Mold runner

Shear rate When, a melt moves in a direction parallel to a fixed surface, such as with a screw barrel, mold runner and cavity, or die wall, it is subject to a shearing force. As the screw speed increases, so does the shear rate, with potential advantages and disadvantages. The advantages of an increased shear rate are a less viscous melt and easier flow. This shear-thinning action is required to move the melt. 

Injection Mold Runner Filling at a Constant Flow Rate Using the viscosity data and runner dimensions used in Example 14.1 calculate the required injection pressure to fill the entire runner at a constant rate of 1.2 x 10 6mVs. 

Filling of an Injection Mold Runner of Noncircular Cross Section Consider the filling of a runner, the cross section of which is formed by three rectangle sides and a semicircle. The filling takes place at a constant applied injection pressure of... 

Ram and piston injection molding units are used in the rubber iudustry [57]. The rubber stock is fed to a heated cyliuder, which is heated to the required temperature aud from there is forced by a hydraulic ram through a nozzle, mold runners, and... 

Several other types of encapsulants have been evaluated for use by the semiconductor industry. Polyphenylene sulfide, a thermoplastic, has the advantage of good thermal characteristics and a low viscosity for device encapsulation. In addition, since it is a thermoplastic, the molded runners can be reused. This improves the material utilization and reduces cost. Unfortunately, this material also contains a significant amount of impurities that caused device reliability problems and efforts to remove them have met with mixed success. 

The sprue is the channel from which the resin will flow through the gate and into the mold. Runners are perpendicular to the direction of draw, and the sprue is parallel to the direction of draw. This portion of the system is often recycled because typical OEMs allow up to 15% of regrind material (the use of polymer salvaged from the runner or sprue and not used in the original part). When injection molding takes place, automotive thermoplastics must be cooled under the Tm or T before ejection. 

Situations where the polymer is subjected to shear forces are the ones most commonly considered. Shear forces are imposed on polymer melts by rotating screws and by flow through dies. In theory the polymer touching the metal surfaces is stationary, and hence a vclocit> gradient occurs between fixed and moving parts, and between the center and edges of a die channel or mold runner as shown in Fig. H. 

When a melt moves in a direction parallel to a fixed surface, such as with a screw barrel, mold runner, or die wall, it is subject to a shearing force. As the screw speed increases. 

As can be seen from Fig. 5, the EYC model allows excellent fits to typical melt flow curves. For this reason, the model has found widespread use in calculations such as in the modeling of flows in molds, runners and dies. We can show that the flow rate-pressure drop relation for the EYC fluid (with rjoo = 0) under steady laminar flow in a pipe of radius R is given by... 

Typical runner sizes range fi-om 4mm to 7nun (0.157 to 0.276 in.) in diameter. Diameters above 7mm (0.276in) tend only to increase the cycle time and do not significantly assist in ihe filling of the mold. Runner lengths should be kept as short as possible and suitably sized ejectors positioned at runner junctions to ensure that the feed system is removed from the mold without difficulty. 

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