What determines the gate location of an injection molded product?

The gate location determines the flow direction, where weld lines arise and, with fiber-filled materials, the fiber orientation and thus the strength. Gating is preferably done in or near the thickest zone, not on a visible surface or functional sealing surface.

What is mold-flow analysis and why does QDP always do this?

Mold-flow analysis simulates how the plastic fills the mold before any steel is machined. This allows us to see filling problems, weld lines, and air entrapments in advance and let us adapt the gate location and wall thickness accordingly, rather than finding this out in the mold.

What is a weld line and where does it form?

A weld line forms where two melt fronts meet, for example behind each hole or in the case of multiple gates. At that line, the material is somewhat weaker; for glass-filled grades sometimes significantly weaker. The position can be controlled via the gate location and wall thickness distribution.

Design for Manufacturing / Injection Point and Mold-Flow

DfM Deep Dive

Injection point and mold-flow: Where the melt enters determines quality and cost

The injection point determines flow direction, weld lines, material orientation, and leaves a visible mark. Where and how the material enters the mold determines weld lines, air entrapment, and dimensional stability. We always verify this with mold-flow analysis before any steel is milled.

You never choose the injection point on a visible area or on a functional sealing surface. Preferably, you inject in or near the thickest zone, so the melt flows from thick to thin and the hold pressure works longest where the shrinkage is the greatest.

Teaching plate injection point and mold-flow: injecting on the visible area leaves a visible mark and weld line, at the edge the weld line remains unobtrusive

What the injection point influences

Effect	Why it matters
Flow direction	Determines whether thin, remote sections still fill well before the melt solidifies
Weld line position	Behind every hole and every core, the flow front meets again; that weld is weaker and sometimes visible
Fiber and shrinkage direction	For glass-filled materials, the injection point determines fiber direction and thus strength anisotropy
Visible mark	The injection point itself leaves a trace on the product

Controlling weld lines rather than hoping

The melt follows the thickest path. A locally slightly thicker "flow leader" attracts the front, a thinner zone slows it down. This way, you can shift a weld line by several centimeters without changing the injection point. Residual strength at the weld line for unfilled materials (PP, ABS, PC) is about 80 to 100 percent of the base material; for glass-filled grades such as PA6 GF30 it can drop to 40 to 60 percent, as the fibers do not cross the weld line.

In every DfM review, we indicate the expected weld line positions and check them against load and visibility. If in doubt, we follow up with a mold-flow simulation.

Gate location is closely linked with the parting line and mold construction: see the full DfM pillar or go to parting line and mold construction.

What does this mean for your design?

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