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Mold design for Metal injection molding

Writer： Admin Source： MDM Date：2019-09-10

Designing molds for MIM (metal injection molding) is not a simple task, with strict tolerance requirements and special attention to complex
details. It is like climbing to the top of an iceberg, which is very difficult.
High tolerance accuracy, no flash, super high surface quality, all these standards are the metal parts die casting process must meet some
requirements. There are, no doubt, a few but very few metal mould makers capable of achieving this level. However, it is "profitable" for
companies to choose this industry. Mr. David Lewis sr. is President of Matrix tools, inc. (Fairview PA), a mold manufacturer of injection
molding and metal die casting (MIM) molds for the radio communications, medical, electronics/electrical, automotive and personal protection
industries."The company started out with precision injection molding products and then moved on to MIM molds," says Mr Lewis.

According to Mr. Lewis's experience, the structure of MIM mould is most suitable for the production of small and medium-sized parts. Matrix
has made a significant contribution to the medical device industry, which USES surgical devices with parts weighing between 0.15"
22.4g."Other parts include watch covers, transmission gears, metal cutting tools, clamps and chisel tips, some of which weigh as much as 60
grams," he said.In short, the manufacture of injection molding and metal die casting requires high quality and service. Mr. Karl Hens, formerly
vice President of technology at MedSource technologies of Minneapolis, minn., agrees with Mr. Lewis. We are a supplier of engineering,
product development and manufacturing services, as well as supply chain management solutions for the medical device industry. "When I look
for an MIM mold maker, the first thing I look for is discipline and organization, whether it's a larger plant, whether it's ISO standard, whether all
its procedures are standard. ""When it comes to complex things like making MIM molds," he says, "the die makers don't have the organizational
skills to do that, and that's a disaster. Quality is the key, and more difficult is that we are engaged in the medical industry, quick and timely
listing is very important. And we also want fast turnaround, and it's often difficult to get quality and fast turnaround to be satisfactory."
"First of all, the design of MIM mold should have a good plan. "Because it's the foundation and the key to success," says Mr Lewis. It should
include excellent cooling water line layout and parting line locking devices to prevent mold misplacement in manufacturing, a mold frame that
supports mold cavities and core side walls well, and a method of mold release suitable for specific parts ."The basic structure of the MIM mold
is similar to that of the injection mold, with a few exceptions. ""These include the selection of die cavity and core steel, the closing Angle fit and
slider, the design of the runner system to make the material flow well, the location of the gate, the depth of ventilation, the surface quality of the
molded area, and the correct coating for the die cavity and core details," Mr. Lewis added. Mr. Hens expressed those views at length. "Because
many die makers start with a blueprint, they give machinists and die makers a lot of freedom to make detailed designs. But this method is
generally not effective for the manufacture of MIM molds." "Instead of looking at the layout of a mold, mold makers and MIM workers look at
a whole set of detailed drawings," he says. The more time they spend designing the details on the front of the mold, the less time it takes to
successfully operate the mold. Detailed design includes selection of die parts materials, mold and cavity tolerances, surface quality and coating,
gate and runner dimensions, vent location and dimensions, pressure sensor location. "In the successful manufacturing of MIM molds, stripping
and cooling have become key issues."
1. Steel used for mold 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 steels in
contact with molded composites (including gates and channels) shall be made of high chromium and high hardness tool steels with high wear
resistance, such as d-2 or a-2 tool steels.""H-13 steel is also available because of its high hardness and has been plated with a layer of hardness
coating. Any detail parts that do not come into contact with the molded composites can be manufactured with more standard s-7 tool steels. "It
can be seen that the material of MIM mold needs great attention.
2. Close corner/slider
"Flash is a serious problem in the MIM mold manufacturing process and a substantial problem for the mold workers. ""In order to prevent this
problem, it is important that all closing corners and sliding areas are well aligned so that a quality MIM part can be produced," said Mr. Lewis.
The die maker must use the triangulation method to calculate the rolling dimensions, and then apply this knowledge to check the closing angles
of the negative and positive steel to ensure that it is within the ±0.0001 tolerance of his calculated dimensions. The tolerances that people need
to maintain are generally quite accurate for metal molds, because MIM mold supplies are sensitive to flash and are prone to flash. "If it's plastic,
it's easy to get rid of it, but if it's metal, it's like a blade. ""You have to make a mold that doesn't produce flaps," he explains.
3. Vent depth
Mr. Lewis explained that in various molds, it is critical to have a good vent in order to release the internal gas generated by the molded material.
He pointed out that the MIM mold in terms of vent depth is very different from the injection mold. "A typical plastic mold, mold resin according
to its different, may be there are 0.0005" 0.002 (1 = 25.4 mm) in deep in the vent, and the MIM mould will flash in the depths of the vent, "he
explained," the MIM mold vent depth is in commonly 0.0001 "0.0002 in between, but may also changes with choose different moulding
compound metal materials. "According to Mr. Hens, the shrinkage of good feedstock during subsequent solidification is extremely low. "In
order to maximize the strength of shape retention, a large amount of metallic powder filler is used, often at close to 70% of the volume. ""In
order to achieve good fluidity of high filler feed, low molecular weight molecules and binders were used to give MIM feed high flash
sensitivity, similar to the properties shown in many nylon filler materials," he explained. Paraffin and polyethylene/glycerine feeds have better
properties than acetyl feeds, but their higher flash sensitivity is affected.
4. The polishing
Another key area for MIM parts is the surface quality of the molding area. "The removal of the mould from the cavity area is largely dependent
on the surface quality of the cavity and core tool steel because of the extremely low shrinkage during the die pressing process. ""The shrinkage
of the die does not really occur unless the binder material is extracted through the post-die forming process," he said. Therefore, the molding
area polishing is very critical. Generally speaking, the molded surface should be 600 composite finish, may need to achieve diamond finish in
very few cases.
5. Electroplating
"High hardness electroplating, such as chromium or nickel, will enhance the efficiency of the mould-pressed metal parts during the stripping
process, combined with a high polishing surface to produce better results. ""Some types of chromium and nickel have release agents added in
order to improve their mold release properties," explains Mr Lewis.

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