Permanent Mold Cuts Machining Costs for Mining Part

Shannon Wetzel, Managing Editor

With a variety of casting processes to choose from, buyers and designers must make their choice based on variables that cover cost, lead times, mechanical and physical properties, volumes, and even available suppliers. For many pieces, as volumes grow or customer demands change, a switch in manufacturing process could be warranted.

Such was the case for a mining part produced for Peak Mechanical LLC (Osburn, Idaho).

The diffuser ring is a wear part for Peak Mechanical’s APEX heavy duty 13 horsepower electric submersible dewatering pump used in the mining and construction industries. This type of pump is left in a mine year-round, even after the area is in use, to remove water from solid material or soil. Although it is a semi-disposable part that operators expect to replace regularly, a part that lasts longer with better corrosion resistance means fewer replacements.

The ring has three sets of holes that originally were machined, so dimensional accuracy was important. As originally designed and manufactured, the sand cast version struggled to keep the dimensional tolerances at two different suppliers. Eventually, Peak Mechanical added a custom machining step to each part, sometimes machining up to 0.12 in. off some areas.

Additionally, some of Peak Mechanical’s international customers wanted higher standards for cosmetic appearances. Sometimes, machining unveiled porosity blemishes that did not affect the function of the part, but left customers annoyed.

Peak Mechanical, which exports to countries worldwide, is keen to expand its global reach. Volumes were also growing enough to make the transition to permanent mold casting cost effective.

“We had looked at permanent mold casting before, but the initial mold cost is substantially more compared to sand casting with this part,” said Tom Reynolds, shop manager for Peak Mechanical. “But eventually we were doing larger quantities and that’s where permanent mold looked more applicable.”

A local machine shop suggested Peak Mechanical reach out to AFS Corporate Member LA Aluminum (Hayden, Idaho).

“They came to visit and walked around and no one there had really seen the permanent mold process,” said Gary Jackson, quality assurance manager at LA Aluminum.

Peak Mechanical did not have a solid model of the diffuser ring, so Jackson looked at a scrap part and the machine print and reconstructed the information digitally to come up with a proposed design and quote. Because of the permanent mold process’’ tolerances, LA Aluminum determined it could cast-in the multiple holes in the ring that were being machined or drilled. Less finish machining would be needed in the proposal, as well. Those factors helped make the permanent mold design an affordable switch for Peak Mechanical.

The diffuser ring features three sets of holes. The first set on the inner most circle were originally drilled and tapped to mount the ring to another component. LA Aluminum was able to cast-in steel nut inserts that eliminated the secondary drilling and tapping operation. The second set of holes were previously machined but now cast-in, and they are present to help the coating of urethane to flow through to cover the casting.

A third set of holes on the outer rim are there for alignment and mounting and were originally drilled. This process of secondary drilling exposed the cosmetic porosity that caused issue with Peak’s customers. Now the holes are cast-in to tolerance, eliminating machining altogether and enhancing the cost savings benefit.

“The cast-in holes are pretty standard, we just had to figure out the size,” Jackson said. “It was a challenge to get them consistently holed at first, but it was just a matter of tweaking the length of the pins.”

Initially, LA Aluminum was not planning on casting in the inserts, but after a few modifications, broached the possibility with Peak Mechanical.

“It is really nice to say, ‘hey, we have figured out a way to save you a few dollars,’” Jackson said.

Once LA Aluminum was awarded the job, Jackson set to work adjusting the design for permanent mold. The sand casting and permanent mold casting processes offer different advantages and design opportunities. The metal mold material in permanent mold eliminates the potential for mold shift, leading to a uniform shape and excellent dimensional properties. For Peak Mechanical’s diffuser ring, dimensional tolerances—particularly around the holes—were more important than having smaller draft angles or thinner walls.

Tooling for permanent mold usually is more expensive than sand tooling, but in this application, that was counteracted by the elimination of machining and other secondary operations. Jackson was able to make other necessary design changes to the ring so that it was more cost effectively cast in permanent mold.

“The sand casting had 1 degree of draft, so I was able to add two or three degrees more draft,” Jackson said. “Some of the ribs and channels also needed tweaking—adding more thickness where we needed it to get the holes lined up.”

Plus the diffuser ring’s volume had grown over time, which made the time to recoup the higher tooling costs shorter.  

“At the time we made the switch, it was going to take us less than a year to pay for the tooling at the volume we were manufacturing the part,” Reynolds said.

The ability to cast in all the holes to tolerances as well cast in stainless steel nut inserts proved to have a big impact on Peak Mechanical’s cost. The part cost is lower and the manufacturer is machining 70% less on each diffuser ring. Now, the only machining is 0.03 in. off full clean-up areas. The permanent mold chill factor helps the casting avoid porosity appearances at the holes, making Peak’s customers happy and opening the door for further expansion into the export market.

Peak also reports longer lasting pump performance due to the tighter tolerance.

“Aesthetics for us are important because when people see our products we want them to see it looks better and has a higher quality than our competitors,” said Jed Hinkins, general manager, Peak Mechanical. “As we move forward, we will continue to transition our highest volume parts to permanent mold and then start exploring the lower volumes. Those will have less drastic savings, so we will do that more slowly.”    

Click here to see this story as it appears in the September 2018 issue of Modern Casting