Radius and Draft in Thermoforming: Two Numbers That Decide Everything

Why a corner thins

Thermoforming stretches a sheet over a shape. The sheet has a fixed amount of material, so wherever it stretches it gets thinner, and the amount of thinning is proportional to how far that region of the sheet travelled. A flat bottom barely moves. A deep side wall moves a lot. A sharp corner at the bottom of a deep wall moves further than anything else, because material from both the wall and the base has to feed into it.

That is the geometric argument, and it is unavoidable in any process that forms a sheet rather than filling a cavity. Injection moulding can put material exactly where it is needed; thermoforming can only redistribute what is already there. A design that asks for a uniform wall in a deep drawn part is asking for something the process cannot deliver.

The practical consequence is that the drawing has to specify a minimum wall at the corner rather than a nominal wall for the whole part. If the corner has to be 1.2 mm, the sheet has to be thick enough that the thinnest point after forming is 1.2 mm. Working backwards from the corner rather than forwards from the sheet is the only way to get a part that performs.

Plug assist changes the arithmetic but not the principle. A plug pushes material down into the cavity before the vacuum pulls it against the walls, which reduces the stretch in the base and moves material into the walls. It improves the distribution, and it does not eliminate the corner thinning. The corner is still the thinnest point, just less dramatically so.

Radius as a multiple of sheet thickness

The useful way to think about a corner radius is as a multiple of the sheet thickness, not as a number in millimetres. A three millimetre radius in a one millimetre sheet is a generous corner. The same radius in a four millimetre sheet is a sharp edge that will thin severely. The ratio is what the material sees.

The chart on the left plots the wall thickness retained at a corner against the radius to thickness ratio, for three draw ratios. At a radius of half the sheet thickness, a part drawn to 0.5 retains 70 percent of the sheet thickness in the corner. At a radius of three times the thickness, it retains 50 percent. A part drawn to 0.7 is worse throughout, dropping to 22 percent at the largest radius shown.

The design rule that follows is a minimum radius of one times the sheet thickness for a shallow draw, and two to three times for a draw ratio above 0.5. Below that, the corner is thin enough that it will crack in service or fail at the trimming station, and no amount of process tuning recovers the material that is not there.

Draw ratio is the other half of the equation and it is defined as depth divided by the smaller of the two opening dimensions. A ratio of 0.3 is shallow and forgiving. Above 0.6 the part needs plug assist, a higher forming temperature and a more generous radius. Above 0.8, most commodity sheet materials are at their limit and the design should be reconsidered rather than pushed.

Draft is not only about release

Draft is the angle on a vertical wall that lets the part slide off the mould. The minimum is usually quoted as one degree per side, and that is enough for a smooth aluminium tool with a polished surface. It is not enough for a textured tool, and it is not enough for a deep wall where the part grips over a long contact area.

Texture is the reason draft requirements escalate. A textured surface is a series of small undercuts, and the part has to be pulled off them. The rougher the texture, the more draft is needed: roughly one and a half degrees extra per 0.025 mm of texture depth is a working rule. A heavy leather grain on a deep wall can need five degrees, which changes the part's footprint and often surprises a designer who specified the texture late.

Draft also affects wall thickness. A drafted wall is not a parallel wall, so the formed wall at the top of a deep draw is thinner than at the bottom, on top of the thinning from stretching. A part designed with a nominal 1.5 mm wall and two degrees of draft will be measurably thinner at the rim, and if the rim is where the part is clamped or sealed, that is where the leak will be.

The forming method changes the requirement as well. Pressure forming pushes the sheet harder against the tool and reproduces detail better, but it also increases the friction on release. Vacuum forming with a good air assist is more forgiving. A tool designed for vacuum forming and then run on a pressure former will often stick, and the fix is draft rather than more release agent.

Radius, draft and the mould material

Aluminium tools are machined easily, which means a radius is cheap to add and cheap to change. That is the argument for prototyping in aluminium and confirming the corner radii before committing to a production tool. A radius that turns out to be too tight on an aluminium prototype can be opened up in an hour; on a steel or a cast tool it is a rework.

Epoxy and cast aluminium tools have a different constraint: they are cast or laid up against a pattern, so sharp internal corners are difficult to produce and difficult to polish. A design with a tight radius on a cast tool will have a rough corner, and a rough corner is a stress concentration on top of a thin wall. Specify a radius the toolmaking process can actually deliver.

Wood and pattern-board tools for prototypes are the least constrained and the least durable. They are the right place to test a radius, a draft and a texture combination before any money is spent on a production tool, and the cost of a prototype tool is usually less than the cost of one rework on a production one.

Table 1 — Radius and draft by draw ratio

Checking both on the drawing

The design review that catches most of these problems is short and mechanical. Take the sheet thickness, divide the smallest corner radius by it, and compare against the table. Then take the deepest draw, divide by the smaller opening dimension, and check that the radius rule for that ratio is met. Two calculations, five minutes, and they find the issues that would otherwise appear at the first trial.

The second check is on the vertical walls. Measure the draft on the drawing, subtract the allowance for the texture, and see what is left. If the answer is less than half a degree, the part will stick and the operator will compensate with release agent, which will then cause paint adhesion problems three months later.

The third check is the wall thickness at the rim and at the corner, calculated rather than assumed. Most CAD systems will not do this for a thermoformed part without a forming simulation, so it has to be done by hand from the draw ratio. A simple spreadsheet with the draw ratio and the radius ratio gives an estimate good enough to catch a design that cannot work.

The fourth check is the trim line. A drafted wall means the trim line moves as the part is trimmed, and a part with generous draft has a trim allowance that varies around the perimeter. If the trim is done in a steel rule die or a CNC fixture, the variation has to be inside the fixture's tolerance or the trim will wander.

Table 2 — Five symptoms, five causes, five fixes

When the design cannot change

Sometimes the radius is set by the product and the draft by the assembly, and the drawing is not negotiable. In that case the process has to absorb the constraint, and there are four levers: sheet thickness, forming temperature, plug assist and the tool surface.

Thicker sheet is the bluntest lever and the most reliable. If the corner needs 1.2 mm and the radius ratio gives 50 percent retention, the sheet has to start at 2.4 mm. The cost is material and cycle time, and it is often cheaper than a tool rework.

Higher forming temperature improves the distribution because the sheet stretches more uniformly before it cools. The limit is sag and, for some materials, degradation. Raising the temperature by ten degrees and slowing the form often converts a corner crack into an acceptable corner without touching the tool.

Plug assist is the third lever and the most powerful, because it puts material where it is needed rather than redistributing what arrives. A plug that is shaped to match the corner and timed to arrive just before the vacuum will thicken a thin corner substantially. Getting the plug shape and timing right is a trial and error exercise, and it is worth the trials.