How to Generate Runner Sizes for Plastic Injection Mold Design

The runner feeds the molten plastic from an injection machine into the mold to form the desired plastic part. The length of the runner is completely dependent on the part design, so there are no specific rules regarding overall length. It is the diameter of the runner that impacts part quality and cost and therefore it should be sized with some care. Once the runner freezes, no more plastic can be injected into the part. The runner should be sized to make sure that part receives sufficient plastic without excessive fractional heating, while freezing quickly enough to minimize cycle time.

Instructions

1.                            Calculating Runner Diameter

1

Determine thickest cross-section of the desired part.

2

Where part design allows, locate runner nearest to the thickest cross section of the part. If the part is of a uniform thickness, locate the runner near the center or along the longest edge, as part design allows.

3

Runner diameter should be between 75% to 100% of the part thickness where the runner is located.

4

Based on your injection machine type, determine the acceptable pressure drop for an injection cycle.

2.                            Calculate Injection Pressure Drop

5

Calculate runner volume. V = pi x r^2 x L, where pi = 3.14159, r = runner radius and L = runner length.

6

Calculate volumetric flow rate (Q) based upon your pre-determined fill time. The fill time is a function of material type, part thickness, and desired cycle time. The molder must know this independent of this exercise. Q = V/t, where t = seconds of fill time.

7

Calculate shear rate (S). S = 4Q/(pi x r^3)

8

Calculate the pressure drop (P). P = (S x m x 2L)/r, where m = melt viscosity of the plastic. The melt viscosity is a function of S, melt temperature, and the material. The molder must know this independent of this exercise, although many plastic suppliers can help with this.

9

Compare pressure drop to predetermined acceptable pressure drop. If improvement is desired, change the runner diameter and repeat the exercise. Repeat calculation until an acceptable runner diameter is found.

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