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.
Polishing the cores and cavities of a new tool is one of the last things that a moldmaker does. This is the finishing touch that will determine the molded part’s appearance. The degree of polish will affect the see-through ability of a transparent part, or how an opaque surface catches and reflects light. The richness of the color of a pigmented part will change with different degrees of polish. These are important considerations, as it is the surface of a product that a potential customer sees first. There must be something in the appearance of the product that encourages a person to take a second look, or to pick up the product. The feel of the product in the hand will enhance or distract from the impression created by the product.
The interior components of a product or parts destined for the industrial market have different surface finish requirements from consumer products. Approximately 25,000 new products were introduced in the U.S. in 1997, and with today’s market cycles that number has undoubtedly increased. Many of these products have similar shapes and features. The visual impression, or the feel created by a product’s surface finish, can make the difference between a successful sale and a failure. No new product is a success unless it sells and money changes hands.
The designers of plastic products are very knowledgeable regarding the marketplace advantages of specifying the ideal surface finish. Selecting the optimal surface finish is challenging, as customer preferences are continually changing. A few years ago, a nonreflecting flat black was the preferred finish for high-tech products. Today, Apple Computer is setting a new trend with a transparent matte finish that allows the customer to see the product’s interior components. This same transparent matte finish is now beginning to appear on irons, flashlights, radios, and printer housings.
Changing Appearance Requirements
The degree of polish on a part is a major contributor to the product’s appearance and marketplace acceptance. It is the product designer’s responsibility to choose and identify the degree of polish, but design engineers are not well informed on how to specify the chosen surface finish. There are many interrelated factors to be considered. Many of these factors are not well defined or described in the literature available to designers.
The surface finish of a molded plastic part is dictated by the polish on the mold’s core and cavity. It is also affected by the type of plastic and the way that material is molded.
Some design engineers have more experience with metal than plastic products. With metal, the specified surface finish is the finish that will be on the finished part. In the case of plastics, the specified finish will be on the mold, but the surface of the molded part may be different.
All of these variables have resulted in the common practice of submitting the part drawing for cost quotations with no surface finish specification. Other drawings contain notes such as "diamond polish," "Class A finish," "high polish," or "matte finish." These designations are inadequate. A moldmaker needs to know specifically what degree of polish will be required in order to prepare an accurate proposal. Hardened, dense metal can be polished to a higher level than softer metals. A high level of polish may require that the mold be built using hardened tool steel, instead of a less costly prehardened steel.
Creative designers have the ability to see things that do not exist. They can create a mental image of a product that is nicely styled with smoothly blended, free-flowing contours. The appearance surfaces of that part might have a rich orange color with a smooth finish. The appearance surface has to be smooth, but not so lustrous that it reflects light. The designer can see exactly what he wants in his mind’s eye, but it is difficult to specify that particular surface finish on a drawing.
Communications between product designers and moldmakers have always been, and continue to be, problematic. Developing a mutual understanding of the surface finish required on both the mold and the molded product is one of these recurring problems.
SPI Mold Finish Guide
In 1962, the Moldmaking Divs. of the Society of Plastics Engineers (SPE) and the Society of the Plastics Industry (SPI) cooperated in establishing mold finish standards for the injection molding industry. This standard specified six finishes, ranging from a high diamond polish to a rough, blasted surface. This standard was controlled by a set of six steel disks containing each of the six finishes.
These finish standards were very helpful in allowing moldmakers and their customers to agree on the polish that would be provided on a new mold. However, there still were differences between the finish on the steel mold and the finish on a part produced by that mold. The misunderstandings created by these differences were minimized in the mid-1970s when SPI introduced an injection molded plaque incorporating the six standard finishes. Designers could then see what the finishes looked like on a molded part. Polish could be specified according to the same numbers one through six that the moldmaker had on his six metal disks. The system worked well. It was widely promoted and well accepted by the moldmaking industry and, eventually, the product design community.
In the mid-1980s, SPE discontinued all standards-setting activities. Fortunately, SPI continues to make the mold finish standards available.
While reintroducing the standard on its own, SPI, based on the experience gained, changed the word "standard" to "guide" and expanded it to include 12 finishes (see Figure 1). The original six SPE/SPI mold finishes were encompassed within the new guide. The SPI Mold Finish Guide has become the internationally accepted standard for specifying surface finishes. Product designers would be well advised to use these guidelines while specifying surface finishes. It also would be a good idea to buy extra plaques for your suppliers.
The last "By Design" article dwelt on the importance of providing molding draft angles on part drawings. The point was made that generous draft angles reduce part cost and increase part quality.
Draft angles and mold polishing go hand in hand. Drafting the sidewalls of a deep draw, 3-D part reduces the force required to pull that part out of the cavity and push it off of the core. The benefits to be derived from draft angles will be lost, however, if those drafted surfaces contain gouges left by machining or coarse polishing. These gouges in the metal of the core and cavity form undercuts. Plastic melt flows into these undercuts and solidifies. This situation increases the force required to eject the part from the mold. This increase in ejection force results in a corresponding increase in the cooling portion of the molding cycle. The benefits to be derived from draft angles will be maximized by polishing the molding surfaces to remove these undercuts.
Product design engineers have strong opinions regarding the finish to be provided on the appearance surfaces of a 3-D part. They are less concerned about the inside nonappearance surfaces of the part. The majority of part drawings that specify a polish are referring to only the appearance surfaces of the part. It is unusual for a design engineer also to specify the degree of polish on the inside, nonappearance surfaces. With no finish specification, the moldmaker polishes these surfaces only enough to get by. This is a common but unfortunate situation that complicates the ejection process.
As a plastic material cools, it also shrinks. This shrinkage pulls the part away from the cavity to grip the core tightly. The maximum demolding force is normally that which is required to eject the part from the core. If the core that forms the inside surfaces is not smoothly polished, the ejection forces and the cycle time will increase accordingly.
There is always a last-minute rush to get a new tool finished and delivered. Polishing is one of the final functions and it is not always done as well as it could be. The cavities that form ribs or bosses and small-diameter core pins often have rough surfaces left by machining or coarse polishing. These rough surfaces increase the force required to demold the part.
In some instances, the functional requirements of a product do not allow for adequate molding draft angles. In these cases, smoothly polished molding surfaces become even more important. Polishing is normally done in a random pattern. Even a high polish leaves behind microscopic gouges or scratches in the surface of the metal. Those scratches, which are parallel to the mold’s parting line, become undercuts that increase the ejection forces.
A single scratch is not significant, but thousands of them will increase the required ejection force. Polishing the mold only in the direction that the molded part moves during ejection eliminates these undercuts. This polishing technique is called "draw polishing." Design engineers would be well advised to specify draw polishing on parts requiring minimal or no draft angles.
Some plastic materials produce glossier surfaces better than others. Regardless of the finish on the mold, low-density polyethylene can never produce the shiny appearance of a polystyrene part. Heavily filled or reinforced materials cannot reliably produce shiny surfaces. Generally speaking, a hot mold will produce better surfaces than a cold mold. A poorly packed out part will not faithfully replicate the mold’s polish on all of the part’s surfaces.
With the vast majority of plastic materials, ejection forces will be minimized by providing smoothly polished surfaces on the mold. There are always exceptions. The softest polyethylenes and polyvinyl chlorides, polyurethanes, and some thermoplastic elastomers have a tendency to adhere to highly polished metal surfaces. Sticky materials of this type will normally release better from molding surfaces with a matte or lightly blasted surface finish.
Strong but somewhat flexible plastics, such as polypropylene, ABS, and impact styrene, are more tolerant of roughly polished molding surfaces than rigid, brittle materials, such as general-purpose styrene and acrylics. Draw polishing is very beneficial with these hard, brittle materials.
Design engineers must be careful to specify a surface finish that is suitable for the plastic material to be molded. Product designers, moldmakers, and polishing companies cannot be expected to know the idiosyncrasies of all of the different plastic materials. Plastic material suppliers and experienced molders can, however, be relied upon to know what mold finish is optimal for a given material.
Polishing Cost Considerations
Many parts of the moldmaking process have been mechanized or automated. Polishing is an exception. It is only the simplest of shapes that can be automatically polished. Polishers now have better equipment and materials to work with, but the process remains labor intensive.
Polishing is a step-by-step process. The polisher starts with files or coarse abrasive, 320-grit stones, which are then followed by repolishing with progressively finer materials, down to an 8000-grit diamond polishing compound. The higher the finish, the more polishing steps. Each additional polishing step represents additional cost.
There is no such thing as an average cost for polishing a mold. Some industry insiders have estimated that, on the average, polishing can represent 10 percent of a mold’s overall cost. This average will obviously vary, depending on the size and shape of the mold, the hardness of the metal, the level of polish required, and how well the mold was finished prior to polishing. Transparent molded parts require a high polish on both the core and the cavity. Optical lens molds are in another whole class by themselves.
Polishing represents a significant part of a new mold’s cost. Among the variables cited here, it is the level of polish being specified that has the greatest effect on the cost of polishing. The level of surface finishing on injection molded parts is frequently overspecified. In many instances, this overspecifying comes from the concept that "my part deserves the best finish that man can provide."
This attitude results in unnecessary added cost. Most consumers will not notice the differences between the SPI Guide’s three highest finishes on an opaque part. The two best SPI finishes should be reserved for transparent, see-through parts. In other words, why spend money on a surface finish that the consumer cannot appreciate? Overspecifying the surface finish on a molded part also results in other less obvious increases in cost. Highly polished surfaces reflect light that highlights surface imperfections. Flow marks, splay, weldlines, and minor scratches or abrasions become more obvious and result in a higher reject rate and increased part cost.
Overspecifying the surface finish not only raises a mold’s initial cost, it also increases the ongoing cost of maintaining that mold. Highly polished metal surfaces do not last forever. Molding surfaces are susceptible to corrosion, abrasion, and mechanical abuse. Highly polished surfaces will magnify these defects. Stainless steel and chrome-plated molds resist corrosion. Hardened steel molds are more tolerant of abrasion and mechanical damage. In spite of these safeguards, highly polished molds do require repolishing.
Selecting the optimal surface finish for a new injection molded part must be based on the careful consideration of three interrelated factors: (1) The marketplace requirements; (2) the efficient molding of the part; and (3) the cost of the mold and the molded parts. Between these three factors, marketplace acceptance has top priority. Cost and ease of molding will cease to be concerns if the product is unsuccessful in attracting the attention of the consumer.
The product designer is the expert on what degree of polish will charm potential customers. It is the designer’s responsibility to specify that surface finish accurately and to communicate that information thoroughly to the molder, moldmaker, and polisher. There is no substitute for a face-to-face discussion with suppliers, but that is not always possible in today’s global economy.
At the very least, the surface finish must be clearly indicated on the part drawing. However, it is strongly recommended that surface finish be specified according to SPI’s Mold Finish Guide. This Guide has now become international in scope. In fact, even those in a small shop in Korea will know what an