MoldMaking Technology

OCT 2017

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Design 16 MoldMaking Technology —— OCTOBER 2017 is compromised, leading to a potential performance failure. All injection molding thermoplastic materials shrink when they cool. As this occurs, the polymer chains try to return to their relaxed state. The combination of orientation, compression and cool- ing prevent the polymer chains from returning fully to that state. Mold Design Mold design, specifically gates and cooling channels, is another factor that has a huge impact on stress within a molded part. Gate size, location and quantity for a mold must be properly designed or the shear heating and stress on the material can exceed rec- ommended ranges quickly. In other words, dimensions, tolerances, per- formance criteria, material limitations (temperature, shear stress, shear rate) and processing parameters (flow rates, pressures, cooling) of a mold require consideration. Otherwise, that material will be compromised, which breaks the molecular chain and leads to part failure. Use the equations in Figure 2 to cal- culate the requirements necessary to avoid shear degradation and stress at the gate for a specific resin and geom- etry. Each material has shear rate and shear stress limits. The equation focus- es on shear rate. With known param- eters, such as part weight, an engineer can select fill times, size and number of gates to design a gate that will not exceed the material manufacturer's recommendations. The equation also gives the molder a process window. For the molder to make fill-time adjustments for cosmetic or machine performance requirements, the mold designer must select the proper size and number of gates. Inappropriate gate size can compro- mise effective pressure transfer from the molding machine to the end-of-fill in the cavity. This includes undersized gating. Incompatible gate size causes problems like voids, sink, warp, dimensional incon- sistency or short shots in molded parts. Use these equations to calculate the requirements necessary to avoid shear degradation and stress at the gate for a specific resin and geometry. FIGURE 2 Q = One Cavity Volume / (Fill Time x Number of Gates/Cavity) Round = 32 x Q Square = 6 x Q (P x Gate ø 3 ) (Gate Width x Gate Thickness 2 ) Key P = Pi or 3.14 ø = diameter 2 = squared 3 = cubed SWT 150 830-1745 LM-B 210 830-1735 Exceptional laser systems, made in Italy and supplied by Gesswein. Our sales team will help find the best laser for your application. SWA 150 830-1710 800.243.4466 www.gesswein.com

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