Processes

Plastics Polishing

The information on this page deals mainly with polishing of acrylic sheet. 

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 build-up.

Polishing Edges 
Polishing creates the best finished edge but requires the most preparation. A well machined (router cut) edge can be polished without prior sanding. However, a saw-cut must be either 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 200 to 350 mm (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 lustre finish, it is best to first use a fast cutting compound to remove all sanding marks, and then a high lustre compound for the final buffing operation.
Be careful to avoid excessive heat build-up when buffing edges. Too much heat can induce stress into the sheet and eventually cause crazing.

Surface Polishing
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 150 to 300 mm (6” to 12”) in diameter, built up to a width of 38 to 50 mm (1½” 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 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 to 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 the Surface Polishing instructions for a higher grade 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 paper 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 the Surface Polishing instructions for a higher grade finish.

Scraping Edges 
Easiest of al finishing techniques is scraping. A scraper can be almost any piece of metal with a sharp, flat edge. 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 lustre 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
Highly polished edges can be obtained by flame polishing with a hydrogen/oxygen-welding torch.  Make sure to reduce the oxygen content to produce a flame that is bright orange/red in colour, as opposed to the bluish, almost invisible flame typically used with standard acrylics.  Acrylic sheet has a tendency to turn a milky white colour when overspray from the flame contacts the surface of the sheet.  It is important to minimize this contact by using a quicker feed rate than would normally be used for standard acrylics.  Hold the torch at an angle 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.  For optimum edge finish, wet sand the edge or pass the sheet through an edge finishing machine or jointer to remove any tooling marks from previous operations prior to flame polishing.

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 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 rotation speeds will permit faster feed rates without chipping.