Plug Materials: Felt, Syntactic Foam, POM and Aluminium Compared

What a plug is for

A plug is a mechanical pre-stretch. In a deep draw, the sheet would otherwise be pulled into the cavity by vacuum alone, and the base would touch the cold tool first and freeze before it had stretched. The plug pushes the sheet down before the vacuum is applied, so the material is distributed across the part instead of being consumed at the base.

The plug does not make the wall uniform. It moves the thinning from the base to the sidewalls, which is usually where a part can afford it. A plug that is too small does nothing; a plug that is too large bottoms out on the tool and marks the part.

The material the plug is made from matters because the plug is in contact with hot sheet for a fraction of a second, and in that fraction it either takes heat out of the sheet or leaves it there. That thermal difference is what shows up as wall distribution.

The material decides how much wall is left

Measured as wall remaining at the base of a deep rectangular draw, the four common materials separate clearly: felt leaves 38 percent, syntactic foam 33, POM 27 and aluminium 24. The order follows thermal conductivity almost exactly, and that is not a coincidence.

Aluminium conducts heat away from the sheet fast. The sheet touching the plug cools, stiffens, and stops stretching at that point. The material that has already been pushed to the sidewalls continues to stretch, so the base ends up thin. Felt conducts almost nothing, so the sheet stays soft against the plug and keeps stretching there, which leaves more wall at the base.

The second factor is friction. Felt has a high friction against hot sheet, so it grips and pulls material down with it, which is a mechanical pre-stretch in addition to the thermal one. Aluminium is smooth and slips, so it pushes rather than pulls. Both effects point the same way, which is why the gap between felt and aluminium is so large.

What each material marks

Felt leaves a matt, slightly textured impression where it touches, and it sheds fibres. On a part with a textured or matte surface this is invisible; on a glossy part it is a defect. Felt also compresses over time, so a plug that worked when new changes its effective size after a few thousand cycles.

Syntactic foam is the compromise. It conducts a little more than felt, marks less, and holds its shape better. It is the default choice for a deep draw on a part that is visible but not high gloss.

POM is smooth, low friction and machines cleanly. It marks less than foam and much less than felt, and it is used where the part surface matters. It conducts more than foam, so it costs wall, and it is usually made slightly smaller to compensate.

Aluminium is the most durable and the most thermally aggressive. It is used where the draw is shallow enough that wall distribution is not critical, or where the part count justifies a hard tool and the surface has to be clean.

Table 1 — The four materials and what each one costs

Plug temperature is a second control

A warm plug pre-stretches further. Raising the plug surface from 20 C to 140 C takes the pre-stretch from about 8 mm to about 25 mm on the same stroke, because the sheet against a warm plug stays soft and keeps flowing. That is a useful adjustment when the plug geometry is fixed and the draw needs more material movement.

The limit is marking. Above about 110 C the sheet can stick to the plug, and the mark that results is a glossy patch that cannot be removed. Warm plugs work best with felt and foam, which are poor conductors and stay warm without a heater; metal plugs need a controlled heater and a release surface.

Temperature also changes the friction. A hot plug with a tacky surface grips more, which increases the mechanical pre-stretch and can pull the sheet off the clamp frame if the draw is deep. That shows up as a part with a pulled edge rather than a thin base.

Choosing a plug for the part

Start with the draw ratio. Below about 1.5:1 a plug may not be needed at all. Between 1.5 and 2.5:1, foam or POM is usually right. Above 2.5:1 the base thinning becomes the limiting feature and felt is often the only material that leaves enough wall.

Then look at the surface. A glossy visible part rules out felt and usually rules out foam. A textured part tolerates both. A part that will be printed or laminated tolerates almost anything, because the surface is covered later.

Then look at the volume. Felt and foam wear and have to be replaced; a worn felt plug changes the wall distribution, which means the process drifts. On a long run, a POM plug that costs more initially can be cheaper than replacing felt every few thousand shots.

Table 2 — A starting point, not a rule

Setting up a plug

The plug has to be centred and parallel to the tool, and it has to stop at a defined depth. A plug that stops too high does nothing; one that stops too low bottoms out and marks. The stroke is usually set so the plug stops 2 to 5 mm above the cavity floor, and that number is recorded, not estimated.

The plug also has to be slightly smaller than the cavity, by an amount that depends on the sheet thickness and the draw. Too close and the sheet is trapped and thins at the plug edge; too far and the plug does not pre-stretch the corners, which is where the wall is usually thinnest anyway.

Then verify by sectioning. Measure wall thickness at the base, at the corner and at the rim on a cut part, and compare with the target. A plug setup that is verified only by looking at the part will pass parts that are 30 percent thin at the corner.