Form-Cut-Stack: How an Inline Thin-Gauge Line Actually Runs

Why thin gauge cannot be trimmed offline

Heavy-gauge thermoforming and thin-gauge thermoforming are different industries that happen to share a polymer. A heavy-gauge shop forms a part, takes it off the machine, and trims it on a CNC router or a saw. A thin-gauge shop does all of that inside one machine, in one cycle, and never touches the part by hand.

The reason is arithmetic. A thin-gauge food container weighs 8 to 20 g and sells for a few cents. At that price there is no labour content available at all: the part has to be formed, cut, counted and stacked without an operator touching it. That requirement dictates the entire machine architecture.

Table 1 — Thin gauge against heavy gauge. Every difference follows from the part weight and the volume.

The in-mould trim is the defining feature. Because the part is cut while it is still located on the mould, its dimensions are set by the tool rather than by a handling operation, and the tolerance is better than an offline trim can achieve. That is also why thin-gauge parts hold ±0.3 mm on the trim outline where a heavy-gauge part holds ±1 mm.

The four stations

An inline thin-gauge machine has four stations and a chain that carries the sheet through all of them. The sheet is gripped at the edges by pins or clamps and indexed forward one station at a time.

Table 2 — The four stations. Heating is the largest share of the cycle and the one with the most room for improvement.

The stations operate simultaneously on different sections of the sheet, which is why the machine's output is set by the slowest station rather than by the sum. A machine with a 20 second heat and a 4 second form runs at 20 seconds per index, not 24 — but only if the heat station is long enough to hold a whole index of sheet.

Heating: the station that sets the cycle

Heating takes the largest share of the cycle because it is the slowest process. Heat has to conduct through the sheet, and plastic conducts heat about a thousand times worse than aluminium.

Table 3 — Heat times for common thin-gauge thicknesses. The times roughly double with each doubling of thickness, because heat has to conduct through.

Thin-gauge ovens heat from both sides, which halves the conduction distance and doubles the rate. They are also zoned: a machine might have 8 to 20 independently controlled zones across the width and along the length, because the sheet loses heat at its edges and the oven has to compensate.

The single most common heating fault is not a wrong temperature but a wrong distance. A heater that has drifted 5 mm closer to the sheet will overheat one zone, and the symptom is a band of parts that are hazy or stuck while the rest of the sheet is fine. Heater distance is a maintenance item, not a setting.

Forming: vacuum, pressure and plug in one stroke

The forming station on an inline machine does three things in a fraction of a second, and the order matters.

The plug descends first and prestretches the sheet into the cavity. Vacuum is applied at or just before the plug reaches its stroke, pulling the sheet against the mould walls. Pressure may then be applied from above to push the sheet into fine detail and to increase the definition of the mould surface.

The whole sequence is over in 0.3 to 1.5 seconds, and the timing between the three is what separates a good part from a webbed one.

Table 4 — Forming parameters. The plug-leads-vacuum timing is the one that is hardest to diagnose and easiest to fix.

The vacuum system on a thin-gauge machine is not a small pump and a hose. It is a receiver tank, a large bore valve, and a pump sized for the volume of all cavities at once, because the vacuum has to reach −0.7 bar in a fraction of a second across a 600 mm wide sheet. A machine with an undersized vacuum system will form slowly, and the parts will be hazy at the corners where the sheet cooled before it reached the mould.

Cutting in the mould

The trim station is where thin-gauge parts get their dimensional accuracy, and it is the station most often compromised to save tooling cost.

There are two cutting technologies. A steel rule die is a plywood or aluminium plate with a sharpened steel blade bent into the part outline; it is cheap and it cuts against a hardened pad. A matched metal die is a male punch and a female cavity machined to the part shape; it costs several times more and it holds a much tighter tolerance for a much longer life.

Table 5 — Trim technologies. The clearance between punch and die is 5 to 10% of sheet thickness, and it is what produces a clean edge rather than a torn one.

The clearance is the number that decides edge quality. For a 0.5 mm sheet the punch-to-die clearance is 0.025 to 0.05 mm per side. Too tight and the die wears fast and the edge fractures; too loose and the edge is torn and stringy. The correct clearance produces an edge with about a third of the thickness sheared cleanly and the rest fractured, which looks like a clean cut.

Cutting temperature matters as much as clearance. PP and PET cut cleanly when they are cold and tear when they are warm, so the trim station is usually positioned after a cooling dwell, and on fast machines with a cooling plate. A part trimmed while still soft has a rounded, drawn edge that will not sit flat.

Stacking and counting

The stacker is the least glamorous station and the one that decides whether the machine can run unattended.

Parts are picked off the web by vacuum cups, counted, and stacked in a magazine. The requirements are that the parts land square, that they do not stick to each other, and that the count is exact so the case packer downstream can trust it.

Sticking is the common problem. Two warm PP parts pressed together will fuse at the contact points, and the stack will not separate at the customer's line. The countermeasures are to cool the part before stacking, to emboss a pattern on the non-visible face, and to use an anti-static ioniser so that thin parts do not cling.

Counting is done by a photo-eye or by counting stacker strokes, and it drifts when a part fails to pick and the machine does not notice. A stacker without a part-present sensor will eventually ship a short case, and the customer will find it.

Where the seconds go, and where to find them

A 6 second cycle on an 8-cavity tool is 4,800 parts an hour. Getting from 6.5 seconds to 6.0 is an 8% capacity increase with no capital cost, which is why thin-gauge shops spend their improvement effort on seconds rather than on machines.

Table 6 — Cycle breakdown for a typical thin-gauge line. Cooling and heating together are over half the cycle.

Cooling is where the easy seconds are, because it is usually limited by water flow rather than by physics. A mould with a blocked cooling channel, or with a circuit that runs in series through eight cavities, will be hot at the far end and the dwell has to be set for the hottest cavity. Parallel circuits and flow meters pay for themselves quickly.

What stops a line

Inline machines stop for a small number of reasons, and they are worth knowing because each has a specific signature.

Web breaks. The trimmed web is thin and weak, and it breaks when the trim is dull or the tension is wrong. The machine stops, the operator re-threads, and the downtime is 5 to 20 minutes.

Sheet jamming in the oven. A sheet that sags too far touches the heaters, sticks, and tears. This is a temperature or a support problem and it usually happens on a grade change.

Plug or cavity damage. A part that fails to eject stays in the cavity, the next shot crushes it, and the mould is damaged. Cavity-present sensors are the only real protection.

Stacker jam. A part lands crooked, the stack leans, and the magazine overfills. Usually a vacuum cup or an alignment guide.

Heater failure. One zone dies, and a band of parts is under-formed. The operator sees the scrap before the alarm.