Chapter 8

How do you achieve a high quality surface?

DuPont reflects on how to enhance polymers to gain a high-quality finish

When processed in a well-designed, well-made tool, moulded parts made of engineering plastics have an attractive surface.

Painting, metallising or sublimation transfer printing can further enhance this surface to suit the requirements of the application. The much-used term “surface quality” encompasses a multitude of properties and attributes. It becomes meaningful only when the user has defined what surface properties are relevant in a specific application.

Test methods

One of the most important tests of surface quality is the measurement of colour on intrinsically coloured or painted plastic parts. Only test methods that take into account how the human eye perceives colour give practical results.

Such tests are based on the colour triangle (CIE colourchromatic diagram or colour board according to DIN 5033), in which every colour is composed of the basic colours red, green and blue. At the same time, the colour diagram takes into account that the human eye’s sensitivity to different colours is not the same.

With colorimetry, colours can be objectively measured, colour differences evaluated and colour tolerances determined.

Gloss level, which is measured with a reflectometer, is also important for the appearance of a surface. It is determined from the spatial dispersion of the scattered light reflected by the surface being checked.

The cross-hatch test is widely used to check the adhesion of surface coatings. In this test, the surface is cut through in two perpendicular directions with a special five-bladed cross-hatch cutter and the sections of the grid thus created are checked with adhesive tape to show whether, and to what extent, the coating separates from the substrate.

Scratch tests are used mainly in the automotive industry; the methodology is generally specific to each company. They provide information about the undesirable lightening which scratches cause in a surface.

In a broader sense, weathering tests may also be counted as surface tests, since they provide information about long-term changes in plastic surfaces caused by exposure to weather.

Surface quality

Figure 1: A silvery metallic paint gives the headlamp bezel of Zytel® nylon on the new Ford Transit the high-quality surface that this eye-catching front-end styling element calls for.

If they are correctly processed, most engineering plastics parts have a smooth, firm surface. The surface quality of an untreated moulded part depends mainly on:

• the wall texture of the moulding tool; the mould’s inner surface can, for example, be highly polished, matte or structured, and
• the position of the gate and the gating system, as well as from the processing parameters, because the latter influences the creation of weld lines, flow marks or burns. Surface faults such as these sometimes become more visible after subsequent surface treatment than they were before.

The “feel” of a plastic’s surface can be influenced through modification with elastomers or, even more, by means of two-component moulding with a thermoplastic elastomer.

Softer surfaces of this kind may be desirable for power tools and hand tools. The automotive industry also requires interior equipment to have a pleasant, softer “feel”.

Painting and metallising

Figure 2: With Delrin® Decorating Solutions, developed by DuPont, the productivity of plastics processing can be combined with the functionality and looks of metals, in applications such as consumer goods and health-related products.

There are many ways to influence or change the properties of plastics surfaces. Cost considerations naturally play a decisive role.

For many applications, such as articles for household use and for large or small power tools, it is enough to polish the walls of the moulding tool, to make them matte or to structure them (grain pattern).

The scratch resistance of plastics surfaces can be improved with additives such as talcum. When considering post-moulding surface treatments, it is important to keep in mind that only perfect mouldings are suitable, because surface faults, including weld lines and flow lines, become more apparent after surface treatment.

Painting is one way to produce surfaces that meet more demanding requirements. When deciding on the substrate and the paint system, it is important to ensure adhesion between the two is good.

It is also recommended – in fact, it is a requirement in the automotive industry – that the moulded part is intrinsically of the same colour as the paint coating, because this makes scratches less obvious.

In the new Ford Transit, the headlamp bezels are a prominent front-end styling element. A silver-metallic paint coating gives them a striking, high-quality surface.

Its application is a highly cost-efficient process, because neither a primer nor any other pretreatment is needed for the plastic substrate, which is Zytel® nylon. This material also has the necessary high-temperature resistance and impact resistance for this application.

Metallising also produces high-quality surfaces which are used mainly in health-related applications, in the automotive industry (chrome trim is made almost entirely of metallised plastic nowadays) and in high-quality packaging.

The choice of different metals makes colours from light silver to dark gold-yellow or copper possible; even multi-coloured parts can be made.

Figure 3: The decoration on this snowboard binding, applied by means of sublimation, is scratch and abrasion-resistant, because with this new technology the inks sink into the surface of the substrate.

Particularly in the case of acetal resins, the Delrin® Decorating Solutions Technology developed by DuPont makes it possible to combine the advantages of plastics processing with a high-quality metal-like surface.

Typical application areas include attractive cosmetics packaging, health-related products, consumer goods and sports equipment. Certain Crastin® polybutylene types are also suitable for highly cost-effective direct metallisation.

Sublimation techniques

Sublimation, a relatively new technology for plastics, produces high-quality, scratch-resistant surfaces. This is because the inks penetrate the surface of engineering plastics to a depth of about 0.2-0.3 mm.

The colours are brilliant and complex patterns are possible. A new technology, developed by DuPont, works with a patented ink transfer medium which can be draped evenly, without folds or creases, even onto complex-shaped three-dimensional objects. This makes sublimation technology useful for complex-shaped parts.

Value in use

Correctly processed plastics parts have attractive surfaces which require no further post-moulding treatment for many applications.

High-quality surfaces can be achieved by painting or metallising. Sublimation can produce multi-coloured scratch-resistant surface decoration, which can now also be applied to complex-shaped objects.

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The examples in this series of articles are intended to illustrate underlying principles and to explain the main influencing factors.

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