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Gate design overview

2016/11/2 10:58:29

What is a gate?

A gate is a small opening (or orifice) through which the polymer melt enters the  cavity. Gate design for a particular application includes selection of the gate  type, dimensions, and location. It is dictated by the part and mold design, the  part specifications (e.g., appearance, tolerance, concentricity), the type of  material being molded, the fillers, the type of mold plates, and economic  factors (e.g., tooling cost, cycle time, allowable scrap volume). Gate design is  of great importance to part quality and productivity.

Single vs. multiple gates

You’ll usually have better success with a single gate, unless the length of the  melt flow exceeds practical limits. Multiple gates always create weld and meld  lines where the flows from the separate gates meet. Except for long, narrow  parts, a single gate into the body of the part (as opposed to an edge gate) will  assure more uniform distribution of material, temperatures, and packing, and  better orientation effects. While a single gate into the body of the part might  incur a higher initial tool cost, lower scrap rates and higher part quality will  quickly justify this expense.

Gate dimensions

The cross section of the gate is typically smaller than that of the part runner  and the part, so that the part can easily be “de-gated” (separated from the  runner) without leaving a visible scar on the part. The gate thickness is  usually two-thirds the part thickness. Since the end of packing can be  identified as the time when the material in the gate drops below the freeze  temperature, the gate thickness controls the packing time. A larger gate will  reduce viscous (frictional) heating, permit lower velocities, and allow the  application of higher packing pressure for a longer period of time. Choose a  larger gate if you’re aiming for appearance, low residual stress, and better  dimensional stability.

Figure 1 below illustrates the terms we use to describe gate size.

1. Gate size terminology

Gate location

Select a gate location that will ensure rapid and uniform mold filling. Position  weld lines and air/gas vents so they have the least effect on the appearance and  strength of the part. Since gates are locations of high residual stress,  position them away from areas that will experience high external stress during  use.

Position the gate away from load-bearing areas.

The high melt pressure and high velocity of flowing material at a gate cause the  area near a gate to be highly stressed.

Position the gate away from the thin section areas, or regions of sudden  thickness change.

This will avoid Hesitation or Sink marks and voids.

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