Polishing Edges

Polishing creates the best finished edge, but requires the most preparation. A well-machined (milled) edge can be polished without prior sanding. However, a saw-cut edge must be sanded, run through a jointer, shaper, router or edge-finishing machine, or be hand scraped before it can be polished. Edge polishing is best done on a stationary polishing head. Use 8" to 14" (200-300mm) diameter bleached muslin wheels designed with bias strips which give the buffing wheel a pleated appearance. This design runs cooler than a stitched buffing wheel design and will do a faster job.

Edge finish quality depends on the selection of the polishing compounds. The use of a medium cutting compound will give a fairly good finish in one operation. For a high luster finish, it is best to first use a fast cutting compound to remove all sanding marks, and then a high luster compound for the final buffing operation.

Be careful to avoid excessive heat buildup when buffing edges. Too much heat can induce stress into the sheet and eventually cause crazing. To reduce the amount of stress to a minimum, if possible, anneal the part after all fabrication steps are complete (including polishing).

Polishing Surfaces

If the scratches or machining marks are not too deep, the surface can be polished without prior sanding. Wheels used for surface polishing can be 6" to 12" (150-300mm) in diameter, built up to a width of 1.5" to 2" (38-51mm). The wheels should be made of soft, bleached muslin for the initial polishing operation and of soft flannel for the final finishing.

For the first buffing operation use a medium-coarse polishing compound or a fine compound depending on the depth of the scratches.

When polishing the surface of the sheet, the piece must be kept in motion at all times. Do not use excessive pressure, as softening from over-heating can result.

Sanding Edges

Wet sanding is desired for finishing acrylics. Normally 180-320 grit "wet or dry" paper is used along with plenty of water. If done by hand, use a sanding block to keep the edges even. Only light pressure should be applied when grinding with power sanders to minimize frictional heat which can cause gumming from over-heating of the acrylic. Follow "Polishing Surfaces" procedure for a higher finish.

Sanding Surfaces

A scratched surface should not be sanded unless the imperfections are too deep to be removed by polishing alone. If sanding is required, it is recommended that wet sanding be used. The application of water makes it possible to produce a smoother finish, because fine-grit sandpaper can be used. Without water, this same fine-grit sandpaper would fill up and over-heat the acrylic.

For very deep scratches, a 240-grit or 320-grit paper will be coarse enough to start the sanding process. This first step should be followed, after rinsing, by a 400-grit, and then by a 600-grit paper. Be sure to use plenty of water and rinse the sandpaper frequently to keep it from clogging. With power sanders, only light pressure should be applied to reduce friction. Follow "Polishing Surfaces" procedure for a higher finish.

Scraping Edges

The easiest of all finishing techniques is scraping. A scraper can be almost any piece of metal with a sharp, flat edge. Special acrylic scraping tools are available from an acrylic sheet distributor. Whatever tool you use, it must have a sharp, square edge. Hand scraping is an alternative to sanding for preparing the edges for polishing. Used in conjunction with flame polishing, high luster edges are achieved, but without the smoothness of sanded and polished edges.

Clamp the work vertically. Start at the rear of the edge with the scraper tilted 45° and draw with uniform speed and pressure to the front. A uniform strip should be removed. Follow an edge polishing procedure for a higher finish.

Flame Polishing Edges

Flame polishing should be done with an oxygen-hydrogen welding torch. It is important to remove the protective masking from the material's edge when flame polishing. The flame should be bluish, nearly invisible, approximately 3" (75mm) long and narrow. Hold the torch even to the edge and at a slight angle. Then, draw the flame along the edge of the sheet. Practice will help you estimate the speed and distance. If the first pass does not produce a completely polished edge, allow the piece to cool, then try a second pass.

Remember that when flame polishing, the sudden heating of the sheet induces a high degree of stress. After flame polishing, be sure to anneal the part. This will decrease the amount of stress and reduce or prevent the chance of crazing over time.

Edge Finishing Machines

Commercially available edge finishing machines offer a fast method of obtaining smooth edges without sanding or scraping. Finishes range from smooth edges with slight machining marks to smooth, almost polished edges, depending on the design of the machine and cutting tools used. Machine finished edges are ideal for cementing or flame polishing, but are sharp and require some additional fabrication to make them safe for handling.

Care should be taken to adjust the feed rate to eliminate chipping and melting. Feed rates that are too fast will result in chipping, while a slow feed rate will result in melting. Edge finishing machines will vary in cutter diameter and rotation speed. In general, machines with larger cutter and higher rotation speeds will permit faster feed rates without chipping.

Written by Grant LaFontaine, Sheet Products Technical Service Manager, CYRO Industries.

For more information, contact CYRO Industries, P.O. Box 5055, Rockaway, NJ 07866, 800-531-6384, FAX 973-442-67117, Web: www.cyro.com.