Mould Texture: Grain, Depth and How It Transfers to Sheet

Texture is a depth, not a look

A texture sample is judged by eye, and the eye is a poor judge of depth. Two grains that look identical on a colour chip can differ by a factor of three in peak-to-valley depth, and depth is what decides whether the texture survives forming, whether it releases from the tool and whether it hides a flow mark.

The industry convention is to describe grain by a depth range in micrometres, or by a named standard such as a VDI or MT scale, and both are attempts to put a number on the same thing. What matters for a vacuum forming tool is the peak-to-valley depth and the draft angle of the grain walls, because those two decide how much of the texture reaches the part and how easily it comes off the tool.

A grain that is deep but has vertical walls will hold the part. A grain of the same depth with walls at 10 to 15 degrees will release cleanly. That is why two tools cut to the same depth can have completely different release behaviour, and why depth alone is not a specification.

How grain is put into a tool

There are three routes, and they cost very differently. The cheapest is to use a grained aluminium plate and machine the pocket into it, which gives a uniform grain on the pocket floor but a cut edge where the machining crosses it. The most flexible is to etch the tool after machining, which puts grain anywhere the resist can be applied. The most expensive is to hand-texture, which is used for leather reproductions and grain that has to match a physical sample.

Etching is a chemical process and it is not perfectly uniform. Grain depth varies with local bath agitation and with the alloy, and on a large tool the variation across the face can be 15 to 20 percent. That variation shows up in the part as a texture that is deeper in one corner, and it is worth measuring on the tool before blaming the forming.

Machining a grain into a tool directly is possible with a small ball-nose cutter and a long program, and it gives a repeatable, defined pattern. It is used for geometric textures such as a diamond or a line pattern, where the geometry can be described exactly. For a leather grain, hand texturing or etching remains the practical route.

Table 1 — How the grain gets into the tool, and what it costs in consistency

What the sheet does with it

The sheet does not copy the tool. It stretches over it, and stretching thins the wall and flattens the detail. A 50 micrometre grain in the tool comes back at about 46 micrometres in the part, because the local stretch over such a shallow feature is small. A 200 micrometre grain comes back at about 152, because the sheet has to bridge a deeper valley and the walls of that valley are where the thinning happens.

The transfer ratio therefore falls as the grain gets deeper, from about 92 percent at 50 micrometres to about 76 percent at 200. This is the opposite of the intuition that a deeper grain gives a more pronounced texture in absolute terms, and it is why a customer who asks for a deeper grain to get a stronger effect often gets a slightly stronger effect and a much thinner wall in the grain valleys.

The second factor is sheet temperature. A hot sheet is soft and copies detail well; a cool sheet is stiff and bridges over it. At 130 C the fidelity is close to 96 percent of what the tool can give; at 190 C it is down to 45, because the sheet is soft enough to sag into the vacuum holes and stretch away from the grain.

Draw ratio and the grain that disappears

Grain survives where the sheet does not stretch, and disappears where it does. On a shallow tray, the base stretches very little and keeps its grain; the corners stretch most and lose it. The result is a part with a textured base and polished corners, which is a common and unwelcome surprise on a first-off sample.

The fix is usually to accept the variation and design around it, or to reduce the draw. Increasing the grain depth to compensate does not work well, because the corners are stretching more than the base by a factor that no depth can match, and the deeper grain costs wall thickness everywhere.

Where a uniform texture matters, the tool is often textured only where the draw is low, and the high-draw regions are left smooth or given a shallower grain. That is a deliberate design decision made on the tool, and it is much cheaper than trying to fix it with process settings.

Textured tools need different venting

A textured surface is a surface with thousands of small closed volumes in it. Air trapped between the sheet and the grain has nowhere to go except along the grain channels to a vent, and if the grain is fine and the vents are far apart, the air does not get out before the sheet sets.

The symptom is a texture that looks right but has a sheen in patches, or a part with visible air entrapment in the deepest grain areas. The cause is not the vacuum pump; it is the path length from the middle of a grain field to the nearest vent.

The remedy is more vents, smaller and more evenly distributed, and a slower evacuation so the air has time to travel. On a heavily textured tool the vent spacing is often halved compared with a smooth tool of the same size, which is a tool cost that is easy to forget when the texture is added late in the project.

Table 2 — Vent spacing has to follow the grain

Specifying texture so it survives

Specify the depth in micrometres, the draft of the grain walls and the area where the texture is required. A sample chip is not a specification, because a chip is flat and unstretched and the part is neither.

Say which surface is textured and which is not, and expect the transition to be visible. A tool that is textured on one face and polished on the other will show a line at the boundary, and that line moves with the draw.

Finally, agree the acceptance method. Grain is usually judged by comparison with a physical master under a defined light, and that master should be a formed part, not a flat chip. A master made from the actual tool, formed at production conditions, is the only reference that settles the argument.