Design of MIM Mold for Metal Injection Molding

Designing molds for MIM (Metal Injection Molding) is not a simple task, with strict tolerance requirements and special attention to complex details. All of this is like climbing to the top of an iceberg, which is very difficult.

High tolerance accuracy, no flash, and super high surface quality are all requirements that must be met in the manufacturing process of metal parts die-casting molds. Undoubtedly, although there are several metal mold manufacturers that can achieve this level, there are very few. However, for companies choosing this industry, it is' profitable '. Mr. David Lewis Sr. is the President of Matrix Tools (located in the Fairview region of PA), a mold manufacturer that produces injection and metal die-casting molds (MIM) for the wireless communication, medical, electronics/electrical, automotive, and personal protective products industries. Mr. Lewis said, "The company initially produced precision injection molding products and later engaged in the production of MIM molds

According to Mr. Lewis' experience, the structure of MIM molds is most suitable for the production of small and medium-sized parts. Matrix has made significant contributions to the medical device industry, The weight of surgical and medical equipment parts used in this industry ranges from 0.15 to 23.4g. "Other parts include watch covers, transmission gears, metal cutting tools, clamps, and chisels, some weighing up to 60g," he said. "I know there are larger parts, 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 strongly agrees with Mr. Lewis' viewpoint, as he was previously the Vice President of Technology at MedSource Technology in Minneapolis, Minnesota. This is a supplier that provides engineering design, product development, and production services, as well as supply chain management solutions for the medical device industry. When seeking a MIM mold manufacturer, I first need to see how disciplined and organized it is, whether it is a large factory, whether it adopts ISO standards, and whether all its procedures are standardized, "he said, If it comes to complex matters such as MIM mold manufacturing, where mold manufacturers have no organizational ability, they will encounter disasters. Quality is crucial, and even more difficult is that we are in the medical industry, and it is important to go public quickly and promptly. Moreover, we also hope for fast turnover, making both quality and fast turnover satisfactory is often difficult.

Firstly, the design of MIM molds requires a good plan, "Mr. Lewis pointed out, Because this is the foundation and key to success or failure. It should include excellent cooling water circuit layout and parting line locking devices to prevent mold misalignment during processing and production, a mold frame that can support the mold cavity and core sidewalls well, and a method suitable for demolding specific parts. "The basic structure of MIM molds is similar to injection molds, with only a few exceptions," Mr. Lewis added, This includes the selection of die cavity and core steel, closing angle fittings and sliders, the design of a flow channel system that provides good material fluidity, the position of the gate, the depth of ventilation, the surface quality of the molding area, and the correct selection of coatings for die cavity and core details. Mr. Hens elaborated on these viewpoints. Due to many mold manufacturers generally starting to manufacture molds based on design drawings, they give mechanics and mold workers a lot of freedom to design details when manufacturing molds. However, this method is generally ineffective for the manufacturing of MIM molds, "he said, Mold manufacturers and MIM mold workers are not observing the layout of a mold, their main task is to study and observe a complete set of detailed drawings. The more time they spend on designing the front details of the mold, the less time it takes for their mold to achieve successful operation. 

Detailed design includes the selection of mold component materials, tolerances between the mold and cavity, surface quality and coating, dimensions of gates and flow channels, and the position of air vents Set and size, position of pressure sensor. In the successful manufacturing of MIM molds, demolding and cooling have become key issues

1. Steel used for mold cavity/core
According to Mr. Lewis, the steel used in the mold cavity/core should be able to withstand the wear caused by the molding material. All steel in contact with molded composite materials (including gates and flow channels) 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 it has high hardness and is also coated with a high hardness coating. Any detail parts that do not come into contact with molded composite materials can be made of more standard S-7 tool steel, The material of MIM molds requires great attention.

2. Close the corner/slider
In the MIM mold production process, flash is a serious problem and a substantive issue for mold workers, To prevent this problem, it is crucial to have a good fit between all closing angles and sliding areas in order to produce a high-quality MIM part. Mold manufacturers must use the triangulation method to calculate rolling dimensions, and then apply this knowledge to check the closing angles of the yin and yang steel to ensure that they are within the ± 0.0001 tolerance range of their calculated dimensions The tolerances that people need to maintain are generally quite accurate for metal molds, as the feeding parts of MIM molds are sensitive to flash and are prone to flash. If plastic parts produce flash, it is easy to remove the flash, but if it is metal, the flash can cause problems like a blade, "he explained." You must create a mold that does not produce flash.

3. Depth of ventilation port
Mr. Lewis explained that in various molds, it is crucial to use a good ventilation port in order to release the internal gas generated by the molding material. He pointed out that MIM molds differ greatly from injection molds in terms of vent depth. A typical plastic mold may have 0.0005 "0.002 inches (1 inch=25.4mm) deep vents everywhere, depending on the resin used for molding, and MIM molds will definitely produce flash edges at the depths of these vents," he explained. "The vent depth of MIM molds is generally between 0.0001" 0.0002 inches, but it may also vary depending on the different composite metal materials used for molding. "According to Mr. Hens, A good feed exhibits extremely low shrinkage during the subsequent solidification process. In order to maximize shape retention, a large amount of metal powder fillers were used, often in amounts close to 70% of the volume, "he explained, In order to achieve good flowability of high filling materials, low molecular weight molecules and binders were used, resulting in high flash sensitivity of MIM feed, similar to the characteristics shown by many filled nylon materials. Paraffin and polyethylene/glycerol based feeds have better characteristics than acetyl based feeds, but this will affect their higher flash sensitivity
4. Polishing
Another key area of MIM parts is the surface quality of the molding area. Demolding from the mold cavity area largely depends on the surface quality of the mold cavity and core tool steel, as its shrinkage rate is extremely low during the molding process, Shrinkage of metal molded parts does not really occur unless the binder material is extracted through the post molding process. Therefore, polishing of the molding area is crucial. Generally, the molded surface should achieve a composite finish of 600, and in rare cases, diamond finish may be required
5. Electroplating
A high hardness electroplating layer such as chromium or nickel will enhance the efficiency of the demolding process of molded metal parts, combined with a highly polished surface, to produce better results. Mr. Lewis explained, "Some types of chromium and nickel have release agents added to improve their demolding characteristics
learning curve
After mastering the basic knowledge, there are still more factors to consider before gradually learning in sequence. Experience in injection mold design requires gradual accumulation. According to Mr. Hens, another challenge for MIM comes from raw materials, which generally refer to the supplied materials. Because it contains a large amount of very fine metal powder inside, often approaching 70% of its volume. It is metal powder that creates the final part, "he explained, Plastic parts increase the fluidity of metal powder and are completely removed after injection molding. The powder has some friction, so it must be processed with harder tools at higher spindle speeds. Many mold manufacturers, without high-speed machine tools, have to use ablation methods for processing. However, rough surface quality cannot be left behind, as MIM requires tight tolerances
Mr. Hens added that different metals basically have the same mold manufacturing requirements, unless sometimes different shrinkage rates need to be inserted. The shrinkage rate referred to here is not the internal shrinkage rate of the mold. It only occurs when the parts detach from the mold and enter the heating furnace. "He explained," In the mold, the shrinkage rate is almost zero, with a shrinkage rate of less than 0.001 inches per inch, so the shrinkage in the mold is minimal. "In addition, Mr. Hens also provided his final advice. Experience has shown that some mold factories have already manufactured some low-quality molds before realizing the challenges they face, Whenever a mold factory manufactures the first mold for a MIM production plant, combined with the design of the mold, a lot of time has already been spent. Generally speaking, the result is that the mold is very good and meets the predetermined work goals, or after the first mold trial, only a few changes have been made. It may seem easy to carefully analyze the details of the first mold, but this will affect the precision of the mold Section generates excessive confidence. It is not unusual to spend a lot of time and money debugging the second mold, sometimes involving rework issues. At this point, mold manufacturers understand the hardships of the road. If they do not give up on MIM molds at this point, then generally speaking, the molds produced afterwards will be very excellent.