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.
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