Cleaning Profile Dies: Burn-Out, Fluidised Bed and the Land You Lose

What actually builds up

A running PVC profile die collects three different things. The first is plate-out: calcium stearate, waxes and stabiliser residues that condense on the cooler surfaces of the land and the flow channel.

The second is degraded polymer. Where flow is slow, in a corner of the channel or behind a restriction, PVC sits long enough to lose HCl and form a dark, cross linked layer that bonds to the steel.

The third is pigment and filler agglomerates. Titanium dioxide and calcium carbonate can collect in low velocity zones and build a hard deposit that is not removed by purging.

Each of the three responds differently to heat. Plate-out burns off at moderate temperature, degraded PVC needs higher temperature and leaves a carbon residue, and filler deposits barely respond at all and have to be removed mechanically.

Three methods and what they cost

A burn-out oven heats the die to 400 to 450 degrees Celsius in air and holds it until the organic material turns to ash, which is then brushed out. It is thorough and it is slow, because the die has to be heated and cooled slowly to avoid distortion.

A fluidised bed does the same job at 400 to 450 degrees but with far better heat transfer, because the aluminium oxide particles carry heat into the channels. Cleaning time drops from about eight hours to three.

Vacuum pyrolysis heats the die in a low oxygen atmosphere and draws the decomposition products away. It leaves less residue and does not oxidise the steel, at the cost of a more expensive plant and a batch process.

Solvent soaking and hand scraping are the cheap options. They work for plate-out and they do not work for degraded PVC, and hand scraping on a hardened land is how a die gets a burr.

Table 1 — Cleaning methods compared on time and on what they do to the die.

The land is the casualty

The die land is the short parallel section at the exit that sets the final surface and the final dimension. It is typically 0.8 to 2 mm long and it is polished to a mirror finish, because any mark on the land prints on every metre of profile.

Every thermal cycle oxidises the land slightly, and every polish to remove the oxide takes a few microns of steel with it. The chart on the left shows 4 microns lost after five burn-out cycles, 15 after fifteen and 30 after twenty five.

Thirty microns off a 1.2 mm land is 2.5 percent, which sounds trivial and is not. The land length controls the pressure drop and the memory of the extrudate, and a land that has been polished short runs differently from the one the die was designed around.

The visible consequence is usually die swell. A die that used to run a 12.0 mm profile at 18 metres per minute needs a higher screw speed to hit the same dimension, and the extra shear changes the surface.

Dirty die or worn die

The two look similar on the profile and have opposite fixes. A dirty die produces lines, a rough surface and a gradual loss of output. A worn die produces a dimension that has drifted and a surface that is still good.

The test is to clean the die and run it again. If the lines disappear, it was plate-out. If the dimension is still wrong after cleaning, the land has worn or been polished away and the die needs rework.

A second test is to look at where the defect sits. A line that stays in the same place on the profile is a die or a land defect. A line that moves is a flow or a temperature problem upstream.

The third is the output rate. A die that needs a higher screw speed each month to hold dimension is losing land, and the trend is more informative than any single measurement.

Table 2 — Telling a dirty die from a worn one.

Making cleaning less frequent

Start with the compound. Plate-out is a formulation event, and a compound that plates out on a hot die in eight hours will keep doing it. Lubricant balance and stabiliser level are worth reviewing with the supplier before buying a fluidised bed.

Then look at the die temperature. A die run 10 degrees hotter than it needs to be degrades material faster in the low velocity zones, and the deposits form sooner. The correct die temperature is the one that gives a good surface at the lowest heat history.

Then look at shutdown. A die left hot with material in it over a weekend is a die that will need cleaning on Monday. Purging with a purging compound before stopping is cheaper than any cleaning method.

And record it. A cleaning log with date, method, cycles and any dimensional check gives you the data to move from cleaning on a calendar to cleaning on condition.

Setting a cleaning interval

Clean on evidence, not on habit. The evidence is a rising screw speed at constant output, a surface defect that will not clear, or a pressure that has moved outside its band.

For a typical PVC profile die the interval lands somewhere between two and eight weeks depending on compound and hours. The number matters less than the fact that it is measured rather than assumed.

Keep at least one spare die per high runner. A die that has to be cleaned in a hurry gets cleaned badly, and a badly cleaned die loses land that cannot be put back.

And track land length. Measuring the land on a new die and again after every tenth cleaning turns an invisible slow loss into a maintenance decision that can be planned.