PRESERVATION OF SPECIMENS IN PLASTICS 67 



In resurfacing cylindrical blocks, it is usually more convenient to em- 

 ploy ashing with wet pumice applied on a power-driven cloth buffing 

 wheel than to use sandpaper. Such procedure may also be employed for 

 rectangular blocks if desired, but it is inadvisable in most cases because it 

 tends to curve the surface and round the edges excessively. 



After the sanding and ashing operations are completed, the block should 

 be well washed with clean water to remove all grit and dust before polish- 

 ing is undertaken. 



In blocks that are in acceptably good condtion when first removed from 

 the mold, minor scratches or irregularities can be taken care of by the 

 final buffing and waxing without the need of any preliminary sanding. 



Polishing 



Properly surfaced methacrylate castings can readily be given a clear 

 lustrous finish by polishing with suitable abrasive compounds on power 

 driven buffing wheels. For this purpose felt wheels are considered to be 

 the most satisfactory, but cloth wheels can be made to give acceptable 

 results if properly used. These buffing wheels may be from 8 inches to 

 15 inches in diameter and should preferably be built up to a thickness 

 of approximately 3 inches. They may be operated at surface speeds of 

 between 1,200 and 1,800 feet per minute, but extra care is needed at the 

 higher speeds to avoid undue pressure of the work against them which 

 might cause excessive frictional heating with consequent scarring or 

 "burning" of the surface of the plastic. In this connection it should also 

 be borne in mind that localized overheating during finishing is one of the 

 main causes for crazing or superficial cracking at a later time. 



Among the satisfactory abrasives for polishing methacrylate plastics, 

 magnesium carbonate and also prepared calcium carbonate (whiting) 

 have been found to be particularly suitable. In using abrasives of this 

 type, it is advisable first to touch a stick of tallow to the revolving wheel, 

 and then to apply as much of the compound as will adhere to the wheel. 

 A number of special polishing compounds for plastics are also commer- 

 cially available, and those that are indicated as being suitable for use on 

 acrylic plastics may be expected to give good results. Jewelers' rouge can 

 be used if desired for this purpose, but it is recommended only where other 

 more satisfactory compounds are not available. 



When a uniformly smooth, clear surface has been obtained with the 

 buffing wheel carrying the polishing compound, it is advisable to transfer 

 the operation to a second wheel, preferably a cloth wheel of open type, 

 which is free from abrasive. This will serve to clean the block and give it 

 a final high luster. 



If it is desired to maintain very true surfaces and sharp edges on the 

 finished block, the polishing operation should be carried out on a large 

 felt pad, preferably on a flat revolving disc. After moistening this polish- 

 ing pad with water, apply a suitable amount of the abrasive compound 

 and distribute it over the surface; and from time to time, as the polish- 

 ing progresses, add more water and compound as needed. The plastic 

 block should be held against the polishing pad with even and moderate 

 pressure, and it should be moved over the surface at a comparatively 



