HDPE Thermoforming: A 20 °C Window, Heavy Sag and Shrinkage That Continues After Demoulding

Why HDPE is the awkward one

Polyethylene is the largest thermoforming material by volume, and almost all of that volume is thin-gauge packaging where the sheet is 0.3 to 1.0 mm and the part is shallow. Move to thick-gauge forming of HDPE and the material becomes difficult quickly.

Three properties cause it. HDPE is semi-crystalline with a sharp melting range, so it goes from stiff to sagging over about 15 °C rather than softening gradually. It has low melt strength, so a hot sheet will not support its own weight. And it shrinks 1.5 to 2.8 percent on cooling, which continues after the part leaves the mould.

The result is a material that forms well in a narrow band and badly everywhere else, and a part that changes size after it has been measured.

Table 1 — Polyethylene grades for thermoforming. Higher density means more stiffness and less forgiveness.

The 20 °C window

HDPE forms between 138 and 172 °C at the surface, and the practical band for thick-gauge work is 145 to 165 °C. Below 145 the sheet is stiff and springs back after demoulding. Above 165 it sags so much that the geometry has changed before the mould arrives.

The window narrows as the sheet gets thicker, because a thick sheet has a larger thermal gradient and the surface reaches forming temperature while the core is still cold. A 6 mm HDPE sheet needs a long soak at moderate temperature rather than a short burst at high temperature.

Two-sided heating is essential. HDPE has low thermal conductivity, and a single-sided oven produces a sheet whose hot face is at 165 °C and whose cold face is at 130. The hot face sags and the cold face resists, and the part that results has a wall thickness gradient that follows the temperature gradient.

Table 2 — HDPE thick-gauge forming parameters. Mould temperature is higher than for thin-gauge work because it controls shrinkage as well as surface.

Sag and webbing

HDPE sags more than any other common thermoforming material at the same temperature relative to its melting point. A 3 mm sheet at 160 °C will sag 40 to 60 mm across a 600 mm span, and once it has sagged the sheet is no longer uniformly hot.

Webbing follows from sag. Where the sheet has sagged between two deep features, the material there is thinner and hotter, so it stretches first and folds onto itself. The fold cools into a permanent crease that cannot be removed and usually has to be trimmed out.

Three levers control it, in order of effectiveness. Reduce the unsupported span by using a plug or a billow. Profile the heaters so the web region is cooler. Reduce the overall temperature and accept a slower cycle. Reducing temperature alone produces a part that does not form, which is why it is the last resort.

Shrinkage that continues after demoulding

A formed HDPE part shrinks 1.5 to 2.8 percent in the first 24 hours, and about 80 percent of that happens in the first four. A part measured immediately after demoulding is larger than the part that ships.

The shrinkage is anisotropic and follows the draw direction. Material that was stretched along the draw shrinks more when it relaxes, so a deep part is shorter in the draw direction than across it by 0.3 to 0.6 percentage points.

The mould must be cut oversize to compensate, and the compensation is specific to the material, the draw ratio and the cooling. A tool cut for 2 percent will produce parts that are right at one draw ratio and wrong at another. Where a part has features at different draw depths, no single shrinkage factor is correct and the design must allow for it.

The practical answer is to measure. Form three parts, measure them at 1 hour, 4 hours and 24 hours, and cut the tool from the 24-hour numbers. A toolmaker who asks for a shrinkage figure should be given three numbers with the measurement times attached.

Moulds and cooling

HDPE moulds are aluminium for most work. Mould temperature is 40 to 75 °C, warmer than for thin-gauge work because a warm mould reduces the thermal shock that produces frozen stress and post-mould distortion.

Cooling must be uniform. HDPE has a high heat of crystallisation, so a thick section releases heat as it crystallises and a mould that removes heat unevenly produces differential crystallinity and therefore differential shrinkage. The part warps in the direction of the cooler region.

Vacuum holes must be large enough. HDPE at forming temperature is viscous and will bridge over a 0.5 mm hole without pulling into it, leaving a dimple or an unformed area. Holes of 0.8 to 1.2 mm at 25 mm spacing are typical for thick-gauge HDPE.

Defects

Table 3 — HDPE thermoforming defects. Two of them are consequences of the material's crystallinity, which is why they are not fixable by tuning alone.