POLISHING EDGES

In many installations, the edges of the Vical acrylicsheet are completely hidden in a mounting frame or channel and smooth edges, free of chips and major irregularities, are entirely satisfactory. Sometimes good machine-finished edges are used as a decorative element in the design of a part made of Vical acrylic. Well polished edges, however, may be required for household accessories, jewelry, and other decorative items.

1. Preparation
2. Scraping Edges
3. Sanding Edges
4. Polishing Edges Polishing
5. Flame Polishing Edges
6. Polishing Surfaces
   

A. Preparation

The amount of finishing required to produce a smooth, transparent edge is dependent on the quality of the machined edge. A sharp and properly designed cutting tool will reduce the amount of the finishing work needed. Finishing work is also reduced when a spray coolant is used with the cutting tool to prevent excessive heat build­up.

B. Scraping Edges

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 your Chemcast ® GP acrylic sheet distributor. Whatever tool you use 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.

C. Sanding Edges

Wet sanding is desired for finishing acrylics. Normally, 180 to 320 grit "wet­or­dry" 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 over­heating of the acrylic. Follow "Polishing Surfaces" procedure for a higher finish.

D. 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 saw­cut must be sanded, run through a jointer or shaper, or be hand­scraped before it can be polished. Edge polishing is best done on a stationary polishing head. Use 8" to 14" 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.

E. Flame Polishing Edges

Flame polishing is a fast method of obtaining a medium high luster edge with little hand work.

Flame polishing should be done with an oxygen hydrogen welding torch. The flame should be bluish, nearly invisible, approximately 3" long and narrow. Hold the torch at the angle shown and draw the flame along the edge of the sheet. Practice will help you to 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..

Saw marks can be removed from the edges of Vical acrylic sheet by scraping with a hard steel or Carboloy scraper square­ground to a straight, smooth edge. After scraping, the edges should be sanded on a wet belt sander with 400 grit sandpaper.

Buffs for polishing Vical acrylic edges can be made of layers of medium density 100% wool felt about 3/16" to ¼"thick. The felt should have a specific gravity of about ..27. The wheels should be 10" to 12" in diameter and should be held between hard face plates about three inches smaller in diameter than the buff. The wheels should be run at a speed of 1800 surface feet per minute.

The edges are first buffed on a felt wheel charged with abrasive and tallow. They are next buffed on a felt wheel charged only with tallow. The final polish is given to the edges with a soft cotton buff. Felt wheels should not be used on large flat areas since there is a tendency to burn and distort the Vical acrylic.

Whenever possible, a number of Vical acrylic parts should be locked together in a jig leaving only the edges exposed so that the edges may be planed, sanded, and polished simultaneously. This technique is faster and gives better results without rounding the edges. Lapidary wheels may also be used for polishing flat surfaces.

F. 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 from 6" to 12" in diameter, built up to a width of 1 1/2" to 2". They are 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 medium­coarse 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 over­heating can result.

Return to Edge and Surface Finishing