cast-vs-welded-dry-screw-pump-housing-pilot

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title: "Casting or Welding: How We'd Pick for Your Dry Screw Pump Housing Pilot"

summary: "Pilot-stage dry screw vacuum pump housings — when to run lost-foam casting, when to weld from plate, and how the schedule vs cost trade actually decides on a 1–5 piece build."

category: Manufacturing Economics

publishedAt: 2026-06-12

coverImage: /images/blog/cast-vs-welded-dry-screw-pump-housing-pilot/cover.webp

coverImageAlt: Dry screw vacuum pump housing pilot blank staged for 5-axis finish machining of the rotor bores, with weld seams visible at the flange joints

author: Teemo Xu

authorSlug: pengye-xu

noindex: true

keywords:

• cast vs welded dry screw pump housing
• lost foam casting pump housing
• dry screw vacuum pump housing prototype
• welded pump housing pilot run
• low volume pump housing manufacturing
• 304 stainless pump housing casting
• pump housing pilot lead time
• rotor bore parallelism welded housing

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The path we went through

Back when I was doing dry screw vacuum pump R&D, my team would put out a new housing version every couple of weeks, hunting for whatever change would push performance or efficiency up. Drawing a new model on screen is the easy part. Getting a blank in your hands to machine and test is the hard part. Years of iterating pushed us toward two routes that both work at pilot quantities — lost-foam casting and welded fabrication. Both get used regularly, and the pick comes down to three things: per-kg cost, blank lead time, and whether the geometry has any complex internal features.

How we choose

Does the geometry need cored internals? Casting is forced only when an internal feature both needs a machined finish and sits where the cutter can't reach it. Internal volumes that don't need precision — cooling jackets are the canonical case, since flow doesn't care if the walls come off as-cast or as-welded — fit inside a welded body just fine. If both conditions hit, plan around the 45–60 day lost-foam lead time and move on. If only one (or neither), this question doesn't force the route — the next two decide.

Is the design frozen? If you're still iterating between unit 2 and unit 3, weld. A plate re-cut and a fresh weld absorb a bore-spacing tweak or a flange relocation inside a week. A new foam pattern is faster than re-tooling a sand mold, but it still adds days, and any change that touched the pour gating needs re-validation.

Does the calendar have room? If the design is locked and the next milestone is 2+ months out, lost-foam pays you back on per-kg cost and saves the re-qualification work when production later runs off the same blank source. If the validation window is tight, weld and accept the ~25% per-kg premium for the 30–45 days of schedule it buys.

Cost, schedule, and capability, side by side

Numbers below are for a representative dry screw vacuum pump housing in the 175–238 mm bore-centerline class, pilot batch of 3 pieces in 304 stainless. Sizes and alloys move the absolute numbers, but the ratios hold.

How the two routes actually differ on a dry screw housing

The cost gap reads backwards from the usual assumption that welding is the cheap pilot option. On a per-kg-of-blank basis it isn't — lost-foam runs ~$20/kg and welded runs ~$25/kg in 304 stainless. The reason isn't anything exotic: the welded route starts from oversize plate, the blank carries large machining allowance and weld-shrinkage margin, and a lot of the material you pay for leaves as chips. Lost-foam comes out close to net shape — typical machining allowance of 3–5 mm on critical surfaces — so the kilograms you pay for mostly end up on the finished part.

Where welding wins decisively is calendar time. A welded blank is 5–15 days from drawing approval to stress-relieved-ready: plate cutting, fit-up, welding, sub-critical anneal at 600–650 °C for 2–4 hours, then slow cool. Lost-foam needs 45–60 days for foam pattern build, coating, mold prep, pour, cooldown, fettling, and dimensional check before the blank reaches our machining cell. On a pilot with engineering hours burning while the team waits on hardware, that 30–45 day gap is often worth more than the per-housing cost delta.

The capability gap is where most buyers expect a difference that doesn't actually show up on the finished surfaces. A dry screw housing has two precision jobs: parallelism and pitch of the two rotor bores, and flatness plus surface finish on the sealing lands where the end covers and discharge flange mate. Both are finish-machined features. The blank only needs to be dimensionally close enough to fixture and carry enough stock on the critical surfaces for the finishing cuts to clean up. Bore both rotor bores in one fixturing on the 5-axis, on one datum off the end-cover mating face, and the parallelism comes off the machine's own positioning — same on a welded blank, same on a lost-foam blank.

What the casting genuinely gives you, and welding can't, is internal geometry the cutter cannot reach. Cored cooling jackets running between the rotor bores. Thin ribbed structures cast in one piece. Curved internal volumes for gas-path tuning. If your design has any of those, welding isn't an option and the lead time is what it is.

When the welded route is the right call

Welding is the schedule answer. Two pilot situations push toward it almost every time:

• The design isn't frozen yet. Pilot units exist so the engineering team can find what to change. A welded blank can absorb a bore-spacing tweak or a flange relocation between unit 2 and unit 3 with a new plate cut and a fresh weld inside a week. A foam pattern change is faster than re-tooling a sand mold, but it still adds days, and any geometry that affected the pour gating needs re-validation.
• The validation window is tight. From drawing-approved to first machined housing, welding lands at roughly 4–6 weeks total. Lost-foam lands at roughly 8–11 weeks total. If your pump build slot, your customer demo, or your test rig commissioning is inside that gap, the cost premium is the cheap part of the decision.

Geometry has to cooperate. A typical dry screw vacuum pump housing — a box-shaped main body with two parallel rotor bores, end-cover mounting faces, suction and discharge flanges, and a handful of bosses for sensor ports and lifting eyes — is exactly what weld fabrication does well. If your design adds internal cooling channels or anything the welder can't fit up, the door closes here and the conversation moves to lost-foam.

For more on how we hold the rotor-bore tolerances on welded housings, see our dry screw vacuum pump housing component page.

When lost-foam casting is the right call

Lost-foam is the cost-and-capability answer when the schedule allows. Three situations make it the obvious pick:

The geometry needs it. If the design includes cored cooling channels, complex internal ribbing, or curved internal volumes the cutter can't reach, lost-foam isn't a choice — it's the route. Foam patterns can carry near-arbitrary internal geometry that no fabrication route reproduces. The pour and the pattern build happen at our foundry partner; we coordinate the dimensional qualification and bring the blank in-house for machining, inspection, and leak test.

The design is frozen and the calendar has room. If engineering signed off on the housing geometry and the next program milestone is 2+ months out, lost-foam saves you the per-kg gap on a 250–400 kg blank, multiplied across a 3–5 piece pilot, against zero schedule downside.

The pilot is the qualification build for production. If the production plan is 50+ housings per year off the same drawing, running pilot units on lost-foam means the pilot blanks and the production blanks come off the same process. A second qualification cycle later — re-running first-article inspection on a different blank source, re-validating the machining setups — costs more engineering hours than the cost delta you saved by welding the pilots.

Hybrid approaches worth knowing about

Two combinations come up often enough to mention.

Cast main body, welded ancillaries. When the housing body needs cored internals but the suction and discharge flange extensions are simple geometry, the body comes from lost-foam and the flange extensions get welded on afterward. Splits the long-lead casting work from the short-lead features so a flange-port change late in design doesn't restart the casting clock.

Machined-from-solid for single prototypes. On a one-off where even a welded blank's 1–3 weeks is too long, we'll sometimes start from a forged or rolled block and mill the whole housing as a single piece. It burns a lot of stock and a lot of cutter time, but on a single-unit build that has to ship in 7–10 working days, it's the only route that hits the date. Above 2 pieces, welding wins on cost every time.

FAQ

Can a welded housing hold vacuum as well as a cast one?

Yes, when the welds are properly executed and the finish machining is done on the right datums. A dry screw vacuum pump housing seals at the mating faces — end-cover joints, flange joints, shaft seal lands — not through the body material. Weld integrity matters for structural and gas-path reasons, but the sealing performance is a function of the finished surface finish (Ra 0.8 µm or better on the sealing lands), the flatness of those faces (within 0.02 mm over the seal width), and the gasket or seal spec. We pressure-decay test every housing before it ships, welded or cast.

Isn't welding supposed to be the cheap pilot route?

It's faster, not cheaper. The intuition usually comes from sand-cast comparisons where the foundry adds pattern-build minimums on top of the casting cost. Lost-foam uses a per-part foam pattern with no hard tooling, so that minimum collapses — even though we run lost-foam through a foundry partner rather than in-house. On a per-kg-of-blank basis our lost-foam route runs ~$20/kg in 304 stainless versus ~$25/kg welded, because the welded blank starts oversize and a lot of the material you pay for gets cut off during machining. Welding wins on lead time and design-change cost, which is often the right trade on a pilot — but it's a schedule trade, not a cost trade.

How much extra machining allowance does a welded blank need versus a lost-foam casting?

We design welded blanks with 5–8 mm of stock on every critical surface — bore IDs, mating faces, flange faces — to absorb weld shrinkage and dimensional drift. Lost-foam blanks come in with 3–5 mm of casting allowance and tighter dimensional repeatability. That difference is most of the per-kg cost gap: more allowance means more material removed during machining.

Does your shop handle the whole job, or do you partner with a foundry?

The welded route runs end-to-end in-house: plate cutting, fit-up, welding, stress-relief, machining, inspection, leak test. For lost-foam, the pattern build and pour happen at a foundry partner we work with regularly; we own the drawing review, the dimensional qualification of the incoming blank, and everything downstream — machining, inspection, and leak test all stay with us. Either way, the finished housing ships from our facility against our QA report.

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