Plug Assist in Thin-Gauge Thermoforming: Prestretch and Wall Profile

What vacuum alone does to a wall

Vacuum forming pulls the sheet into the mould. On a shallow part that works, because the sheet only has to travel a short distance and the stretch is spread over a large area. On a deep part it fails, and the failure is arithmetic.

Take a cup 100 mm deep and 80 mm across, formed from a 1.0 mm sheet with vacuum alone. The base of the cup is the last part of the sheet to enter the mould and the first part to be stretched, and it is stretched against nothing. By the time the base reaches the bottom of the cavity, the material that started at 1.0 mm has been drawn down to 0.15 to 0.25 mm.

That is a 75 to 85% reduction, and it produces a part with a base so thin it flexes under its own contents, a barrier layer that is no longer continuous, and a bottom corner that cracks on demould.

Table 1 — Wall distribution with and without plug assist, 100 mm cup from 1.0 mm sheet. The range is what matters, not the absolute values.

The last row is the real argument. A wall that varies from 0.15 to 1.0 mm is a part with a weak point that decides everything else: the load it can carry, the drop height it survives, the barrier it provides, the weight it can be specified at. A wall that varies from 0.45 to 1.0 mm is a part that can be engineered.

How plug assist changes the profile

A plug is a male tool that descends into the sheet before the vacuum is applied. It does two things at once, and they act in opposite directions, which is why the setup is a balance rather than a maximum.

It pre-stretches mechanically. The plug pushes the centre of the sheet down into the cavity, so the material that will become the base is stretched against the plug face rather than against air. Because the plug is a solid surface, the stretch is distributed over the whole contact area instead of concentrating at the base corner.

It chills the contact area. The plug is cooler than the sheet, so the material that touches it solidifies slightly and resists further thinning. That is what stops the base from drawing down to nothing — but it also means the contact area ends up thinner than the material immediately around it, because the chilled material cannot flow to relieve the stretch.

The result is a characteristic thickness profile: thin at the point of plug contact, thicker in a ring around it, then thinning again down the side wall. A well-set plug makes that profile gentle. A badly set one makes it a step.

Plug material: the four options

Plug material is chosen for its thermal conductivity, its machinability and its friction against hot plastic. Those three properties point at four different answers.

Table 2 — Plug materials. The conductivity column explains why aluminium only works when it is heated.

Synthetic material — an epoxy or polyurethane loaded with hollow glass microspheres — is the default for thin-gauge work. Its low conductivity means it does not chill the sheet on contact, it machines like hardwood, and it can be shaped to almost any profile. The limitation is that it is brittle at thin sections, so plug edges chip and the chips cannot be filled.

Hardwood is the cheapest option and it works well for a trial. It absorbs moisture from the air and from the sheet, which changes its dimensions by a few tenths of a millimetre over a week, and that is enough to change the wall profile. A wooden plug is a prototype tool, not a production one.

Aluminium is the opposite problem. Its conductivity is 500 times that of the synthetic material, so an unheated aluminium plug chills the sheet on contact and produces a hard, thin, glossy patch that will not stretch. An aluminium plug has to be heated to within 20 to 40 °C of the sheet temperature to behave, and at that point it is a hot tool with all the plumbing that implies.

PTFE and PTFE-coated plugs are used where the material sticks. PET at forming temperature is tacky, and a bare plug will drag and leave marks. A PTFE surface eliminates that, at the cost of a soft surface that wears and needs periodic replacement.

Plug temperature is the real control

Most plug problems are temperature problems, and the direction of the error decides the defect.

Table 3 — Plug temperature against result. "Same as sheet" is a real technique for surface-critical parts, at the cost of base thickness.

The counter-intuitive row is the last one. A plug held at the same temperature as the sheet does not chill anything, so the material flows freely around it and the base ends up thinner than with a warm plug. What it buys is surface quality: no chill marks, no drag, no matte patch where the plug touched. For a cosmetic part that is the right trade; for a structural tray it is not.

The practical target for most thin-gauge work is 60 to 100 °C, achieved by a cartridge heater in the plug body with a thermocouple. A plug without temperature control is a plug running at whatever the shop is, which in winter is 18 °C and in summer is 35 °C, and the wall profile changes with the season.

Geometry: size, radius, shape

Plug geometry is set by three numbers, and each has a range rather than a value.

Clearance to the cavity wall. The plug has to be smaller than the cavity by enough to let the sheet pass, which means the sheet thickness plus a margin. Too little and the sheet is trapped and tears; too much and the plug does not prestretch the side wall at all. For a 1 mm sheet the radial clearance is typically 1.5 to 2.5 mm per side.

Base clearance. The plug must not touch the cavity base in normal operation, or it will punch through. A gap of 0.8 to 1.0 times the sheet thickness at the bottom of the stroke is standard, and it is set by the machine's stroke stop rather than by the plug length.

Top radius. A sharp plug top concentrates the stretch and produces a ring of very thin material at the contact edge. The radius should be at least two to four times the sheet thickness, and more for a deep draw.

Table 4 — Plug geometry. The area ratio is the parameter that best predicts whether the plug is doing its job.

The area ratio is worth calculating because it is the single number that describes how much prestretch the plug provides. A plug whose cross-section is 60% of the cavity area will prestretch the sheet by a factor of about 1.7 before the vacuum does anything. At 85% the factor is 1.2, and the plug is mostly acting as a chill surface rather than a stretcher.

Setting up a plug, step by step

A plug is not a part you fit and forget. It is a tool that has to be tuned against the wall thickness you want, and the tuning is empirical because the material behaviour is not predictable enough to calculate.

Start with the geometry from Table 4. Cut the plug to the nominal dimensions, machine the top radius generously, and leave the length slightly long so it can be shortened.

Set the stroke so the plug stops short of the base. Bring the plug down by hand with no sheet, measure the gap, and set the machine stop to leave 0.8 to 1.0 times the sheet thickness.

Run one shot and cut the part. Section it through the centre and measure the wall at the base centre, the base corner, and three points up the side wall. That profile is the only data that matters.

Adjust in one direction at a time. A thin base centre means the plug is too cold or too small; a thick base with a thin corner means the plug is too large or the top radius is too sharp. Change one thing, run again, measure again.

Record the settings that worked. Plug temperature, stroke, and the resulting wall profile belong in the tool record. The next person to run the job will not rediscover them.

Which materials need a plug

Table 5 — Draw ratio at which each material needs plug assist. PP is the earliest by a clear margin.

The thresholds are rules of thumb and they assume a warm plug. A cold plug effectively raises the threshold because it stiffens the sheet on contact, which is why a cold plug on a PP part at 0.7 to 1 produces a part that looks plugged but still has a thin base.

Defects and what they point at

Table 6 — Plug defects. The last row is the most common and the cheapest to fix.

Webbing is the one that a plug sometimes fails to solve. Webbing happens when the sheet folds between two deep cavities before it can be drawn into either, and it is caused by the sheet being too hot, too slack, or the cavity spacing being too tight. A plug helps because it takes up the slack, but if the web area is too narrow no plug will fix it and the cavity layout has to change.