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Molding defect: Hesitation

2017-7-7

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Molding defect- Molding Hesitation
Solving molding hesitation problems
Definition
Hesitation is when flow slows down or stops along a particular flow path.
If plastic filling a cavity has the option of filling either a thin section or a thick section, the plastic will tend to fill the thick section first as this route offers less resistance to flow. This can result in plastic in the thin section stopping or slowing significantly. Once the plastic starts to slow down, it will cool more rapidly, so the viscosity will increase. This higher viscosity will inhibit flow further causing even faster cooling and so the problem is self propagating.

Hesitation can occur in ribs and in thin section of parts that have significant changes in wall thickness.
In the image above, the rib circled in red, offers a higher resistance to flow because it is much thinner than the rest of the part.
Look at these animations to see how hesitation in a part can be avoided by using a different
injection location. When there is no alternative route available, the flow in the rib will be
continuous and not hesitate as seen in the right hand example.Hesitation can reduce part quality due to variation in surface appearance, poor packing, high
stresses and non-uniform orientation of the plastic molecules. Alternatively, if the hesitation allows the flow front to freeze completely, part of the cavity may remain unfilled (short shot).
Viewing the fill time and temperature results may help explain why the hesitation occurred.

The fill time plot will show hesitation by a very narrow spacing of fill time contours. The
temperature plot will show hesitation by a low temperature and a large temperature gradient.

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