Chapter 35

1. Chapter 35 Processes Used to Finish Plastic Materials

2. Objectives • Applying protective coating of phenolic resin to interior of large chemical storage tanks • Metallic coatings applied to plastics • Electrostatic powder coating for irregularly shaped parts

3. Material removing processes • Tumbling: It is used to remove flash, smooth rough edges, grind, and polish. • The parts to be finished are placed in a drum with finishing materials (tumbling compounds). • The compounds may be abrasive particles, waxes, sawdust, etc. • Dry ice is sometimes used to remove thin flash on products. The ice chills the flash, making it brittle, and tumbling separates it from the product.

4. Material removing processes • Smoothing and polishing: The primary step in finishing rough plastic parts is rough sanding. • Final finishing processes can be finished with fine sanding paper. • Buffing is done when small surface defects must be removed, or when a polished surface is needed. • Polishing compound is applied to half of the wheel, while the other half is left clean. The part is first held against the waxed half, and then against the unwaxed half to wipe off excess.

5. Coating Process • Dip Coating: Poly vinyl chloride (PVC) can be dissolved in plasticizer to create plastisol. • An important use of plastisol is dip coating the gripping surfaces of metal products such as tool handles, kitchen utensils, and dish draining racks. • After curing the material becomes solid and rubbery to the touch.

6. Coating Storage Vessels in the Field • Because of the size, large storage vessels and holding tanks are sometimes coated in the field. • The phenolic coating is sprayed on the inside surface. • Hot air generators are used to bake the phenolic material on the inside surface.

7. Electrostatic Powder Coating • Particles of plastic powder are given a negative charge as they are sprayed on a positively charged part. • Once the part is sprayed, it must be cured in an oven to fuse the plastic to the part. • Powder that falls off the part is collected and reused. • Powder coatings are hard, tough, and attractive. • Unlike conventional spray painting there are no air pollution worries. The powder contains no solvents, so no solvents are given off into the air during curing.

8. Fluidized bed coating • Polyethylene is a popular polymer used with this process. • The part to be coated is heated to a point above the fusion temperature of the powder and is then dipped for a few seconds into the fluidized bed. • The powder attaches itself to the part, and then melts and bonds to the heated surface in a layer of uniform thickness. • Vinyl, epoxy, and nylon coatings are available.

9. Flame Spray Coating • Flame spray coating is used to apply a plastic coating to tanks, vessels, or other products too large to cure in an oven. • A fine plastic powder is heated and dispersed through a specially designed spray gun burner nozzle. • The flame also heats the part to be coated. • Thus the hot plastic is sprayed on to a heated substrate to form a smooth uniform layer. • When the plastic cools it forms a hard permanent coating.

10. Electroplating • Before electroplating can occur, a conductive coating must be applied to the plastic. • The electroplating process is used to deposit metals from solutions onto metallic surfaces using electrolysis. • Electroplating begins with careful cleaning of the part. • A chemical bond of thin copper is then created. • Once the copper coating is completed, electroplating can be carried out using the same methods for plating metals. • Electroless plating: The process uses chemical rather than electrolytic action to produce the desired metallic coating on a substrate.

11. Other Finishing Processes • In-Mold Decorating with Foils: In-mold decorating can be done by applying foils to compression molded and transfer molded products made with melamine, urea, or ammonia free phenolic thermoset and thermoplastic materials. • A foil is a printed melamine impregnated paper that is introduced into the mold.

12. Summary • Tumbling: It is used to remove flash, smooth rough edges, grind, and polish. • Smoothing and polishing: The primary step in finishing rough plastic parts is rough sanding. Final finishing processes can be finished with fine sanding paper. • Dip Coating: Poly vinyl chloride (PVC) can be dissolved in plasticizer to create plastisol. An important use of plastisol is dip coating the gripping surfaces of metal products such as tool handles, kitchen utensils, and dish draining racks. After curing the material becomes solid and rubbery to the touch. • Powder coatings are hard, tough, and attractive. Unlike conventional spray painting there are no air pollution worries. The powder contains no solvents, so no solvents are given off into the air during curing. • Flame spray coating is used to apply a plastic coating to tanks, vessels, or other products too large to cure in an oven. • The electroplating process is used to deposit metals from solutions onto metallic surfaces using electrolysis.

13. Home Work • 1. What are the applications of plastisol? • 2. What is the pollution advantage of powder coating over spray painting?