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Vladimir State University named after the Stoletov brothers Vladimir, Russia

Finishing, assembly and decorating

Treskov D.V.

Отделка, сборка и декорирование

Тресков Д. В. Владимирский государственный университет имени А.Г. и Н.Г. Столетовых Владимир, Россия

This chapter will provide practical information and guidance on several important plastic processes that occur only after the part has been formed. The following processes are sometimes referred to as post processing or secondary operations: polymeric resin plastic thermal

1) Machining and finishing

2) Assembly

3) Decorating

They are common operations essential to producing practical commercial products from plastic materials [1].

Fortunately, many of the processes and tools that are satisfactory for working with metals, wood, and other common engineering materials also apply to plastics. Although there are similarities in these processes, there are also some critical differences that must be considered due to the unique nature of polymeric materials. The material properties of the polymeric resins will dictate the processing parameters that can be used. There are also certain unique assembly and finishing opportunities available for the designer because the material is a plastic. It is these material properties that must occupy the designer's early attention, and this chapter will elaborate on these.

Machining and Finishing

This section will discuss the major machining and finishing operations commonly used in the manufacture of plastic products [1]:

1) Smoothing and polishing

2) Sawing and cutting

3) Filing, grinding, and sanding

4) Routing, milling, and turning

5) Drilling

6) Tapping and threading

7) Cleaning

8) Annealing.

Almost all molded articles require finishing to some extent, if only to remove flash and gates from the molding operation.

Machining of plastic parts is normally employed only if there is no other way of designing the functional requirement (hole, rounded edge, etc.) into the primary molded part. Some finishing operations on molded articles can be avoided or reduced by careful design of the part and the mold. For example, placing the flash lines and gates in selected areas could greatly reduce filing needed to remove excess material, and holes molded into the part could eliminate the need for postmold drilling [2].

Machining requires skilled labor, equipment, and other production resources.

There are several factors to consider when machining plastics: the material's physical properties, such as toughness or modulus; the material's thermal properties, such as thermal expansion coefficient, thermal conductivity, and glass transition temperature; and stress effects on the plastic, either internal (molded-in) stress or external stress from a postprocessing operation.

Internal stresses that are molded into the plastic part can be released when in contact with heat, chemicals, or during cutting. These stresses, when released on the surface of the part, cause stress cracking or crazing. They are noticeable as microcracks or fractures on the surface of the sections under highest stress. When large internal stresses are released by heating through a finishing operation, it could result in warpage and deformation [3]. Like some metals, plastics may need to be annealed before finishing to avoid warpage or undesirable stress relief during postprocessing. Plastic resin suppliers can provide specific annealing instructions for individual plastics.

Some important guidelines should be remembered when machining plastics:

1) Compression, thermal expansion, etc., can result in poor dimension-

2) Heat buildup must be minimized (coolants are usually recommend-

3) Fast tool speed and slow material feed are generally recommended.

4) Parts may need annealing to relieve internal stress.

Assembly of Plastic Parts -- General Considerations

The next four sections of this chapter provide practical information and guidance on how to join parts made from plastics to themselves and to other substrates. Several processes are available for joining these materials:

1) Mechanical fastening

2) Adhesive bonding

3) Thermal welding

4) Solvent cementing

Solvent cementing and thermal welding use the resin in the part itself as the “fastener” to hold the assembly together. Adhesive bonding and mechanical fastening use another substance as the “fastener.” The design engineer must determine the joining method that best suits the purpose. The choice will often depend on the type of plastic, the service environment, economic and time constraints, and production parameters. The designer should not force an assembly method on a plastic product originally designed for another assembly method. Usually, parts must be specifically designed to an assembly method. In fact, certain plastic materials are specifically chosen for an application because of their capability for assembly [4]. For instance, in the automotive industry, plastics are often chosen based on the fact that they can be assembled with very fast processes such as ultrasonic welding.

It is important that the designer realize the unique opportunities and problems posed by each method of assembly. To do this well, he or she must have an understanding of materials science, chemistry, surface science, physics, mechanics, and industrial engineering since all of these disciplines will come into play. Even with this background, final selection of the most desirable assembly method involves some trial and error that can become costly and time consuming. The purpose of these next sections is to give the designer a foundation for finding the right assembly system for any particular combination of plastic material, part design, service environment, and production constraint.

The joining of plastics is generally more difficult than joining of other substrates because of their low surface energy, poor wettability, and presence of mold release agents and other contaminants that can create a weak interface [4]. The relative differences in thermal expansion coefficient and elastic modulus also make joining of plastics to nonplastic materials difficult. They may also cause very high loads or loose fitting fasteners in parts assembled with mechanical fasteners. The nature of the polymeric material could also change with the service environment. Parts may swell in solvent, become brittle when exposed to UV, lose plasticizer on aging, gain a plasticizer (water) during exposure to humidity, and go through many other changes. All of these will have an effect on the joint.

With plastic materials, the designer also has a greater choice of assembly techniques than with many other materials. Thermosets must be adhesively bonded or mechanically joined, but most thermo-plastics can be joined by solvent or heat welding. Plastic parts can also be designed for assembly by means of molded-in snap-fit, press-fit, pop-on, and threaded fasteners, so that no additional fasteners, adhesives, solvents, or special equipment is required.

Decorating Plastics

Several decorating processes can be accomplished with plastic parts-- either during processing of the part, directly afterward, or before final assembly and packaging. The most inexpensive method of providing decorative designs on plastics is to incorporate the design into the mold or to apply the decoration as part of the molding operation. However, often this is not possible either because of mold complexity, the need to apply decorations to customers' specifications, or other reasons.

This section will discuss common decorative processes that are generally used with plastic parts. The most widely used decorating processes in the plastic industry are:

1) Painting

2) Hot decorating (hot stamping, in-mold decorating, heat transfer)

3) Plating (electroless, electrolytic, vacuum metallizing)

4) Printing

5) Application of labels, decals, etc.

As with adhesive bonding, surface treatment and cleanliness is of primary importance when decorating plastic parts. Prior to decoration, the surface of the plastic part must be cleaned of mold release, internal plastics lubricants, and plasticizers. Plastic parts can also become electrostatically charged and attract dust. This could disrupt the even flow of a coating or interfere with adhesion. Solvent or destaticizers may be used to clean and eliminate static from plastic parts prior to decorating. Cleaning of the plastic part requires an understanding of the plastic material to be cleaned and the effect of the solvents and processes on that plastic [5].

Some plastics may need to be roughened or chemically treated to promote adhesion of the decorating medium. Molded and extruded plastic parts tend to have glossy resin-rich surfaces. This is desirable if the part is used in the as-molded condition, but the glossy surface may require abrading or etching to hold paint or printing media or prior to application of an adhesive [5]. Polyolefin, polyacetals, polyamides, fluorocarbons, and other surfaces that are difficult to wet might require special surface treatments before decorating processes can be completed satisfactorily. Many of the cleaning methods and surface treatments that are described in the previous sections of this chapter for surface treatment prior to bonding are also applicable prior to decorating.

References

1. Terry A. Richardson. Machining and Finishing // Modem Industrial Plastics. - College Audience, 2003. P. 13-22.

2. John L. Hull. Design and Processing of Plastic Parts. // Handbook of

Plastics Elastomers and Composites. / Charles A. Harper, ed., McGraw-Hill - New York, 2005. P. 247-269.

3. J. O. Trauemicht. Bonding and Joining, Weigh the Alternatives, Part 1 // Solvent Cements, Thermal Welding / Plastics Technology. - 1970. P. 125-128.

4. Charles A. Harper. Engineer's Guide to Plastics // The Complete Guide to Properties and Performance / Materials Engineering - 1972. P. 101-118.

5. Edward A. Muccio. Finishing and Decorating Plastic Parts. // Plastic Part Technology. / ASM International - Materials Park. - Ohio. - 1991. P. 21-28.

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