Chapter 17,Gate design overview

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.

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.