Making Mass Produced Parts Distinctive

A special dip bath technique enhances the appearance of injectionmouldings, turning mass-produced components into what are virtuallyunique specimens. The contract painter Ritzi of Tuningen, Germany,has acquired a sublicense from a Japanese company for the useof its cubic-print process. This Japanese company is representedby the company Color-Dec in Italy. A total of 10 people are currentlyengaged in this decorative coating technique. IMI visited Ritzito find out how the technique works, what it can do, and whereit comes up against its limits.

The Ritzi Group, a family-owned company dating back more than100 years, had its origins in a painter's workshop. Today, thecompany's 250 employees achieve sales of some DM 40 million (US$24 million) per year in the fields of display production, therepainting of repaired automobiles, industrial painting, and surfacetechnology. A staff of approximately 100 is employed at the RitziLackiertechnik GmbH subsidiary in Tuningen, where annual salestotal some DM 13 million (US$ 7.6 million). Josef Brunner, generalmanager, explains the way in which this finishing technique works.

Decoration on a Film Substrate

The decoration itself can convey a marble or wood effect, createthe impression of a carbon fiber mat (Figure 1), or take the formof a fashionable decor. Six hundred different decorations arecurrently available, which are applied to transparent film inJapan. The films come in a standard width of 1m. Since the rolllength is 1,000m, efficient use means at times it would be appropriateto coat more than one type of part with the same decor in orderto make ordering an entire roll of film worthwhile. Standard decorations,such as marble and wood grain, however, are always in stock atRitzi.

Since the hiding power of the paint on the thin decorative filmis limited, it is necessary for the moulded part to be paintedin the same basic shade as its final appearance. Either a lightor a dark coat of paint may be required here. For example, fora portable telephone housing or interior automobile parts (Figure2) intended to convey the impression of real wood grain, onlythe dark brown grain will be applied to the film. The plasticor preliminary coating of paint must then be in the light brown.In the case of light switch covers (Figure 3) intended to looklike black marble, the substrate is in black and only the whitemarble pattern will be printed on the film.

Today, injection moulded parts account for half the substratescoated, while metal and wooden articles make up the rest. Partsin nonpolar polymers, such as polypropylene or polyethylene, alwaysrequire painting prior to coating in order to ensure that thecoating adheres at all.

The Underlying Principle

The steps for the process are shown in Figure 4. film is cut offthe roll in the approximate size of the moulded part being coated,placed on the surface of a large water bath, and sprayed fromabove with a solvent-known as the activator. The carrier layerof the film dissolves in the water, leaving just the ultrathindecoration layer to float on the surface.

The injection moulded part is then lowered slowly into the waterbath. The activated paint floating on the water hugs the surfaceof the part and immediately bonds to it. The pressure of the waterforces the decoration against the contour of the part. Once theentire part has been immersed and coated with the decoration,it is removed from the bath and rinsed with cold water. A coatof clear lacquer, which dries to a glossy or matt finish, is thenapplied by way of wear protection for both the moulding and thedecoration; this gives the finished part an impressive opticaldepth, conveying an excellent impression of finely oiled realwood.

The chemical properties of the water are decisive for a successfulcoating. The hardness and pH-value of the water, and also itstemperature, must fall within a narrow range. Dissolved partsof decorative film and any paint remaining on the surface of thewater are rinsed out of the water bath by a surface current aftereach individual coating operation.

Part Design: the Crucial Factor

Not all geometries can be coated by the cubic-print method. Thefact that the parts have to be immersed in water imposes limitson the applicability of the process. Flat surfaces do not poseany problems. The moulding is held at an angle and-commencingwith one of the corners-is gradually immersed by a handling unit.If air bubbles become trapped between the surface of the waterand the moulded part at undercuts, then the coating will not work.This is why holes must be provided, even in small undercuts, toensure that the air can escape.

There is also a limit on the depth to which the decoration canbe applied inside large undercuts. Since the thin decoration possessesonly a limited elasticity, it will tear if subjected to too higha load and thus is overstretched. Prior to this, the regular decorativepattern (carbon fibers, artificial stone) will have already beenseverely distorted. A high-precision, uniform decoration can thusonly be achieved on relatively flat moulded parts. In the caseof irregular decorative patterns, such as wood grain or marbleveins, a greater degree of stretching can be accepted. A switchbutton that is more spherical than flat can be readily coatedwithout forsaking the impression of a wood decor. The film onlytears at the bottom end, at a point that is concealed by a sleeveanyway.

If long, rod-shaped parts are immersed at an angle, then the decorationwill form a slight knit line on the side pointing upwards. Sincethis can impair the appearance of both regular and irregular patterns,and a small ridge will remain even after the lacquer has beenapplied, knit lines of this type should be placed in areas whichare not on the main visible surface or are located in the handleregion, wherever possible.

Appearance has its Price

This surface finish is anything but cheap. To coat an eyeglasscase will cost some DM 5 to 6 (US$ 3.00 to 3.50) for a medium-sizedseries. Smaller parts in bigger series that require less pretreatmentcan be coated at a considerably lower cost. Individual items andsample parts can essentially be coated without any problems aswell, although at a high price.

The cost of the process can thus only be justified on parts thatneed a unique appearance or that will really fetch the extra valueon the market (Figure 5). Whether or not the high cost is worthit will depend on the case in question.

The chief application in volume terms today is parts for automobileinteriors where a plastic part with a wood decor offers majorcost savings over real wood. Ritzi intends to continue focusingon the refinement of high-grade consumer goods, together withelectronic components, sports equipment, and domestic installationfittings. There would seem to be a sufficient number of applicationsfor the cubic-print process, and the technique itself appearsto be mature.