Gates in injection molding

Nozzle, mold-gating In injection molding, a nozzle whose tip is part of the mold cavity, thus feeding material directly into the cavity, eliminating the sprue and runner. 

Sprue Gating n (direct gating) In injection molding with single-cavity molds, particularly those with circular symmetry, filling from the center with the sprue connected directly to the gate and mold cavity. 

In injection molded composites of polypropylene containing short glass fibers, the fiber orientation depended on the flow pattern (which, in turn, is related to mold thickness, the position of the gate, and flow rate)." Substantial variation was detected along the thickness of the sample. Crystallites followed a pattern of fiber distribution but they grew in a direction perpendicular to the direction of the fiber and specimen surface. The direction of spherulite growth was different in neat resin where crystallites grew parallel to the surface of the mold (specimen). 

Table 6.25. Relationship between Gate Size and Table 6.26. Gate Locations in Injection Molding the Part Thickness ...

To minimize pressure and heat losses, the runners should be round and as short as possible. Another preferred choice is trapezoid runners that are easy to machine and provide the same amount of resistance to flow as round runners. They should blend smoothly into the gates, without abrupt cross-sectional changes. The runner s length should be reduced and its diameter increased when the thickness of the molded part increases. A runner diameter of at least 6 mm is required for a 10 mm thick part. Thicker parts require runners that are 50%-100% larger in diameter than the thickness of the part. As in injection molding, pressure drop and material loss are de-... 

Center-gated mold n. In injection molding, a mold in which each cavity is fed through an orifice at the center of the cavity. This type of gating is employed for items such as cups and bowls. 

Runner system n. This term is sometimes used for the entire mold feeding system, including sprue, runners, and gates, in injection or transfer molding. 

Short shot n. In injection molding, failure to fill all cavities of the mold completely, caused by too low melt temperature, too low injection pressure, insufficient plasti-cation time, too constricted gates, too viscous resin, inadequate venting of cavities, etc. Short shots are often made deliberately when testing a new multicavity mold to reveal the pattern of runner flow and the sequence of cavity filling. 

Tab gate n. In injection molding, a small removable tab of approximately the same thickness as the molded item usually located perpendicularly to the item. The tab is used as a site for edge-gate location, typically on items with large flat areas. 

Barrel temperature in injection molding should gradually increase from the hopper to the nozzle. It should be controlled according to the injection molding machine, shape and size of the product, and the mold structure. Injection pressure should be controlled to mold friU parts with acceptable appearance. Many parameters affect injection pressure, which include injection temperature, product shape, nozzle and gate size, runner dimensions, and mold temperature. It is important that injection pressure should drop off to holding pressure immediately after filling up. 

Figure 1 Two types of weldlines in injection-molded parts (mold-filling patterns in double-gated rectangular cavities), (a) Melt fronts flowing in opposite directions. (b) Mold with two gates side by side. Weldlines are indicated by dashed lines.

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