Best discusses the mold design concept of metal injection molding MIM
time：2020-04-27Views：858 Author：Best Seiko
Designing molds for MIM (Metal Injection Molding) is not a simple task. Tolerances are strict and special attention needs to be paid to complex details. This is as difficult as climbing to the top of an iceberg. High tolerance accuracy, no flash, and super high surface quality, all of these standards are some of the requirements that must be met during the manufacturing process of metal collar die casting molds. There is no doubt that although there are several metal mold manufacturers that can reach this level, there are very few. However, it is "profitable" for companies that choose this industry. Mr. David Lewis Sr. is the president of Matrix Tool Company (in the Fairview area of PA), a mold manufacturer that produces injection molds and metal die-casting molds (MIM) for radio communications, medical, electronic/electrical, automotive and personal Protective products industry provides molds. Mr. Lewis said: "The company initially produced precision injection moulding products, and later engaged in the production of MIM molds." According to Mr. Lewis' experience, the structure of MIM molds is very suitable for the production of small and medium-sized collars. Matrix has made a great contribution to the medical device industry. The weight of surgical medical instruments used in this industry is between 0.15"23.4g. "Other collars also include watch covers, transmission gears, and metal cutting tools. , Clamps and chisel tips, some weigh up to 60g," he said, "I know there are larger collars, such as replacement parts for golf club heads. In short, the manufacturing of injection molds and metal die casting molds has high requirements for quality and service. Mr. Karl Hens agrees with Mr. Lewis. He was formerly vice president of technology at MedSource Technology (in Minneapolis, Minnesota). This is a provider of engineering design, product development, and production services. The medical device industry provides supply chain management solutions. "When seeking a MIM mold manufacturer, I first have to look at its discipline and organization, whether it is a larger factory, whether it adopts the ISO standard, and whether all its procedures are standardized. "He said, "If it involves complicated things such as MIM mold manufacturing, mold manufacturers have no organizational ability, then it will encounter disaster. Quality is the key, and even more difficult is that we are engaged in the medical industry, and quick and timely listing is very important. And we also want quick turnaround. It is often difficult to achieve satisfactory quality and fast turnaround. Basic knowledge of MIM "First of all, there must be a good plan for mold design." Mr. Lewis pointed out, "Because this is the basis and key to success. It should include an excellent cooling water circuit layout and parting line locking device to prevent the misalignment of the mold during processing and production, a pair of mold frames that can support the cavity and core sidewalls, and a suitable for specific collars Demoulding method. "The basic structure of MIM molds is similar to injection molds, with few exceptions." Mr. Lewis added, "This includes the selection of mold cavity and core steel, closing angle fittings and sliders, the design of the flow channel system to make the material have good fluidity, the position of the gate, the depth of ventilation, the shape of the molding area The quality of the surface, the coating applied to the details of the cavity and core are selected correctly. Mr. Hens described these points in detail. “Since many mold manufacturers generally start manufacturing molds based on design drawings, they give mechanics and molders a lot of freedom to make detailed designs when manufacturing molds. But this method is generally ineffective for the manufacture of MIM molds. He said, "Mold manufacturers and MIM molders are not looking at the layout of a mold. Their main task is to study and observe a detailed set of drawings. The more time they spend on the design of the front of the mold, the less time it takes for their mold to succeed. The detailed design includes the selection of the mold collar material, the tolerance of the mold and the cavity, the surface quality and coating, the size of the gate and runner, the position and size of the vent, and the position of the pressure sensor. In the successful manufacture of MIM molds, demolding and cooling have become key issues. 1. The steel used in the cavity/core According to Mr. Lewis, the steel used in the cavity/core should be able to withstand the wear caused by the molded material. "All steel materials (including gates and flow that come in contact with the molded material) Road), should be made of tool steel with high wear resistance, high chromium content and high hardness, such as D-2 or A-2 tool steel. "He commented," H-13 steel is also acceptable, because of its high hardness, and a layer of high hardness coating. Any detail collars that are not in contact with the press-formed composite material can be manufactured from the more standard S-7 tool steel. 2. Close the corner/slider “In the MIM mold production process, flashing is a serious problem, which is a substantial problem for molders. Mr. Lewis said, "In order to prevent this kind of problem, it is very important that all the closing angles and sliding areas are well coordinated. Only in this way can a high-quality MIM collar be produced. The mold manufacturer must calculate the rolling dimension by using the triangulation method, and then apply this knowledge to check the closing angle of the steel on the female and male sides to ensure that it is within the tolerance of ±0.0001 of the calculated dimension. The tolerances that people need to maintain are generally quite accurate for metal molds, because the feed parts of MIM molds are very sensitive to flash, and it is easy to produce flash. "If plastic parts produce flash, it is easy to remove the flash. Remove it, but if it is metal, its flash will cause problems like a blade. He explained, "You must make a mold that will not produce flash." 3. Vent depth Mr. Lewis explained that in various molds, in order to release the internal gas generated by the molding material, it is very important to use a good vent. He pointed out that the depth of the vent in the MIM mold is the same as injection molding. The molds are very different. "A typical plastic mold, depending on its molding resin, may have 0.0005"0.002in (1in=25.4mm) deep vents everywhere, and MIM molds will definitely be in these vents. Flashing occurs in the depth of "," he explained, "The depth of the vent of the MIM mold is generally between 0.0001" and 0.0002in, but it may also vary with the different molded composite metal materials selected. According to Mr. Hens, the shrinkage rate of a good feedstock during the subsequent solidification process is low. “In order to strengthen the shape retention strength as much as possible, a large amount of metal powder fillers are used, and the dosage is often close to 70% of the volume. He explained, "In order to obtain good fluidity of the high-filled feedstock, low-molecular-weight molecules and binders are used to produce a high flash sensitivity of the MIM feed, similar to that shown by many filled nylon materials. characteristic. Paraffin and polyethylene/glyceryl feeds have better characteristics than acetyl feeds, but will affect their higher flash sensitivity. 4. Polishing Another key area of the MIM collar is the surface quality of the molding area. "Removal from the cavity area depends largely on the surface quality of the cavity and core tool steel, because the molding process Medium, its shrinkage rate is low. He said, "The shrinkage of metal molded parts does not really happen unless the binder material is extracted through the post-molding process. Therefore, the polishing of the molded area is critical. In general, the molded surface should achieve a composite finish of 600, and diamond finish may be required in rare cases. 5. Electroplating “A high-hardness electroplated layer such as chromium or nickel will enhance the efficiency of the molded metal parts during demolding, combined with a highly polished surface, it will produce better results. Mr. Lewis explained, “Some types of chromium and nickel have added release agents, the purpose of which is to improve their demolding properties. "Learning curve" After mastering the basic knowledge, there are still more factors that need to be considered, and then learn in order. According to Mr. Hens, another challenge of MIM comes from raw materials, generally referring to the supplied materials. Because of its internal Contains a lot of very fine metal powder, often close to 70% of the volume. "It is the metal powder that makes the collar. He explained, “The plastic collar adds fluidity to the metal powder and is completely removed after injection molding. The powder has a little friction, so it must be processed with a harder tool at a higher spindle speed. Because many mold makers do not have high-speed machine tools, they have to use ablation. However, rough surface quality cannot be left behind, because MIM requires tight tolerances. "Mr. Hens added that different metals basically have the same mold manufacturing requirements, unless sometimes it is necessary to insert a different shrinkage rate. "This does not refer to the shrinkage rate inside the mold. Only when the collar is detached from the mold and enters the heating furnace, shrinkage will occur. He explained, "In the mold, the shrinkage rate is almost close to the collar, and the shrinkage rate per inch is less than 0.001in, so the shrinkage in the mold is small. In addition, Mr. Hens also offered a piece of advice. "Experience shows that some mold factories have already made some molds of poor quality before recognizing the challenges they face. "He said, "At any time, when a mold factory manufactures the first mold for a MIM production plant, plus mold design, it has taken a lot of time. In general, the result is that the mold is very good, reaching the predetermined work goal, or after the first mold trial, only a few changes have been made. It may seem easier to analyze the details of the first mold carefully, but this will create excessive confidence in the details of the mold. It is not abnormal to spend a lot of time and money to debug the second mold, and sometimes it involves rework. At this point, mold makers understand the hardships of the road. If they do not give up MIM molds at this point, then in general, the molds produced afterwards will be very good. "