In dry drawing the lubricating film forms when powder picked up from the soap box is pressed onto the surface at the die entry by pressure and heat. We look at why that film breaks at high speed, the symptoms it shows, and the discipline required on the soap box side.
In dry drawing, lubrication is achieved not by feeding a liquid lubricant to the die but by the wire carrying its own lubricant to the die entry. As it passes through the soap box immediately before the die, the wire picks up powder or granular drawing soap on its surface; that loose powder is compressed in the converging zone at the die entry under pressure and frictional heat, softens, and becomes a continuous film by adhering to the phosphate layer. In other words the powder you see in the soap box is not yet a lubricant; the lubricant forms at the die entry.
Film stability means that this formation can be reproduced without interruption along the line, at every pass and every speed. As line speed rises, the time available for the film to form shortens, the temperature in the contact zone climbs and the system's margin for error narrows. This article looks at how the film is established, under what conditions it breaks, the marks that breakdown leaves on the wire and in the die, and the discipline required on the soap box side, from the point of view of a wire drawing line. For a basic introduction to what drawing soap is and its types, see what is drawing soap. Kimfosan's drawing soap range, Kimkal, is the reference for the behaviour described here.
The film is established by three consecutive steps overlapping, and each step depends on the output of the one before:
The role of the phosphate layer in this chain is critical. The layer is the store from which the soap is carried: the porous crystalline skeleton of the zinc phosphate coating holds the soap and, under the pressure inside the die, feeds it back to the surface from those voids. If the phosphate side is weak, no improvement on the soap side will rescue film continuity. Our broader treatment of the film's effect on the friction regime, friction in cold forming, completes this context.
Film breakdown does not have a single cause; it usually emerges when several borderline factors coincide. Line speed is the multiplier that strains all of them at once.
A phosphate layer with low coating weight, a coarse crystal structure or a tendency to dust cannot hold enough soap on the wire surface. When the amount of soap entering the die falls, the film stays thin and is punctured at the first high-reduction pass. Whether coating weight is in the target range needs regular verification; for the method see coating weight measurement.
This is the most common and most overlooked problem in dry drawing. Because the wire always runs on the same path through the box, it opens a tunnel for itself through the mass of powder. The walls of that tunnel compact and harden over time; the wire is then contacting a hard crust rather than loose powder. Visually the box is full, yet the amount of soap transferring to the wire surface is steadily falling. Drawing that deteriorates gradually through a shift is usually a sign of exactly this.
Drawing soaps are hygroscopic; powder left open takes up ambient moisture. Soap that has absorbed moisture loses its flow, cakes and bridges inside the box. Caked soap transfers unevenly to the wire: thick build-up in some areas, almost bare surface in others. The film becomes discontinuous. Moisture entering the die also flashes off in the contact zone and punctures the film locally.
It is desirable for the soap to soften and adhere to the surface; it is not desirable for it to melt outright and run. At high speed and high reduction the contact zone temperature rises. A soap whose melting/softening point is too low for the application becomes fluid under these conditions, is squeezed out of the die throat and leaves no film behind. Conversely, a soap with too high a melting point cannot become plastic enough, stays as loose powder and never forms a true film. The sodium/calcium balance of the soap, its reactive or neutral character and its softening behaviour are therefore selected together with line speed; we set out the selection logic in drawing soap selection.
The entry cone angle directly affects how the soap is dragged into the die throat. Too steep an entry makes it harder to draw powder into the die; too wide an angle lowers the compaction pressure. Equally, a reduction ratio distributed unevenly between passes pushes temperature and stress to a peak at certain dies. Film breakdown usually starts not across the whole line but at a single die, where reduction or wear is highest. When hunting the problem, assess the line die by die rather than as a whole.
When the lubricating film is interrupted, metal and die make direct contact. The consequences leave readable marks on both wire and die: longitudinal scratches and scuffs on the wire surface with occasional bright friction bands; pick-up and material transfer in the die; noticeable heating of the die body; dimensional drift and finally wire breaks. The earliest sign an operator notices is usually a rise in the current drawn by the drawing motor and a die that is hotter than normal.
| Symptom | Likely cause | Action |
|---|---|---|
| Longitudinal scratches and scuffs on the wire surface | Film thin or discontinuous; coating weight insufficient | Verify phosphate coating weight and crystal structure; check the fill height in the soap box |
| Pick-up in the die, die running hot | Film broken in the contact zone; metal-to-die contact | Check the wear and entry cone of that die; review the reduction distribution |
| Drawing deteriorating gradually through the shift | Tunnelling and crusting in the soap box | Empty the box and clean out the compacted crust; define a filling and stirring interval |
| Irregular, locally thick soap build-up on the wire | Soap has taken up moisture and caked | Take the opened pack out of use; correct storage conditions and stock rotation |
| Soap smearing and running out of the die throat; no film on the surface | Softening/melting point too low for the line conditions | Re-assess the soap type for the line speed and reduction with the technical team |
| Soap falling off the wire without adhering | Surface polished / coarse-crystalline; soap too hard | Check the degreasing and activation stages before phosphating; review the soap character |
| First passes fine, breaks at the last passes | Cumulative deformation and temperature rise; soap reservoir exhausted | Consider moving coating weight to the upper band and redistributing the pass schedule |
When you see pick-up in the die the reflex is to change the die first; yet pick-up is usually a fault of the film, not the die. The new die will run with the same film.
The soap box is the simplest-looking and most neglected piece of equipment on the line. Much of film stability is won or lost here. A few rules need to become established practice:
Film stability cannot be managed on a “fine until it fails” basis. There are a few practical methods for making the soap film trackable under production conditions:
When making changes, change one parameter at a time. If soap type, coating weight, die schedule and line speed are all changed at once, the source of an improvement or a deterioration cannot be identified and the line ends up running on a setting nobody can reproduce.
In dry drawing the soap film is a result produced not by the soap alone but by the phosphate layer and the soap together. If the carrier surface cannot hold the soap, if the box has tunnelled, or if the soap has taken up moisture, the film cannot be established at the die entry. Every link of the line sequence (degreasing → rinsing/activation → phosphate → soap → drawing) is written into the quality of that film.
If you would like to assess the soap character and coating weight together for your line speed, reduction schedule and material, talk to our technical team. Producing phosphating chemicals and drawing soaps since 1982 and Türkiye's leading drawing soap manufacturer, Kimfosan provides process support tailored to field conditions from its plant in Kartepe / Kocaeli.
The wire picks up powder or granular soap on its surface as it passes through the soap box before the die. That loose powder is compacted in the narrowing gap of the die entry cone; under pressure and frictional heat it softens, fills the pores of the phosphate layer and becomes a continuous film adhering to the surface. The lubricating film forms at the die entry, not in the box.
As speed rises the time available for the film to form shortens and the temperature and pressure in the contact zone climb. Inadequate phosphate coating weight, tunnelling in the soap box, caked soap that has taken up moisture, a softening/melting point unsuited to line conditions and an uneven reduction distribution all lead to the film being punctured under these conditions. It is usually not one cause but several borderline conditions coinciding.
The most typical sign is drawing performance that deteriorates gradually as the shift goes on even though the box looks full: rising motor current, a hot die, scratches on the wire surface. A compacted, hardened crust has formed along the wire path. The fix is not to add soap on top but to empty the box completely, clean out the crust and put filling on a defined routine.
Powder and granular drawing soaps take up moisture; soap that has absorbed moisture cakes and transfers unevenly to the wire, breaking film continuity. Packs should be kept closed, on pallets, in a dry and cool area away from heat and steam sources. An opened pack should be consumed quickly and stock rotated on a first in, first out basis.
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 →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 →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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