Mold-gate location

Figure 3 Thickness of the plaque varies significantly depending on the location of measmement with respect to the molding gate location. Contour m of the thickness show consistent pattern as that of the LET contour m. ...

To meet the products functional needs the wall thickness of the part can be varied. However, to obtain optimum processing cycles and reduce the chance of sink- and flow-mark formation, wall thickness should be kept as uniform as possible. Where variation in wall thickness is unavoidable, gradual transitions in the wall thickness should be provided to ease the polymer melt flow transitions. Part design, mold design, and gate location should work together to allow the material from heavier section into thinner walls for best appearance and process latitude. 

Common problems like insufficient filling-packing and poor dimensional control are often related to the gate size and design. Similarly, gate location is another important factor. They should be located in areas having heaviest cross-section of the part to assure fill-out and elimination of sink marks. Also their position should not facilitate the residual molded stress formation in the part, knit line formation. 

Figure 1 details the 3-dimensional nature of the linear CTE s for an LCP molded part. These linear CTE s can be very dependent upon part geometry, gate location, and molding conditions. From reference f71 and Figure 1, the following definitions can be made ... 

Diaphragms or ring gates can be used for molding parts where concentricity is important such as cylindrical parts or parts where weld lines can not be tolerated. Pinpoint gates should not be used unless parts (small) are injected rapidly. Tunnel gating can also be used in such cases. Gate location should be at one of the points listed in Table 6.26. 

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

Typically, the selection of gate location and the decision between cold runner molds (two-plate or three-plate molds) or hot runner molds are important to the product design when considering the mold cost and the plastic flow within the mold. Today, about 90% of all molds built are still two-plate molds, which are considerably cheaper than three-plate or hot runner molds. For many reasons, in most cases, hot runner molds are better, but these questions must always be asked ... 

There are many considerations in specifying the shape of the counterbore. In molding, every hole in a plastic piece is created by a pin (the core pin). The core pins are often subjected to unbalanced side forces, caused by high injection pressures and poor flow conditions in the mold as a result of the gate location, which will deflect these pins. 

A small removable tab of approximately the same thickness as the mold item, usually located perpendicular to the item. The tab is used as a site for edge gate location, usually on items with large flat areas. 

SO the mold designer can lay out the mold accordingly. This issue can normally be addressed with the following specification Gate location must be approved by engineering. 

The diameter D) of a round gas channel should not exceed 1 in (Z) < 1 in). Gas channels in wall sections are determined experimentally and are very much a function of the wall thickness. The height H) of the gas channel should not exceed 3 to 4 times the nominal wall T) of the part (if s 3 to AT). The width (W) should not exceed 2.5 to 3 times the nominal wall (T) (W < 2.5 to 3T). The basic rules regarding uniform wall and inside radii outlined in Sec. 8.2 apply to gas-assisted injection molded parts, except for the gas channels. However, careftil consideration must be given to gate location, gas nozzle location, and channel path location to assure proper application of the principle. The prospective molder should be consulted during the design phase to get it right. 

PTA and the customer engaged early in the project to determine material selection. The decision must be based on chemical resistance, material strength, and weight. To ensure all critical conditions are met, PTA performed a complete Design for Manufacturability review on every part, which included draft angle, wall thickness, gate location, and mold flow analysis. 

A stripper plate mold is a two-plate mold in which the core and ejector plates are combined into a stripper plate. It is used for thin wall parts and parts with symmetry and no undercuts. A three-plate mold has two parting Unes, thereby providing automatic separation of the parts and runner system. This does not work with all materials brittle materials tend to fracture on ejection. It also limits the selection of gate locations and does not eliminate scrap. A stack mold is used to mold two layers of parts without increasing clamp force. Because stack molds are not as precise as two-plate molds, they are used for low-tolerance parts such as polystyrene drinking cups. 

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