How to polish acrylic: flame, buffing or diamond

The three methods at a glance

Method 1: sanding and buffing

This is the method for a home workshop, a single part or a repair. It needs patience, not machines.

1. Remove the saw marks. Scrape or file the edge flat, or start with a coarse wet-and-dry paper.
2. Wet sand in steps. Work through finer grits, for example 320, then 400, then 600, with plenty of water. Sand until the marks from the grit before are gone, then move on. For the clearest result, continue to 800 or 1200.
3. Buff. Use a soft cotton wheel with a cutting compound to take out the sanding marks, then a finishing compound for the shine. By hand, a plastic polish on a soft cloth does the same job more slowly.

What can go wrong: pressing too hard on the buffing wheel heats the edge. It can melt, smear or build up stress. Keep the part moving and the pressure light. The edge always ends up a little rounded, which is fine for most parts but not where a sharp, square edge is wanted.

Method 2: flame polishing

In flame polishing, a small, very hot flame from a hydrogen-oxygen torch is passed along the edge. The surface melts for a moment and flows into a glossy skin. It is fast, it follows curves and inside corners, and it needs no tool contact.

The edge must be prepared first. The flame does not remove saw marks; it only glosses what is there. A cleanly machined or scraped edge polishes well, a rough sawn edge does not.

What can go wrong:

• Stress and crazing. The sudden heat puts stress into the edge. Later, contact with alcohol, glass cleaner, solvents or glue can open a network of fine cracks, called crazing. Sheet makers recommend annealing parts after flame polishing to reduce this.
• Gluing and printing. A flame-polished edge should not be solvent cemented, for the same reason.
• Overheating. Moving too slowly burns, bubbles or whitens the edge. If one pass is not enough, let the part cool before the next.
• Shape. The edge rounds over and the corners soften. On thick sheets the surface often looks slightly wavy.

Method 3: diamond polishing

Diamond polishing is cutting, not melting and not rubbing. On an edge polishing machine, the sheet is clamped and its edge is fed past a spinning cutter head. Pre-cutters take off the saw marks, and a finishing diamond takes a last, very fine cut. The edge leaves the machine flat, square and clear, in one pass.

The finishing diamond is a single crystal: natural diamond or MCD, a grown single crystal. A single crystal can be polished to a far sharper and smoother edge than any other cutting material, and that edge is copied onto the acrylic. This is why the result is clear without any polishing afterward. For how the two diamonds differ, see our guide to natural diamond and MCD.

Why workshops choose it:

• The same edge every time. The result does not depend on the operator's hand.
• Square and flat. Corners stay sharp, which matters for display cases, blocks, awards and furniture.
• Little heat. The edge is cut, so it carries far less stress than a flame-polished one.
• Thick sheets and stacks. A thick block polishes as evenly as a thin sheet, and thin sheets can be clamped and polished as a stack.

Its limits: an edge polisher handles straight edges and bevels. Curves, inside corners and pockets are done on a CNC machine with a diamond end mill or chamfer cutter, or by flame. The diamond is also sensitive to knocks and dirt: a chipped diamond leaves a fine line along every edge.

Which method should you use?

• One part, or a repair at home: sand and buff.
• Curved shapes, fast, and the edge is not the main feature: flame polish, and anneal the part if it will be glued or cleaned with alcohol.
• Straight edges in production, thick sheets, parts that are glued, or anywhere the edge must look like glass: diamond polish.
• Clear pockets, outlines and chamfers on a CNC machine: a single crystal diamond end mill or chamfer cutter as the last pass.

Cast or extruded acrylic

All three methods work on both. Cast acrylic is the easier material: it machines cleanly and tolerates heat and solvents better. Extruded acrylic softens at a lower temperature and holds more stress from its production, so it melts and smears sooner under a buffing wheel or a dull tool, and it crazes more easily after flame polishing. On extruded sheet, a sharp tool and a light cut matter even more.

When the diamond-polished edge is no longer clear

If an edge polisher that used to give clear edges starts to leave a haze, fine lines along the edge or a slightly milky surface, the finishing diamond is worn or chipped. It does not need to be replaced: a single crystal diamond can be resharpened several times, for a fraction of the price of a new one.

Diamond tools for acrylic

Bulunmaz makes finishing diamonds for edge polishing machines, and diamond end mills, chamfer cutters and face cutters for CNC machines, in natural diamond and MCD. See diamond tools for acrylic edge polishing, or send us your machine type and a photo of the tool you use now.

We do not polish parts for customers and we do not sell edge polishing machines.