“Good Investment” Castings

by Ed Sullivan

Used to form precise and intricate metal shapes, investment casting offers tool & die makers, machine shops and their customers superior finishing plus significant savings of time, material and labor.

If you’re fabricating parts out of bar metal, chances are it’s costing you a lot of extra time and money. If those parts require extensive machining, you could be losing significant money in scrap, especially if you’re using pricey metals or alloys.

An increasing number of shops that make metal parts that are intricate, or require extensive machining, are produced repetitively or in limited quantities, are finding that investment casting is the ideal solution.

Investment casting offers broad flexibility of alloys while saving finishing time and material waste. The process can also enable you to combine two or more parts into a single piece, saving on fabrication, welding or assembly and machining time. The range of metals and alloys that can be investment-cast is very broad, including low cost alloys such as carbon and many tool steels or costly alloys such as aluminum, stainless steel, hastalloy, cobalt and Inconel.

When it comes to making metal parts, investment casting can provide a surprisingly high return on investment for your shop and customers. Somewhat overlooked today, investment casting – forming metal parts in disposable molds - offers opportunities to create “near net shape” parts of virtually any metal, even in very low quantities.

Investment casting is an ideal process for those who fabricate or use metal parts repetitively, whether intricate shapes or components that require precise tolerances. If those parts are between one ounce and 30 pounds in weight, they can be investment cast in remarkably close tolerances with surfaces that require little finishing.

Here, the wax is removed from the pattern material by heating the ceramic mold in a high-heat furnace. The material is lost, and molten metal is poured into the ceramic pot.   After the metal parts solidify, they are removed from the investment material and are inspected for quality to ensure complete satisfaction.

As opposed to forming parts from bar metal, investment casting is also beneficial for fabricators who want to combine components into a single piece, or use pricey metals and want to avoid wasted material while minimizing machine time, which can run $85 to $100 per hour including machine cost.

“I suppose that some fabricators look at the somewhat higher initial cost and don’t realize all the savings of investment casting in time and materials, saving money in the long run. Plus they can produce a better part,” says Carl Johnson Jr., vice president of Staten Island Machine Shop Inc. (Staten Island, NY).

Johnson, whose shop produces metal shafts as well as plate and sheet metal, explains that the stainless shafts he fabricates in relatively small lots are investment-cast rather than cut from bar stock or formed by sand casting and then finished.

“For one thing, in this part of the country it is becoming difficult to find qualified machinists,” Johnson says. “There are few machinists or CNC operators coming out of the schools today, and that – as well as the cost of equipment and labor – has become a problem for many machine shops. To an extent, investment casting alleviates this problem, because it eliminates some of the burden of machining.”

Several years ago Staten Island Machine Shop began having some of the parts they previously had sand cast instead supplied by Rimer Enterprises (Waterville, OH), a state-of-the-art investment-casting specialist that serves a variety of industries ranging from railroad to food processing.

Johnson adds that the stainless steel gears his shop now gets from Rimer, typically for marine applications, are high-precision parts that slide over or under other components. In the past, when the gears were made from sand castings, there could be significant shifting or other movement.

“This problem is far less likely to happen with investment-cast gears because the rotations are right on, the holes are exactly where they should be, and all critical dimensions and tolerances are very close, which also minimizes the need for machining,” he says.

While reducing the demand on machine time is a significant savings, there is also substantial added savings in costly metals used to fabricate many parts. Chuck Myers, president of Rimer Enterprises, says depending on the metals and alloys used to make the castings, the differences in material costs could be stunning.

“For example, if you are machining a piece of stainless steel that costs $5 per lb., you might be machining 80% of the steel out for your finished product,” Myers explains. “By the time the part is finished you’ve got 4 lbs of stainless steel chips that you end up selling to a scrap dealer for $2 per lb. If the same part is investment-cast, the near-net shape virtually eliminates the scrap, which could represent many dollars in savings per part in alloy cost as well as labor.”

The general manager of an Ohio-based machine shop says that one of the main reasons he buys investment castings is that he can’t get the needed material in bar stock and prefers not to use sand castings. However, the savings on materials is also significant.

“The advantage of getting a near-net shape, means less machining and also material savings,” the GM explains. “So, when you make parts with alloys, such as the nickel-based alloys that we use, there is a pretty significant cost savings because you don’t have to throw half of the metal away in chips. And of course, the machine time is less when you have parts that are cast pretty close to size.”

This shop, which also has its investment castings made by Rimer, recognizes that the consistency of investment-cast products is a noteworthy benefit.

While investment casting may provide quantum savings in terms of time, material and labor, some people have concerns about turn-around time. The GM, for example, uses limited quantities of orifice rings that one of his glassmaking customers uses in making bottles. “When we need castings it is usually because a customer is running the same part except that the dimension may change,” the GM says. “We try to stay ahead of the game but we can’t anticipate how long their production runs are going to be. So, if we get caught short, any delay in turnaround time can really hurt.”

In anticipation of such problems, Rimer made substantial new investments in their in-house capabilities when they took over ownership of the business several years ago. For example, in 2006, they installed a robot dipping system to reduce lead-time through the shop and improve the consistency of products. The newly expanded facilities also include a very modern CNC tooling shop and a CNC machine shop for machining castings.

“Turnaround time in our industry is often 10-12 weeks,” Myers says. “We have been able to cut that time more than 60 percent. In emergency situations, we will do everything we can to turn around the needed castings as quickly as possible.”

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