ABS Sheet Extrusion: Gloss, Grain and the Polishing Stack

The third roll is the one that matters

An ABS sheet line ends in a three-roll polishing stack. The sheet leaves the die and passes over the top roll, under the middle roll and over the bottom roll, and by the time it leaves it is solid. Each roll does a different job, and the one that decides the surface is the one that touches the sheet while it is still soft enough to take a finish.

The top roll is usually chrome and polished, and it sets the face that will be seen. The middle roll is often rubber-covered, and it can carry a textured sleeve that embosses a grain. The bottom roll pulls and cools. Change which face runs against which roll and the same sheet comes out looking like two different products.

Roll temperature matters more than most settings on the line, because ABS is amorphous and its surface freezes over a narrow range. A roll at 20 C chills the skin almost instantly, and the skin that forms is smooth and glossy. A roll at 100 C lets the skin stay mobile for longer, and the surface that forms is duller and slightly hazy.

Gloss comes from the roll, grain comes from the sleeve

Gloss against chill roll temperature: 92 units at 20 C, 86 at 40, 72 at 60, 58 at 80 and 44 at 100. The relationship is monotonic and it is steep in the middle, which is why a line running at 60 C is the most sensitive place to be: a 10 C drift moves the gloss by seven points.

Grain is a separate mechanism. A textured sleeve presses a pattern into the sheet while it is above its softening point, and the depth of that pattern depends on the sleeve, the nip pressure and the temperature. Gloss and grain are not independent: a deeply grained sheet reads as matte even if the resin would have produced a glossy surface, because the texture scatters light.

That is why a grained ABS sheet can be sold as matte without any change to the roll temperature. The grain is doing the work, and the roll temperature is only setting the base gloss underneath it. When a customer asks for a specific gloss on a grained sheet, the number is really a statement about grain depth.

Why the draw eats the grain

The grain you buy is not the grain the part keeps. Forming stretches the sheet, and stretching a textured surface flattens the texture. Grain depth retained against draw ratio: 100 percent at 1.0, 88 at 1.5, 72 at 2.0, 55 at 2.5 and 40 at 3.0.

The relationship is worse than linear at high draw because the stretch is not uniform. The area over a male plug thins most, and that is exactly where the grain disappears first. A part with a 2.5 draw ratio will show a glossy, grain-free patch on the top of the form and a fully grained surface on the flange, from the same sheet.

There are three ways to deal with it, and only one of them is a sheet decision. Deepen the grain so that 40 percent of a deep grain is still visible. Reduce the draw ratio by changing the part. Or accept the patch and design it in, which is what most appliance and automotive interior parts do, because the eye reads a consistent gradient as normal and a sharp boundary as a defect.

Cap sheet and the cheaper route to gloss

A glossy ABS sheet can be made two ways. The whole sheet can be polished on a chrome roll, or a thin cap layer of high-gloss ABS can be coextruded over a cheaper, recycled or foamed core. The cap route gives a better gloss than polishing, because the cap resin can be formulated for surface and the core for cost.

The cap layer is typically 50 to 150 microns. It carries the surface, it carries the colour, and it is the only part of the sheet that has to be UV-stable if the part goes outdoors. The core can then be a lower grade, which is where the saving is.

The catch is that a cap layer is a thin skin over a different material, and the two have different shrinkage. A cap that is too thin will show the core's texture through it; a cap that is too thick costs more than it saves. The practical window is where the cap is thick enough to be opaque and thin enough not to delaminate on a deep draw.

Table 1 — Polishing stack arrangements and the surface each one produces

Reading a surface defect

Surface complaints on ABS sheet are almost always one of six things, and five of them are on the stack rather than in the resin.

Table 2 — ABS sheet surface defects and the setting behind each

The banding defect is the most diagnostic. A roll with an internal water channel partly blocked runs cool in a stripe, and the sheet comes off glossy in that stripe and dull either side of it. It looks like a resin problem and it is a plumbing problem, and it will persist through three resin changes before anyone puts a thermocouple on the roll.

What to specify

An ABS sheet surface specification has to describe the part, not the sheet, because the two are not the same surface.

• Gloss target with a tolerance, measured at 60 degrees, and on which face.
• Grain depth or a grain reference sample, with the depth the sleeve actually produces.
• The draw ratio of the part, because it predicts how much grain will be left.
• Whether the sheet is capped, and the cap thickness if it is.
• Colour under a stated light source, since a grained surface shifts colour appearance.
• Which face is the polished face and which face goes out on the part.

Then agree the acceptance sample on a formed part rather than on a flat sheet. A flat sheet at 88 gloss that forms at a 2.5 draw ratio gives a part at roughly 55 percent grain, and if the customer's reference was the flat sheet, the part will be rejected for a property the sheet never had.