Centrifugal casting delivers a consistent, quality tubing product at a controlled cost. But a big part of what makes this such an effective process may come as a surprise: Permanent steel molds.

What’s so special about permanent molds – and why are they so specific to the centrifugal casting process? To answer that question, let’s look at the difference in mold types.

But first, here’s a quick overview video on permanent molds.

Expendable vs. permanent molds

Both expendable and permanent molds use a casting process that starts by pouring molten liquid metal into a specifically designed empty cavity. Each process also allows the metal to solidify. The similarities, however, stop there. 

Expendable (non-permanent) molds

With expendable mold casting, the molds used during the pour process are broken apart after the part solidifies. Therefore, they cannot be used to make another part.

It’s an ideal method for complex shapes because the sand allows for a lot of flexibility in the design. It also allows for small batches to be made at a fairly reasonable cost. Because of the nature of the process, a unique mold has to be made for every part.

It can be time-consuming to set up and requires a lot of labor to reproduce parts. There’s also a fair amount of waste, which can impact costs.

In the end, though non-permanent mold techniques provide some design flexibility, it’s not usually a good choice for big orders of the same part.

Sand casting and investment casting are examples of processes that use expendable molds.

Permanent molds

A permanent mold can be used and reused over a thousand times. And if any surface warping or changes occur, it can be resurfaced, then possibly used a thousand more times.

Made from a robust metal material, the mold can withstand a repeated heating and cooling process involved in large volume production.

The process produces metal tubing with better dimensional tolerance, superior surface finish, and higher and more uniform mechanical properties than expendable molds. You’ll also get higher strength, toughness, and ductility due to a fast solidification rate producing a fine grain structure in the cast metal.

Centrifugal casting is a good example of a popular process that uses permanent molds.

The benefits of permanent molds

There are many benefits to using permanent molds for a metal cast tube part, but the most notable are:

• No mold-related production delays
• Creates less variation and waste
• Works with all alloys
• Highly repeatable and documentable

No mold-related production delays

Because a permanent mold only needs to be created once, you aren’t left with the time and expense it takes to recreate it for repeated parts. That also means the method, material and secondary processes can all be documented for future production runs.

Many foundries will have some molds already made. In fact, the Spuncast foundry has multiples of the most popular diameters and lengths on site, which can eliminate the wait time for creating a new mold.

“The more popular the size, the more variation in length we’re going to have for that diameter – and the more backup molds we’re going to have in case something happens to the primary so that we’re never without,” says Greg Whitman, VP of sales and engineering at Spuncast.

“You’re not going to have to wait for a pattern to be designed and then a mold to be created,” he points out. “We just take the mold from the mold yard, and we put you right in the schedule for the day the parts are ordered. So there’s no additional waiting time for the tooling.”

Less variation and waste

If you tried to make metal tubing using investment or sand casting, you probably wouldn’t be satisfied with the results.

You’d have porosity in places you didn’t want porosity, and the end product would vary from piece to piece. There’s also a fair amount of waste, which can have a cost impact.

With a sand casting, you have to recreate it out of sand every time, and that typically speaks to cost, time, and money. In addition, the sand that gets broken away from the casting only has a finite life period, and it can create a huge mess.

Using a permanent mold, however, represents the ultimate in consistency. And permanent molds exhibit a denser and finer granular structure with improved physical qualities (tensile strength, flow resistance, elongation, uniformity).

If cast with an efficient process like centrifugal casting, the outer surface areas of your tube will have a higher density than the inner surface areas. The result is a seamless, more robust cylinder.

“With the permanent steel molds used in centrifugal casting, there’s almost no waste or byproduct,” says Greg.

Works with all alloys

The material used in permanent mold casting is selected from an extensive range of metals. However, it’s critical to choose the metal that suits the application because the mold materials’ composition and structure affect the final part’s result.

But most permanent mold castings are made of steel and its alloys. That’s because alloys are specifically made to enhance the properties of the metals and elements they contain. They also work well when the need to engineer specific characteristics is important.

Repeatable and documentable

For many customers looking for cast metal tubing, satisfaction starts with getting exactly what they asked for every time.

Using permanent molds, particularly in centrifugal casting, can create a huge sense of security for tube designers. They don’t have to worry about all the variation and potential quality issues that could otherwise be there.

And because it’s always a consistent process, it’s easily documented. That information helps gain long-term primary and secondary benefits like:

• Automating the workflow
• Removing operational ambiguity
• Creating availability for analysis
• Improving communication
• Reducing errors and costs

“It’s a very repeatable operation for any specific size, based on the weight of the material we put inside the mold. You’re going to get that same tube every time,” Greg said.

Quality tubes start with quality molds

For metal tubing, the permanent steel molds used in a centrifugal casting process provide undeniable advantages. They offer efficient production with less variation, the ability to work with all alloys, and a documentable process, which are tangible and real difference-makers.

However, even with all the benefits of using a quality permanent mold, it takes an experienced foundry to repeatedly produce a better tube.

With over 40 years in business, Spuncast can provide that expertise. They offer over 1,000 permanent onsite mold options, with outside diameters ranging from 2.5” to 44” and all sizes in between.

And the process never varies – because the molds never change.