.
In addition, this type of injection machine usually consists of a mold closing 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 throughout the entire process.
Key points of injection molding process
Finally, let's take a look at the requirements of micro injection molding in terms of injection molding process. In the injection molding process, we need to consider the issues of air marks and stress in the gate, usually using multi-stage injection molding to ensure that the material can be filled in a stable flow state.
In addition, it is also necessary to consider the size of the holding time. A too small holding pressure can cause shrinkage of the product, but an excessive holding pressure can lead to stress concentration and size enlargement.
In addition, the time the material stays in the material tube also needs to be strictly monitored. If the material stays in the material tube for too long, it can lead to material degradation and affect the functionality of the product. It is recommended to implement standard parameter control in process parameter management. It is best to conduct DOE verification for each product before mass production, and all changes during production should be retested for size and functionality.
Micro injection molding, as a branch of the injection molding field, is developing towards high dimensional accuracy, high functional requirements, and high appearance requirements. Only through strict control and continuous improvement of various processes such as molds, equipment, materials, and processes can the market development be met.