In tube drawing the die does not touch the outside surface alone; a mandrel or plug creates a second contact zone on the inside. That is precisely where lubricant is hardest to deliver. We look at lubrication strategy and soap selection by drawing method.
In wire drawing there is a single contact zone: the die and the outer surface of the wire. In tube drawing, most methods involve two — the die on the outside, a mandrel or plug on the inside. The centre of gravity of the lubrication problem is that second zone, because the inside surface is where lubricant is hardest to reach, and once drawing has started there is no way to intervene there.
This article covers the differences between tube drawing methods from a lubrication standpoint, why inside surface preparation matters as much as the outside, soap selection, and the causes of the inside-surface defects most often seen in the field.
The lubrication requirement changes fundamentally according to whether there is a tool on the inside surface:
As a rough rule, the more the tool contacts the inside surface, the greater the lubrication requirement. In sinking, outside-surface lubrication largely solves the problem, while in mandrel drawing inside-surface lubrication becomes the main constraint on the line.
Soap works to the extent that it can key into the surface. With its porous structure the phosphate layer is the ground that holds the soap; we explain the mechanism in what is phosphating. On a tube, that ground has to exist on the inner wall as well.
In practice the real difficulty is this: the bath solution not circulating adequately inside the tube. In narrow-bore, long tubes filled from one end, an air pocket can remain on the inside surface, the solution is not renewed and the layer on the inner wall is either very thin or absent altogether. Viewed from outside the tube looks properly phosphated; the problem only emerges in drawing.
Dry drawing soaps applied over a phosphate layer divide broadly into reactive and neutral. Reactive soaps interact with the phosphate layer to build a more strongly adhering film; neutral soaps adhere physically and behave differently in terms of the residue they leave. We cover that distinction and the selection criteria in drawing soap selection.
| Method | Inside surface contact | Lubrication requirement | Key criterion |
|---|---|---|---|
| Sinking | None | Medium — mainly outside surface | Film continuity on the outside surface |
| Fixed plug | Yes, narrow zone | High | Continuity of phosphate + soap on the inside surface |
| Floating plug | Yes, dynamic | High | Film stability at high speed |
| Mandrel (rod) | Yes, large area | Very high | Inside coating quality and ease of stripping |
The “key criterion” column shows which property should take priority in soap selection. Mandrel drawing carries one further constraint: the rod has to be stripped out of the tube after drawing. The lubricant must therefore build a film strong enough while not leaving a residue so tacky that stripping becomes impossible. The two requirements pull against each other and the right balance is product specific.
Managing friction in tube drawing rests on the same physics as in wire drawing: the lubricating film gets between metal and tool and prevents direct contact. The moment the film breaks, friction rises, friction raises temperature, and the temperature rise accelerates the breakdown of the film — the cycle feeds itself. We set that mechanism out in friction in cold forming.
On a tube, the most dangerous place for that cycle is the inside surface, because the heat generated there cannot easily escape and is invisible. A film loss beginning on the inside surface usually reaches the operator looking from outside only as a surface defect, after the job is finished.
In tube drawing the outside surface shows you the problem; the inside surface only tells you once the job is done. That is why inside surface preparation is a parameter to be audited, not a result to be hoped for.
| Symptom | Likely cause | First check |
|---|---|---|
| Scratches and sticking on the inside surface | Insufficient phosphate/soap on the inside surface | Internal circulation and air pockets in the bath |
| Unexpected rise in drawing force | Film breakdown, inadequate soap transfer | Soap type and application temperature |
| Difficulty stripping the mandrel | Excess residue or a tacky film | Soap type and inside surface coating weight |
| Bright rubbing marks on the outside surface | Film continuity broken on the outside surface | Soap box fill level and die condition |
| Different surface between tube end and middle | Filling/draining imbalance in the bath | Immersion angle and dwell time |
| Rusting shortly after drawing | Rinse water or acid left inside | Drying and neutralising stages |
The last row is a particular risk in tube drawing: water left inside is invisible and leads to corrosion working from the inside out in the store. We cover the storage side separately in why phosphated wire rusts.
Lubrication in tube drawing is the job of managing two contact zones at once. In sinking the load is mainly on the outside surface, while in plug and especially mandrel drawing the inside surface becomes decisive. Without a sound phosphate layer on the inside surface, even the right soap will not work; internal circulation in the bath, rinsing and drying are therefore parameters as critical as soap selection. In mandrel drawing the balance between film strength and ease of stripping must also be struck product by product.
Kimfosan manufactures zinc phosphate coatings (KİMSOL) and drawing soaps (Kimkal), and provides field support on inside surface preparation and soap selection for tube drawing lines. Talk to our technical team with your tube dimensions, method and reduction figures. In production since 1982, Kimfosan offers technical support tailored to your process conditions from its plant in Kartepe / Kocaeli, Türkiye.
In wire drawing there is a single contact zone: die and outer surface. In tube drawing, when a plug or mandrel is used, a second contact zone forms on the inside surface. Getting lubricant there is difficult, no intervention is possible during drawing, and the heat generated cannot easily escape. Inside surface preparation is therefore the most critical parameter.
The most common cause is the bath solution not circulating adequately inside the tube. In narrow-bore, long tubes an air pocket can remain inside, the solution is not renewed and the layer on the inner wall stays thin or does not form at all. Immersion at an angle, complete filling and, where necessary, internal circulation are required; the same problem occurs in rinsing and drying.
Mandrel drawing has to satisfy two requirements at once: a strong, continuous film on the inside surface, and the ability to strip the rod out after drawing. Because these pull against each other, the right balance is determined by product, tube size and reduction; it would be wrong to give a single general-purpose soap recommendation.
In sinking there is no tool contact on the inside surface, so the lubrication load is mainly on the outside. However, phosphating the inside surface and, particularly, rinsing and drying it properly still matter; acid residue or water left inside leads to corrosion working from the inside out during storage.
The most common cause is water or acidic residue left inside the tube after rinsing not being fully dried out. That residue is invisible and starts corrosion working from the inside out in the store. The drying stage and neutralising need to be validated against the tube geometry.
Reactive soap reacts with the phosphate layer to form zinc soap; neutral soap leaves a physical film. Which suits which line is decided by drawing speed, reduction ratio, wire diameter and what the wire has to go through after drawing.
Read →Drawing soap is a metallic-soap based dry lubricant that forms a lubricating film between die and metal in wire and profile drawing. It is not a cleaning soap that dissolves in water; it is a consumable that keys into the phosphate layer and forms a film under pressure and heat.
Read →In fastener production the surface expands several-fold; a liquid oil cannot carry that contact pressure. We explain how the porous phosphate layer and the soap film work together, and how the pair is selected by severity of deformation.
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