HDPE Sheet Extrusion: Thickness Control on a Wide Die

A wide die is a series of narrow dies

A sheet die is a manifold that distributes melt across the width and a lip that sets the gap. The lip is not one gap: it is a gap controlled by a row of adjuster bolts, and between two bolts the lip flexes. That is the whole geometry of thickness control, and it explains why the number of adjusters matters more than the width of the die.

Cross-direction thickness spread against adjuster spacing: 0.12 mm at 100 mm spacing, 0.07 at 75, 0.04 at 50 and 0.02 at 25. A die with adjusters every 25 mm can hold a 0.05 mm specification over a metre and a half. A die with adjusters every 100 mm cannot hold it over any width, because the lip between two bolts is uncontrolled.

This is why a cheap wide die is a bad purchase for a tolerance-critical job. The width is what the marketing sheet quotes, the adjuster count is what the process needs, and the two are independent numbers.

Adjuster spacing is the tolerance

The relationship is close to linear over the range that matters, and the reason is geometric rather than metallurgical. Between two adjusters the lip deflects under melt pressure, and the amount of deflection depends on the span between them. Halve the span and the deflection falls by roughly a factor of eight for the same load, which is why the improvement is faster than the spacing suggests at first.

In practice the improvement is limited by other things: the flatness of the lip itself, the temperature uniformity across the die, and the flow balance of the manifold. Below about 0.02 mm of cross-direction spread those become the dominant terms, and adding more adjusters stops helping.

The specification limit in the chart is 0.05 mm, and it is crossed between 50 and 75 mm of spacing. That is the number to look for when buying a die for a job that has a thickness tolerance: not the width, not the output, the adjuster pitch.

Width buys output, not accuracy

Output against sheet width: 320 kg/h at 1,000 mm, 470 at 1,500, 600 at 2,000 and 700 at 2,500. The curve flattens, because the extruder has a maximum output and the die can only use what arrives. Widening the die increases output until the extruder becomes the limit, and after that it just spreads the same melt over more width.

That flattening is a useful signal. If a line's output is not rising with width, the extruder is saturated and a wider die will buy nothing. If output is still rising steeply, the die is the limit and there is capacity to sell.

What width does not do is improve thickness accuracy. A wider die has a longer flow path in the manifold, more opportunity for temperature variation across the width, and more lip to deflect. In practice wide dies are harder to hold to tolerance, not easier, which is why the two numbers in the chart move in the same direction and mean different things.

HDPE specifics: die gap and drawdown

HDPE is a semi-crystalline material with a sharp melting point and a strong tendency to orient. It is extruded hot, at 200 to 230 C, and drawn down from the die gap to the finished thickness. The drawdown ratio is the die gap divided by the finished thickness, and it sets the amount of machine-direction orientation in the sheet.

A high drawdown ratio gives a sheet with good machine-direction strength and poor cross-direction strength, and with a strong tendency to shrink along the machine direction when it is reheated. A low drawdown ratio gives a more balanced sheet and a better surface, because the melt has less distance to travel in air before it reaches the rolls.

The practical range is 3:1 to 8:1. Below 3:1 the sheet is hard to control and the surface is rough; above 8:1 the orientation is severe and the sheet will not form predictably. For HDPE the drawdown also interacts with the die gap: a wide gap with high drawdown gives a better surface than a narrow gap with low drawdown, at the same finished thickness.

Table 1 — HDPE sheet line settings and what each one is actually controlling

Reading a thickness scan across the web

A single thickness measurement tells you almost nothing about a sheet. What matters is the profile across the width, and the profile along the length, and they have different causes.

A profile that is high in the middle and low at the edges is a die lip or manifold balance problem. A profile that is high at one edge and low at the other is a temperature gradient across the die, or a die that is not square to the rolls. A profile that wanders along the length is a line speed or a melt pump problem, not a die problem.

The measurement has to be taken the same way each time: a contact gauge on a flat section, or an online scanner, at a known distance from the edge. Measurements taken at the very edge are meaningless because the edge is where neck-in and trim happen.

Table 2 — Thickness defects in HDPE sheet and where they come from

What to specify

An HDPE sheet specification should describe the profile, not just a nominal thickness.

• Nominal thickness with a tolerance, and whether that tolerance applies to any point or to the average.
• Cross-direction spread, which is the number the adjuster spacing buys.
• Width, with the trimmed width and the edge condition.
• Drawdown ratio or the machine direction shrinkage on reheating, since that is what the thermoformer feels.
• Which face is the chill roll face, if the sheet will be formed or printed.
• Weld line or die line acceptance, with a limit on how visible a line can be.

The point that causes the most rejections is the difference between a thickness tolerance and a thickness spread. A sheet at 2.00 mm nominal that varies by 0.12 mm across the width may be within a plus or minus 0.15 mm tolerance and still be unusable for a formed part, because the thin band forms differently from the thick one.