By Design: Part design 109 -- Textured finishes

In this bimonthly column, Glenn Beall of Glenn Beall Plastics Ltd. (Libertyville, IL) shares his special perspective on issues important to design engineers and the molding industry.

The use of plastic materials is increasing at a rate of almost double the gross domestic product. This impressive growth is due in no small part to the acceptance of these materials by the product design community. Design engineers normally select the material to be used. They can choose any material that will work, but they continue to show a fondness for plastics. The two primary reasons for this preference are that plastic products are economical to produce and they provide the maximum design freedom. 

This freedom provides an outlet for the designer's natural tendencies to create unique products. The designer's attitude is that if it can be envisioned, their suppliers will figure out a way to make it. Many of these creative shapes are a challenge to the injection molding industry. 

One of plastic's important attributes is its ability to mimic other materials. Adding color to a plastic can create subtle hues of natural materials such as ivory, tortoiseshell, and flower petals. 

The pigmenting and staining of plastics can replicate the color of leather and different kinds of wood. In these instances, the color is the same, but something is missing. The missing element is the 3-D graining that gives wood and leather its unique appearance. This graining cannot be produced by pigmenting the plastic. It has to be provided by the mold. 

In the beginning, this graining was hand cut into a mold by a master engraver. The process worked well, but it was labor intensive. The uniformity of the pattern varied, depending on the skill and care of the engraver. No two engravers produced the same results. It was also difficult to get exactly the same patterns on multiple molds. These difficulties were overcome in 1966 with the issuance of a German patent for the controlled chemical etching process. 

The photo chemical etching (PCE) process starts by photographing the required pattern. The photograph is then used to create a mask that is transferred to the surface of the mold. The mold is then subjected to a chemical etching bath that corrodes or eats away the metal in the unmasked portion of the pattern. 

Other surface finishes are produced by blasting the mold surfaces with various abrasive materials at varying pressures. Other molds rely on the patterns left by electrical discharge machining (EDM). All of the surfaces produced by these different techniques are loosely referred to as textured finishes. 

The most common surface finish on plastic products is a polish of varying degrees of smoothness (for more on polishing, see "By Design," April 2000 IMM, pp. 56-60). After a while, all of these shiny, polished surfaces begin to look the same. They have that plastic look. It is generally agreed that consumers consider a textured product as having greater value. Textured parts are perceived as being more durable, more resistant to scuffs and scratches, and easier to keep free of fingerprints. The skillful use of polished surfaces, combined with textured surfaces, can give a low-cost plastic part a rich appearance. 

Why Texture?
Designers normally specify textures in order to make a plastic part look like some other material. The marine and construction industries specify woodgrain textures. The luggage and automotive industries favor the look of leather. Furniture manufacturers use both wood and leather grain textures to give their plastic products a more traditional look. 

The skillful use of textures can also result in a lower part cost by reducing reject rates. Surface imperfections such as minor sink marks, weldlines, flow marks, and minor scuffs are minimized by a textured finish. Some soft, sticky materials will demold better with a light matte finish. Textures are also used to improve the speed and strength of ultrasonic welds and adhesive bonds. Dull textured surfaces can eliminate light reflection on photographic equipment and instrument panels. Very light textures are specified in order to control the degree of see-through ability of transparent parts. 

Both EDM'ing and blasting the surface of a mold can produce surface finishes. These finishes will, however, be uniform matte finishes of varying degrees of roughness. The PCE process is not limited to uniform matte finishes. This versatile process can reproduce the smooth graining of goatskin or the rough, 3-D shape of alligator leather. Aged wood, paint splatters, pebble, stipple, hair cell, fabric, wicker, geometric patterns, and varying degrees of matte finishes are all widely used. 

Pattern Selection
Selecting the surface finish to specify on a plastic part is an important decision. This determination must be based on the part's intended use. Industrial components and interior, nonappearance parts may require nothing more than stoning and smoothing out the grooves left by machining. Products destined for the consumer market require more careful consideration. 

The exterior surfaces of a product are important, as they are what the potential customer sees first. The size, shape, color, and surface finish of the part must first attract and then hold the customer's attention. These considerations take the designer out of the realm of engineering and into the field of marketing. Industrial designers are normally more attuned to current and future customer preferences. The insight of an industrial designer can be very helpful in selecting the optimal surface finish. First select among varying degrees of polish or texturing. The only justification for the added cost and time associated with texturing is that a finish will provide cost, functional, or marketing benefits. 

If the decision is to texture the part, the question then becomes which texturing method to use. There are exceptions but, generally, the cost and delivery time increases with the decision to go from EDM'ing to blasting to PCE'ing. All three techniques can provide varying degrees of matte finishes. The more detailed finishes, such as fabric or weathered wood, are available only with PCE'ing. 

If the decision is to use a PCE texture, the next question has to be which pattern. The PCE process is capable of duplicating any surface that can be photographed. New patterns are being created all of the time. However, the texturing companies already have thousands of patterns in inventory. Specifying one of these standard patterns minimizes both cost and delivery time. 

Texture suppliers provide molded plaques that show their most frequently used patterns. These plaques are very useful in selecting a specific pattern. These plaques are free for a phone call, but most good designers already have a collection of them. No supplier can afford to make plaques of all of the patterns that are available. This always leaves the designer with the thought that there may be a better pattern that he or she doesn't have a sample of. 

The answer to this dilemma is to review the project with a texture supplier. Another approach is to send the texturer a sample of fabric or a piece of wood, or a competitive product that has just the right finish. The texturer will then respond with whichever of his patterns comes the closest to that sample. 

The selection of a specific texture pattern must be done during the product design process, and before the part design phase of the project. The worst thing that can be done is to wait until the part design is finalized before selecting the patterns. The reason for choosing the pattern early is that the part design will have to adapt to the requirements of the chosen texture pattern. 

Draft Considerations
There are many benefits to be realized by providing a textured surface on a molded part. However, these highly desirable 3-D patterns have the disadvantage of forming undercuts on those surfaces that are perpendicular to a mold's parting line. Undercuts of this type can be seen in Figure 1, p. 60. These undercuts require more than the normal amount of molding draft angle in order to allow them to be demolded. This increase in draft angle is the reason why the surface finish has to be chosen before the part design is finalized. 

Draft angles affect part cost. Specifying the optimal draft angle is an important consideration for parts with textured or polished surfaces. (For more on draft angles, see "By Design," February 2000 IMM, pp. 52-57.) Many of the texture patterns have unique draft angle requirements. These differences originate with the varying shapes and depths of the individual patterns. Most of the texture plaques label each pattern with its identifying number and its depth. A few plaques also provide the recommended draft angle for each pattern. 

In general, textured parts must be designed with more draft than polished parts. The question then becomes how much draft is required. Each part must be considered individually, but the rule of thumb is a minimum of 11/2° per side for each .001 inch of texture depth (Figure 1). This general rule applies to the sidewalls of cavities that the material shrinks away from as it cools. 

The advent of high-speed cavity filling and thin-wall molding have resulted in the development of injection molding machines with injection pressure of more than 40,000 psi. These higher packing pressures result in less shrinkage. This, in turn, affects the way parts release from textured cavities. In these instances draft angles have to be increased. 

There is no general industry agreement for draft angles on inside surfaces that the material shrinks onto as it cools. Some texture suppliers ask for twice as much draft on inside surfaces. A safer approach is to avoid using textures on inside surfaces. 

Knowing the recommended draft angle is convenient, but there are two other major considerations. The design of the part and nature of the plastic material being molded must also be considered. 

Material Considerations
Thick-walled parts produced with high-mold-shrinkage plastics are easier to demold than thin-walled parts molded in low-mold-shrinkage materials. The very strong, rigid materials are more difficult to pull out of texture undercuts than softer, ductile plastics. 

There are hundreds of different plastic materials and thousands of texture patterns. Each combination of material and texture pattern has to be considered separately. This has made it difficult to establish general design guidelines for designing parts that are going to be textured. The best information on how to design for texturing is available from the texturing suppliers. 

Specifying Textures
Another consideration is how the texture is specified on the drawing. In most cases, it is identified as a pattern number in a general note. A note of this type, by itself, specifically indicates that all surfaces on the part are to be textured. When the designer means to texture only the appearance surfaces, this should be specified. By whatever method, the designer must indicate where the texture stops and starts. 

A good texturing note should indicate the pattern number, depth of the texture, its location, and-if appropriate-the direction of the pattern. 

Many patterns, including wood graining, are directional. In some cases, directional patterns can be aligned parallel to the direction the molded part moves during ejection. Aligning the texture in this direction minimizes demolding problems. 

The rush to be the first to market a new product has resulted in having several different toolmaking companies simultaneously building the molds required for an assembly. In those instances where several parts fit together in one product, the direction of the texture pattern absolutely must be specified. Failing to include this detail can result in unpleasant surprises when the different parts are assembled. 

Providing a textured surface on a part can create a pleasing appearance. However, there is more to texturing than meets the eye. This short review does not include all of the nuances required to best utilize the texturing process. The single most important thing that a designer can do to optimize the use of texturing is to review the project with a knowledgeable texture supplier before finalizing the part design. 

Editor's note: A one-day seminar covering the part design details presented in this series of "By Design" articles will be conducted by the author during the Appliance Manufacturers Conference & Exposition in Cincinnati, OH on September 11, 2000. For registration information, contact Bernice Sharpe (440) 349-3060, ext. 303; fax (440) 498-9121; or e-mail [email protected]