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Mold Polishing: Still a Viable Option By Steven J. Bales, President & Harry Raimondi, Technical Services Manager, Bales Mold Service, Downers Grove, ILToday, mold makers and molders are continuously on the lookout for the very latest coating technologies that will improve mold performance and help their customers get to market faster. It’s true that coatings play a significant role, especially with the ever widening variety of materials being molded and the inherent challenges they can present. But sometimes the latest, greatest solution to better mold performance lay right in your hands – or, rather, in the hands of a skilled mold polisher, that is. How does one know if polishing is the answer? It comes down to a couple of factors, not the least of which is what materials you are molding and what grade of tool steel you are using. Just as toolmakers say early communications from their customers helps them do their jobs better, the same can be said for mold polishers getting early communications from the toolmaker or molder. In fact, a marked blueprint is ideal and appreciated. Polishing in and of itself can be an excellent release aid. But it also can provide an excellent base for some coatings, adding additional benefits that coatings alone cannot provide. It helps to understand the different types of mold finishes and how they work. The most widely known finishes are categorized into four types: Diamond Polishing, Paper, Stone and Dry Blasted, and each of these have been broken down into more refined versions of itself. It would be difficult, if not impossible, for the naked eye to tell a difference. These are the standard SPI finishes used in the mold making and molding industries.
Because it produces a lens quality finish, A-1 Diamond Polishing is considered to be the best. It is ideal for any molded part that is meant to be decorative and highly visible. A diamond finish can be used with chrome or nickel coatings and can enhance release due to the high quality of the finish. It can also be the most costly finish due to the time and skill required to achieve it. A paper finish, produced using emery paper of different grits, is the second most refined finish. It is often recommended for electrical and automotive components or any product that does not require a visual impact because it can make the tool slightly reflective or glossy. Paper finishes provide good release. Stone finishes typically remove all tool and machining marks and provide a rougher surface finish than paper or diamond polishing. The stone finish is ideal for strictly functional molded parts where aesthetics is of no concern, and can also be used as a pre-finish before heat-treat. The molded part will experience adequate release and show minimal gloss or shine. Last is the Dry Blasted finish. This is commonly used to produce various textures and stipples on molded surfaces. By using different medias, such as glass bead or aluminum oxide, the dry blasted finish can vary the gloss or shine on the molded part. Because of its uniform, textured result, it is ideal for some decorative finishes and is great for camouflaging slight imperfections on the mold. Dry blast finishes also prevent fingerprint marks from occurring when handling plastic parts and they give good part retention on mold cores in lieu of using mechanical detents or undercuts. As mentioned earlier, there are many types of metals used in mold making, and each takes a finish differently. For example, P-20 grade steel offers the ability to accommodate a diamond finish. An A-1 diamond is possible, but the steel must be properly prepared. With 420 SS, you can achieve an excellent finish in all grades including optical when it is polished correctly. A good, commonly used steel that handles heat treat and EDM well is S-7. This steel can also establish a good finish in all polishing grades including possibly a lens quality A-1 finish. There are many different grades of aluminum, but generally speaking, an A-3 diamond finish is obtainable. On some grades an A-2 can be achieved but only with much work. Aluminum is not usually prone to pitting, but it can show grain. To really get the scoop on metals and polishing, consult with a reputable polishing vendor who knows his craft. An important additional point to make is that the time and money it takes to get the best quality finish for your customer’s tool depends on the quality of the finish on the tool to begin with. In other words, prepping a mold before it is sent to be polished can be a smart move. Whether a mill, lathe or EDM is used, take a little extra time to slow down the feed of the tool so that a smoother finish is created. This little “head start” will result in less time and effort on the part of the polisher and can save time and money. A book could be written about these finishes and how they work with certain tool steels, resins and coatings. The possibilities are endless. What is written here is meant only to provide a basic understanding of mold finishes so that the reader will be able to communicate more effectively with his polishing vendor and therefore provide customers with the very best quality tools and molded parts possible. Bales Mold Service, Inc., is an ISO 9002-certified provider of engineered finishes and coatings for the precision tooling industry and manufacturers worldwide. For more information, contact Bales Mold Service at 800-215-MOLD or visit us online at www.balesmold.com. Be a TPN Guest Speaker! If you'd like to submit an article for a future issue of the TPN, please contact Editor Cyndi Kustush at editor@tooling-product-news.com for editorial guidelines. Be sure to provide complete contact information and any proposed topics or ideas. |
