Mold Building for Metals

Designing a mold for MIM is no easy task - tight tolerances and paying the utmost attention to intricate details are just the tip of the iceberg.
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Higher tolerances. No flash. Superior surface finish. All of these criteria are just some of the requirements that must be met when building a mold to run metal parts - it's no wonder that metal moldmakers are few and far between.

However, for the companies that have chosen to make it a specialty, it is a lucrative business. David Lewis, Sr., president of Matrix Tool, Inc. (Fairview, PA) - a plastic injection and metal injection molding (MIM) moldmaker for the telecommunications, medical, electronic/electrical, auto-motive and personal care industries - notes that his company was first built around precision plastic injection molds before it delved into the intricacies of MIM tooling.

In Lewis' experience, MIM mold construction is best for small- to medium-sized parts. Matrix has done a great deal of work for the medical device industry, where the size of the surgical device components has ranged from 0.15 to 23.4 grams. "Other parts have included watch bezels, drive gears, metal cutting tools, pliers' jaws and chisel bits weighing as much as 60 grams," he states. "I do know that many larger parts exist - such as golf club heads and knee and hip replacement parts." Both plastic and metal injection moldmaking require the highest commitment to quality and service, he adds.

In agreement is Karl Hens, formerly the vice president of technology at MedSource Technologies, Inc. (Minneapolis, MN) - a provider of engineering, product development and manufacturing services, as well as supply chain management solutions to the medical device industry. "When looking for moldmakers for MIM, I first look for the discipline and the organization - if it's a bigger shop, it ought to have ISO certifications and all of its procedures in place," he states. "When it comes to something complicated like building molds for MIM - and the moldmaker is not organized - you are headed for disaster. Quality is key. Plus, what makes it more difficult is that we are in the medical industry where quick time-to-market is critical, so we also want fast turnaround. It's often tough to get both quality and fast turnaround."

The ABCs of MIM

"First, a well-planned mold design is required," Lewis points out, "since this is the foundation for success or failure. It should include good water line patterns for cooling, parting line locks to prevent movement during processing, a mold frame that allows for good side wall support around the cavity and core pockets, and a suitable part ejection method for the given part.

"The basics of MIM tool construction are identical to that of plastic injection molds - with a few exceptions," Lewis adds. "These would include choice of cavity and core steel, fits for shut-off angles and slides, a runner system designed for good flow of the material, gate locations, venting depths, surface finish of the molding areas, and the correct choice of plating to be applied to the finished cavity and core details."

Hens expands on these sentiments. "While many moldmakers start constructing tools from general layout prints - giving the machinist and toolmaker a lot of liberty to perform detail design as they are building the tool - this approach does not typically work for MIM molds," he states. "Instead of the moldmaker and the MIM molder reviewing a layout of a tool, it becomes essential to jointly review a set of fully detailed prints. The more time that is spent upfront, the less time it will take to get the mold operational successfully. Details involving material selection for each mold component, mold and cavity tolerancing, surface finish and coating, gate and runner sizing, vent location and sizing, pressure transducer location, ejection and cooling become key issues in building an MIM mold successfully."

Cavity/Core Steel
The cavity/core steel must be able to withstand the abrasive wear of the molding material, according to Lewis. "Any steel that comes in contact with the molding compound - including the gates and runners - should be made from a high-wear resistance, high-chromium base and high-hardness tool steel such as D-2 or A-2," he comments. "However, H-13 also will work at a high hardness and plated with a high-hardness coating. Any details that do not come in contact with the molding compound can be constructed from a more standard tool steel like S-7."

Shut-Off Angles/Slides
"Flash is a real issue with the MIM process and can be a real problem for the molder," Lewis says. "To prevent this, good fitting on all shut-off angles and slide areas is paramount to produce a good MIM part. The moldmaker must calculate roll dimensions through the use of trigonometry and then apply this knowledge to checking the male and female steel shut-off angles so they are within plus or minus .0001 of his calculated dimensions."

Hens adds that the tolerances that one has to hold are typically much tighter with a metal mold because MIM feedstock parts are very flash sensitive and will flash very easily. "If a plastic part flashes you can get rid of the flash easily, but if it's metal it's like a knife edge and it's a problem - you don't want flash," he explains. "You have to make a flash-free mold."

Venting Depths
Lewis notes that good venting is critical in all molds to release the internal gases produced by the molding material, but points out that MIM tooling is very different in respect to depth of vents versus the plastic injection mold. "A typical plastic mold - depending on the molding resin - may have vents anywhere from .0005" deep to .002" deep, while an MIM tool vent would certainly flash at these depths," he explains. "Venting for an MIM tool would be more like .0001" to .0002" deep. This too can vary with the choice of molding metal compound."

Premium feedstocks exhibit a minimum amount of shrinkage during the subsequent sintering process, according to Hens. "To maximize shape retention, a maximum amount of metal-powder filler is used, often approaching 70 percent by volume," he explains. "To be able to obtain good flowability of highly filled feedstocks, low molecular molecules and additives are used, resulting in a high flash sensitivity of MIM feedstocks not dissimilar to that exhibited by many filled nylons. Wax and polyethylene/glycol-based feedstocks tend to have better flow characteristics than acetyl-based feedstocks at the expense of a higher sensitivity to flashing."

Another critical area for MIM parts is the surface finish of the molding areas. "Release from the cavity areas is very dependent on the surface finish of the cavity and core tool steel since the shrink factor is very minimal as molded," he states. "Shrink of a metal molded part does not really happen until the binder material is extracted through a post molding process. Therefore, polishing of the molding areas is critical. Normally, molding surfaces should be brought up to a 600 compound finish and possibly in rare occasions to a diamond finish. When in doubt - polish more."

"A good high-hardness plating like chrome or baked nickel will enhance the release process of the molded metal part and works well in conjunction with the high polished surfaces," Lewis notes. "Some types of chrome and nickel have release agents designed into their properties specifically to enhance the release process."

Learning Curve

After mastering the basics, there are still more factors to consider before graduating from this class. According to Hens, another challenging aspect of MIM is that the raw material - typically referred to as feed-stock - contains high amounts of very fine metal powders, often approaching 70 percent by volume. "It is the metal powder that yields the final part," he notes. "The plastic component gives the metal powder flowability and is completely removed subsequent to the injection molding process. The powder is a bit abrasive, so one must use a harder cutting tool at higher spindle speeds. Many moldmakers do a lot of burning if they don't have a high spindle machine. However, you can't leave a rough surface finish behind because of the tight tolerances that MIM requires."

Hens adds that different metals basically have the same mold build requirements, with the exception of sometimes having to plug in a different shrinkage factor. "It's not shrinkage in the mold. All of our shrinkage happens once the part is out of the mold and it goes into the furnace," he explains. "In the mold, you have almost zero shrinkage, less than one thousandths of an inch per inch shrinkage, so in the mold, shrink is very small because you have so many fill lines and material."

Using valve-gated hot runners provides better shot control, Hens points out. "However, when we use valve-gated hot runners, we typically cheat and don't go directly into the part," he notes. "We just use a small subrunner that goes into the part. Remember that these parts must be molded stress-free because they all get heated afterward during heat-treating. So, if you mold in any stresses, parts come out distorted. You will get the best parts by using reasonably large diameter cylindrical, but very short length, runners. Processing windows are wider when short runners are used rather than hot-gating right into MIM components."

Additionally, Hens has one last piece of advice. "History has shown that several mold shops have had to build a poor tool before they recognize the challenge at hand," he says. "Whenever a mold shop builds a first tool for an MIM producer, a lot of time is spent together in the design of the mold to ensure that all of the topics are covered. Typically, as a result, the mold is excellent and works right from the get-go - or maybe with one minor change after the first mold trial. Then the MIM producer assumes that the tool shop understands what the demands are and lets it work more independently on the second and third mold, relying on the success of the first one. This is when disaster often strikes. As the first tool seemed easy as all of the details were analyzed carefully, an over-confidence has been created and less attention is paid to the mold details. It is not uncommon that a lot of time and money was spent debugging a second tool, often involving major rework. At this point the moldmakers have learned the hard way - and if they have not given up on MIM molds at this point, subsequent molds will typically be excellent."