Gate location

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. 

Consider a straight tubular runner of length L. A melt following the power-law model is injected at constant pressure into the runner. The melt front progresses along the runner until it reaches the gate located at its end. Calculate the melt front position, Z(f), and the instantaneous flow rate, Q t), as a function of time. Assume an incompressible fluid and an isothermal and fully developed flow, and make use of the pseudo-steady-state approximation. For a polymer melt with K = 2.18 x 10 N s"/m and n = 0.39, calculate Z(t) and Q(t)... 

The synthesis is carried out in a cascade of three consequtive reactors 9 of the same type. The parent substances are continuously sent through collector 8 into first reactor 9 from there, the products of the reaction flow into the second and the third reactors. The given level in the first two reactors is supported by pouring the products through the piping for the third, a pressure regulator is used with a gate located on the outlet pipe. Strict observance of the level is necessary to keep the calculated synthesis time. 

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 ...

Sink marks Melt temperature too high Insufiticient material injected Insufiticient dwell time Premature gate Ireezing Sharp variations in wall thickness Wrong gate location Part ejected too hot Cavity pressure too low... 

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