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moldmakingtechnology.com 21 on the raw material's performance in the molding process. This performance can ultimately impact many part properties, including surface quality, knit line strength, part ejection force (sticking), dimensions, tolerances, total cycle time and overall mechanical properties. Terminology Injection molders expect mold builders to be experts in the raw materials used in their molds and to have relationships with material suppliers. This expertise leads to molds optimized for high-volume production with large processing windows, which results in on-time delivery of high-quality parts that meet proj- ect requirements at lower cost. In order to meet this demand to be well-informed about the raw materials, it is important for moldmakers to understand and apply the following terms in their day-to-day mold-design and building processes: Shear is the change in viscosity brought on by one layer of polymer sliding against another layer of polymer. Shear occurs in multiple locations within the molding process, including: 1. Injection mold machine screw, barrel and nozzle. 2. Sprue bushing, cold or hot runner system. 3. G ate point on the part (typically, the highest shear is at the gate). 4. Part geometry of the cavity. Some poor injection molding results such as splay, gate blush, halo, degraded material, and brown or color streaking are tied directly to uncontrolled shear. Only area number 1 is controlled by the molder; the other three areas are the mold builder's responsibility. Knowing and understanding flow, shear, viscosity and the polymer limits can help determine hot runner drop location, gate type, gate sizing, venting and vent placement, etc. Viscosity and viscosity curves refer to a polymer's "molten thickness" or resistance to flow. High viscosity means less fluidity (flows like concrete), while low viscosity equals more fluidity (flows like water). Understanding this gives the mold builder a point of reference for best practices in mold design and the relative tolerances needed to manufacture the mold for the specific applications and raw materials. For example, too much clearance in the components or vents for a low-viscosity (high-flow) polymer could lead to flash. Melt Flow Index (MFI) and Melt Flow Rate (MFR) are two specific tests used to determine the ease of flow for the given polymer at very low shear rates. These tests were developed to check the flow behavior consistency of a polymer from one sample to another, but they can be misleading if used to com- pare flow characteristics of two different polymers at the high shear rates normally achieved in injection molding. Next Step The goal of this background on raw material properties is to provide insight on how they interact with one another and, ultimately, how they influence the molded product. Understanding and applying this knowledge is the next step in designing and building molds for performance and reliability. In this way, a mold builder can eliminate potential problems before they get to the injection molder, becoming a more val- ued partner in the complex injection molding process. The mold is one part of the entire injection molding process that needs to interact seam- lessly with the other components. It can't limit the process window or impact production uptime in a negative way. It also must meet market demand on price point, tool quality and lead time. Simple, right? The mold's value should be measured by its direct influence on the total net profit of the molded piece part, sustained over the life of the mold or the length of the part program. This thinking requires a cultural shift within the plastics industry. So are you up for the challenge of change? CONTRIBUTOR Rich Oles has had direct roles in moldmaking, mold design, IT, hot runner manifold, hot runner control and injection molding, and currently is working with a Tier II injection molder in Zeeland, Michigan. Scott E. Powers is techni- cal service manager for Ascend Performance Materials. FOR MORE INFORMATION: Rich Oles / 616-610-7050 / richoles@gmail.com Ascend Performance Materials / 616-307-9069 spower@ascendmaterials.com / ascendmaterials.com LEARN MORE Raw Material Extra Visit short.moldmakingtechnology.com/ p2praw for more on the information presented here. 1000 100 10 Shear Viscosity Curves for Two Grades of Nylon 6,6 Polymer A Polymer B MFR/MFI Shear Region Injection Molding Shear Region Viscosity 10 100 1000 10,000 100,000 From Pellet To Part Shear Rate Graph courtesy of Ascend Performance Materials.