jetting - Plastics Training Centerpl .Page 39 Jetting Notes Gate Thickness Plastic leaving a gate

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Transcript of jetting - Plastics Training Centerpl .Page 39 Jetting Notes Gate Thickness Plastic leaving a gate

  • Page 37

    Jetting

    Notes Jetting

    I. Example of Jetting

    Jetting

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

    II. How the Problem Develops

    Jetting occurs during mold filling when plastic squirts or jets through thegate and on into the cavity without hitting any obstruction. Eventually,the plastic forms a flow front and the cavity is filled. But in themeantime, the surface of the jet of plastic has cooled slightly, and someof it is pushed to the surface. The appearance defect is the jet of chilledplastic 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 format 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 anobstruction. This will stop the jet from forming.

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

    B. Gate thickness:

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

  • Page 39

    Jetting

    Notes Gate Thickness

    Plastic leaving a gate will typically expand to about twice the gatethickness. If this expansion is enough so that the plastic touches bothwalls 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 plasticflow 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 thecavity wall thickness

    plasticmelttemperature

    plastic flow rate

    Jetting