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POLISHING EDGES In many installations, the edges of the
Chemcast® GPsheet 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
Chemcast® GP. Well polished edges, however, may be required for
household accessories, jewelry, and other decorative items.
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
Chemcast® GP 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 Chemcast® GP 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 Chemcast® GP. Whenever possible, a number of Chemcast® GP
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. |
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Copyright © Plastiglas de Mexico 1999
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