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### Transcript of jetting Jetting is a surface appearance defect caused during cavity filling. It is also called jet...

• Page 37

Jetting

Notes Jetting

I. Example of Jetting

Jetting

Jetting is a surface appearance defect caused during cavity filling. It is also called jet marks or worm tracks.

II. How the Problem Develops

Jetting occurs during mold filling when plastic squirts or jets through the gate and on into the cavity without hitting any obstruction. Eventually, the plastic forms a flow front and the cavity is filled. But in the meantime, the surface of the jet of plastic has cooled slightly, and some of it is pushed to the surface. The appearance defect is the jet of chilled plastic at the surface of the part.

• Injection Molded Part Problems & Solutions Study Guide

Notes

Page 38

What we want to happen during the start of filling, is a flow front to form at the gate and the cavity to fill as an unbroken flow front.

III. Cause of Jetting

A. Unrestricted flow through the gate at the beginning of cavity filling.

IV. General Solution to Jetting

A. Restrict the initial plastic flow into each cavity.

V. Mold and Part Design Solutions

A. Gate placement:

1. Gates should be placed so that the plastic entering the cavity hits an obstruction. This will stop the jet from forming.

a. The obstruction can be a wall, a pin, or any interference that will stop the jet.

B. Gate thickness:

1. A gate thickness of one half to two thirds the wall thickness will usually stop any jetting.

• Page 39

Jetting

Notes Gate Thickness

Plastic leaving a gate will typically expand to about twice the gate thickness. If this expansion is enough so that the plastic touches both walls of the cavity, the jet will be stopped and a flow front will form.

VI. Operating Solutions

Problem Analysis

A. Reduce plastic flow rate - A jetting problem is caused by a high plastic flow rate when the plastic first enters the cavity.

1. Reduce the initial fill rate (on machines with programmed injection).

T

2T

Gate size 1/2 to 2/3 the cavity wall thickness

plastic melt temperature

plastic flow rate

Jetting