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Injection Molding 101
Published: Wed Nov 26, 2008 2:00 AM MST Before jumping into how Nordic ski gear is made, I thought it prudent to spend some time explaining several manufacturing techniques. These methods are used in almost every piece of gear we use. While eager to start explaining individual pieces of ski gear, having the back ground and basic understanding of manufacturing techniques will keep all of us speaking the same language and hopefully give the reader a better insight into the production and hidden costs of our beloved equipment.  Each mold starts out as two solid blocks of high-grade pre-hardened tool steel. The mold for the split rim weighs approximately fifty pounds. The cavity, or shape of the final piece is machined into these blocks along a parting line which is essentially the largest part of the finished piece. The parting line is evident on most IM parts by a thin ridge of plastic running all the way around the part such as a ski pole grip. On some parts, like pole baskets, the parting line is hidden along the edge.  Figure 1 - Both halves of the mold. The pins are retracted and the cooling ports on top have hose connectors. In addition to the shape of the cavity, the mold contains areas for the plastic to enter and fill the cavity. Ejection pins are used to push the cooled part out of the mold and are positioned to apply even pressure, and to minimize being seen. The ejection pin marks are the small, approximately ¼” diameter, circles found on IM parts.  Figure 2 - Mold with ejector pins out. Each IM mold is also made with positive relief, meaning there is a slight taper from the parting line down into the deepest recess of the cavity. If this relief was not present, parts would lock into the mold or be very difficult to remove. The mold maker/designer must also account for the shrinkage of the plastic as it cools. Because of shrinkage, the mold might not be the exact shape of the finished piece. The back-side of the mold has channels cut, allowing cooling water to circulate speeding the cycle time and preventing the mold from warping. Smaller molds are machined to fit into a larger carrier frame on the injection molding equipment. The cavity is roughed in using computer numerically controlled (CNC) milling equipment and carbide cutting tools before being sent to the polishing department for finishing.  Figure 3 - CNC milling machine. Operator's station on right. The inside of the molds have an almost mirror finish which helps the machinist inspect the cavity. If any texturing is needed it is added after the polished inspection. Steve was not sure of the exact time needed to create the mold for the Jenex split rim wheel but figured “…at least 60 hours goes into a mold of this size and complexity. And honestly, this is a simple mold. It’ll last for approximately 100,000 cycles before needing to be replaced.” Steve brings me into the molding room to the Nigata MD385Wi22 molding machine. At somewhere near twenty feet long, eight feet tall and six feet wide I am amazed at how clean and quiet it is while operating. The air handling system for the room creates more noise and there is only a slight hint of the odor of melting plastic.  Figure 4 - Nagata MD385. This MD385 is one of the new generation, energy efficient, plastic injection molding machines. According to Steve, this machine uses “Tens of thousands of dollars less of electricity each year than the older hydraulic machines.” Here’s how it works: The mold is placed into the carrier frame; one half is fixed to the injection ram, the other is stationary. Plastic beads (the Jenex rim uses a glass reinforced 612 nylon) are sucked through a hose and deposited into a metering hopper which determined how much plastic is injected per cycle. Any coloring or reinforcement is added here. The glass reinforcement in the Jenex rims make up 33% of the volume of the part and are approximately ¼” long and much finer than human hair. From the metering hopper, the plastic mixture moves by crew augur into the melting chamber. The augur moves and mixes the plastic as it transforms from room temperature beads into a homogenized liquid with a temperature of 500 degrees F. waiting its turn to be forced into the mold. The mold halves come together and are held with a clamping force of 385 tons. When the augur chamber moves into the mold the plastic is forced into the cavity at predetermined pressures between 8,000 and 30,000 PSI and held for one to three seconds while cooling water begins to circulate. Cooling takes about 30 seconds. The mold opens and the ejection pins push the cooled part out of the mold.  Figure 5 - Metering and additive hopper.  Figure 6 - Mold open and red hoses are for cooling. A human operator inspects the part and keeps or rejects it. The total cycle time is forty-five seconds. If the mold contains more than one cavity the pieces are attached together by short lengths of sprue which are trimmed off.  Figure 7 - sprue and taper. The part is now ready to be included as a component of the Jenex line.  Figure 8 - The finished split rims. This explanation of IM is oversimplified and intended to make future installments easier to read. While not as sexy as other portions of the equipment manufacturing spectrum, the next time you see thin ridges and circles on your gear you’ll have a better idea of how it came to be.  Figure 9 - And you thought they only made aspirin. 
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