What makes a deep drawn housing leak-tight?
A drawn housing is leak-tight when the wall has no through-thickness defect and no seam, and when the surfaces that seal against an O-ring, a gasket or a mating part are sound. Unlike a housing welded up from tube and end caps, a deep drawn housing is one piece of metal, so there is no weld to leak, no heat-affected zone to crack and no filler metal to react with the medium. What then decides tightness is the wall itself: whether the draw ratio and the number of stages kept the wall thickness uniform, whether the material had enough elongation and the right temper for the reduction, whether the radii were generous enough to avoid thinning, and whether a pre-form defect such as casting porosity or an inclusion has been carried into the finished wall. Most leaks in drawn housings trace back to one of those, not to the draw itself.
Why a drawn housing has one less leak path than a welded one
A welded assembly has three potential leak paths that a drawn part simply does not have. The weld itself can be porous, undercut or incompletely fused, and a weld that passes a pressure test cold can still weep after thermal cycling. The heat-affected zone beside the weld is a different microstructure from the parent metal, and it is where fatigue cracks begin. And the filler metal adds a third material to the joint, which matters when the housing carries a fluid that attacks the filler but not the base. Drawing removes all three by making the shell in one piece: the material flows rather than being joined, and the grain follows the contour of the part instead of being interrupted by a fusion line. That is the main engineering reason solenoid valve and sensor housings are drawn rather than welded, and it is worth keeping in the drawing notes when a supplier proposes a welded alternative for a deep part.
What the wall has to survive
- Thinning at the radii. Material thins where it is stretched hardest, most of all at the punch nose and the die radius. A wall that thins unevenly is not automatically a leaker, but it is where a leaker will appear once pressure or thermal cycling is applied.
- Draw ratio per stage. Pushing too much reduction into one stage thins the wall and can tear it. Splitting the same reduction across two or three stages, with an anneal between them, keeps the wall more even than a single aggressive draw.
- Material elongation and temper. Draw-quality strip such as DC04 has the elongation and the r-value the process needs. A harder temper of the same grade may form without cracking and still leave a thinner, less even wall.
- Grain size and inclusions. A coarse grain or a stringer of inclusions gives the wall a line to open along. This is a mill property, and it is why the material certificate matters on a pressure-carrying part.
- Pre-form defects. A casting that arrives with porosity, or a forging with a lap, carries that defect into the finished wall where machining will not remove it.
- Annealing. Where the route needs an interstage anneal, skipping it or running it badly leaves the wall hard and the next stage tears rather than flows.
The sealing face is a separate requirement
A housing can have a perfectly sound wall and still leak at the joint. The surface that an O-ring, a gasket or a mating component seals against needs its own attention: flatness if the seal is face-to-face, roundness and a diameter tolerance if the seal is radial, and a finish the seal material can bite into without tearing. Scratches, a rolled edge, a burr from trimming, or a surface polished so smooth that the elastomer slides, all create leaks at a seal that the wall itself would have passed. Where a drawn housing is machined afterwards, the sealing face is usually one of the features that is machined rather than left as formed, and it should be called out as such on the drawing rather than left to the general tolerance block.
Key point
When a drawn housing leaks, the cause is nearly always upstream of the leak. Ask three questions in order: was the wall thickness uniform after forming, was the material the right grade and temper for the reduction, and was the sealing face made as a controlled feature? A supplier who answers only the third has not looked at the part.
How to specify tightness on the drawing
- State the requirement, not the test. A leak rate at a defined pressure and medium is a requirement. A note saying only that the part is leak tested is not, because it does not say what passes.
- Give the wall thickness that matters. If the wall carries pressure it needs its own minimum, not a nominal buried in the general tolerance block.
- Name the sealing surfaces. Mark the faces that seal and give them flatness, finish and diameter tolerances separately from the rest of the part.
- Say whether the part is machined after drawing. A sealing face that is turned after forming behaves differently from one left as drawn, and the drawing should not leave the choice open.
What we do and do not claim
Balford deep draws housings on presses from 25 t to 350 t up to a 250 mm maximum draw diameter, and inspects to the drawing with dimensional, geometric and surface records taken per lot. We do not publish a rejection percentage or a blanket leak-rate guarantee, because whether a housing is leak-tight is a property of the design, the material and the process route together, and it is verified by the test the customer specifies rather than by a general claim. Where the requirement is a pressure or leak test, it is stated on the drawing and agreed in the quotation. Quality management runs to ISO 9001:2015 with IATF 16949-aligned controls for the automotive programmes we serve. Where a part is cast, forged, cold headed, cold extruded or produced by powder metallurgy before machining, that pre-form is supplied by a qualified subcontractor and the route is named in the quotation.
Balford has built more than 4,000 single-hit drawing dies since 2000, operates a 12,000 m2 plant in Zhuji, Zhejiang with 500+ machines and 120+ engineering and production staff, and produces roughly 50 million parts a year. Housings are quoted from the drawing, usually within 24 hours, and the DFM review comes back with the price - including, where it applies, the wall-thickness and draw-ratio observations that decide whether the part will hold tightness in production.
Frequently asked questions
Is a deep drawn housing always more leak-tight than a welded one?
No, not automatically - but it has one fewer failure mode. A drawn housing has no weld, no heat-affected zone and no filler metal to leak through, so the remaining questions are the wall and the sealing face. A welded assembly can be perfectly tight when the welding, the material and the joint design are all right. The reason drawn housings are common for solenoid valve and sensor bodies is that drawing removes the weld from the list of things that can go wrong, which matters most on a part that sees pressure or thermal cycling.
What wall thickness can a deep drawn housing hold?
It depends on the material, the starting thickness, the draw ratio and the number of stages, so there is no single number. What matters is uniformity rather than a nominal figure: a wall that starts at 1.5 mm and thins to 0.9 mm at the punch nose is a different part from one that holds 1.4 mm throughout, even if both are drawn from the same blank. Give the minimum wall that carries the pressure or the thread as its own tolerance on the drawing, and the route can be designed around it.
How do I stop a drawn housing cracking at the bottom radius?
Cracking at the punch nose is almost always the draw ratio being too aggressive for the material and the thickness, or the punch and die radii being too tight for the reduction being asked of them. The fixes are to split the reduction across stages, open the radii, or anneal between stages if the material work-hardens enough to need it. If the part has already been tooled, the drawing can usually be revised to allow a larger radius at the bottom, which is cheaper than rebuilding the die.
Does the pre-form affect leak-tightness?
Yes, and this is the part that is easy to miss. If a housing is machined from a casting, porosity in the casting becomes a potential leak path through the finished wall, and no amount of downstream machining removes it reliably. A forging can carry a lap or a fold in the same way. That is one reason a drawn or cold-formed pre-form is often preferred for a pressure-carrying housing: the material is wrought and consolidated rather than cast, so there is no trapped porosity to open up.