Webbing in Thermoforming: When the Sheet Folds Onto Itself

What webbing actually is

Webbing is a fold. When a sheet is drawn into a deep cavity, the material at the corners has to travel further than the material at the flats, and if it cannot stretch enough it buckles and folds onto itself. The fold then gets pressed into the wall and cures there, leaving a stiff ridge that no amount of trimming removes.

It is often described as a vacuum problem because it appears at the corners where the vacuum is weakest. That diagnosis leads to more vacuum holes, more vacuum, and a bigger pump, none of which fix it. The vacuum is not what folds the sheet; the geometry is.

The reason corners are where it happens is simple. In a rectangular box, the corner is where two side walls meet, and the material there has to form both. Its path length is the diagonal, which is up to 1.41 times the path along a flat wall, so it runs out of material first.

That is why webbing is a design problem before it is a process problem. A corner radius large enough to spread the draw, or a chamfer that turns two walls into three shallower ones, removes most of the risk before the tool is cut.

The forming window is narrow

On a 1.2 mm HIPS sheet formed into a 100 mm deep tray, webbing appeared in a band of draw ratios between about 1.2 and 3.6, and the band moved with temperature. At 120 C it sat between 1.2 and 1.5; at 150 C between 2.3 and 2.9; at 180 C between 3.2 and 3.6.

Below the band the sheet tears, because it is too cold to stretch. Above the band it sags, because it is too hot to hold its own weight between the clamps. The usable window is the strip between those two failure modes, and it is only about 0.3 to 0.6 of a draw ratio wide.

The window moves upward with temperature, which is the useful part. A deeper draw needs a hotter sheet, and a hotter sheet needs better sag control. The two constraints trade against each other, and the trade is what makes deep-draw thermoforming difficult.

That is why the answer to webbing is often not 'form it hotter' but 'form it hotter and support it'. A plug assist, a pressure box or a billow gives the sheet support while it is hot enough to draw, and that widens the window from both sides.

Sag sets the clamp frame and the heater

Sag is the other half of the window. A 1.2 mm HIPS sheet held in a clamp frame sags 4 mm at 130 C, 17 at 150, 46 at 170 and 96 at 190 over a 600 mm span. Once it sags past about 40 mm it touches the heater or the mould before forming starts, and the part is scrap.

Sag is what limits the sheet temperature, and the sheet temperature is what allows the draw. So a deep draw on a large sheet is a narrow corridor: hot enough to stretch, cool enough not to sag, and the corridor closes as the sheet gets bigger.

The countermeasures are mechanical. A shorter heater-to-sheet distance reduces the time to sag. A clamp frame with more clamps, or a frame that moves down as the sheet sags, holds the sheet flatter. A sag sensor that drops the heater when the sheet reaches a set deflection is the automated version.

Zone heating is the other tool. A sheet that is hotter in the middle and cooler at the edges sags less overall and draws more where it needs to. On a large part with a deep centre, zone heating is worth more than any amount of vacuum.

Table 1 — Temperature against sag and the webbing band

Tooling changes that remove it

The first is corner radius. A corner radius of at least the wall height, and ideally half the cavity width, spreads the draw over more material and removes the sharp path length difference that causes the fold. A 5 mm corner on a 100 mm deep box will web; a 25 mm corner usually will not.

The second is draft. A cavity with 1 degree of draft forces the sheet to slide against the wall as it draws, which drags material from the flange and starves the corner. Three degrees or more lets the sheet move without friction, and on a deep part 5 degrees is normal.

The third is the plug. A plug assist pushes the sheet into the cavity before the vacuum comes on, and a plug with the right shape pre-stretches the corner material so it arrives with thickness to spare. A cold plug marks the sheet; a heated plug at 80 to 120 C does not.

The fourth is the venting pattern. Vacuum holes at the corner draw the sheet into the corner before the rest of the cavity has pulled, which concentrates the draw exactly where it cannot be afforded. Moving the holes away from the corners, and using a vent slot along the flats instead, changes the order in which the sheet touches the mould.

Process changes, in order of effect

The first is to slow the draw. A fast draw strains the sheet before it has time to distribute the stretch, so the corners take all of it. Halving the forming speed often removes a web that no amount of heat would fix.

The second is to use pressure instead of vacuum. A pressure box pushes the sheet into the cavity evenly from above, which is a different loading than pulling it from below. On deep draws, pressure forming routinely achieves draw ratios that vacuum cannot.

The third is a billow. Blowing the sheet up into a bubble before drawing pre-stretches it uniformly, and the bubble then collapses into the cavity with the material already thinned everywhere. It is the cheapest way to get more draw from a given sheet.

The fourth is a thicker sheet. A 1.5 mm sheet where 1.2 was specified draws 25 percent deeper before webbing, at the cost of material and cycle time. It is the last resort, and it is often the one that gets used because it needs no tooling change.

Table 2 — Fixes ranked by effect and cost

Diagnosing which one you have

A web and a wrinkle look similar and have different causes. A web is a fold that has been pressed flat and is stiff along its length; a wrinkle is loose material that has not been stretched. A web appears at corners and along the junction of two walls; a wrinkle appears wherever the sheet had excess material and no tension.

A web also has a thickness signature. Cut through it and the fold is roughly twice the wall thickness, because two layers have been pressed together. A wrinkle is single thickness and often has a void behind it.

The third thing to check is whether the web is on the plug side or the vacuum side. A web on the plug side comes from the plug pushing material ahead of it. A web on the vacuum side comes from the sheet being drawn into a corner that is too tight. The two have different fixes.

And check the sheet, not just the machine. A sheet with uneven gauge webs where the thin side is, because the thin side stretches first and the thick side has to fold. A gauge profile across the sheet is worth measuring before the tool is blamed.

Designing a part that will not web

The design rules are short. Keep the draw ratio under 2.5 for vacuum forming and under 3.5 for pressure forming. Keep corner radii at least equal to the depth. Keep draft above 3 degrees. Avoid rectangular corners where a radius will do.

Where a deep corner is unavoidable, add a gusset or a radius that turns the corner into a series of shallower curves. The material does not care about the shape, only about the path length, and a gusset shortens the path at the point where it is longest.

The last rule is about the flange. A wide flange holds the sheet and gives it somewhere to slide from. A narrow flange starves the draw and webs the corner. Three times the sheet thickness is a reasonable minimum, and more is better on a deep part.