Page 111 - Injection Molding Advanced Troubleshooting Guide
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11.7 Orifice Size 97
3. Valve gates are used to direct the gate onto the part with minimal vestige and
are also used to control the flow when using multiple valve gates. Because they
can be shut off independently you can control flow fronts and knit line. When
doing a color change with a valve gate the previous color may stick to the pin
and continue to drag out. Cycling the valve open and closed can help with break-
ing free of the previous material or color.
11.5 Drooling, Stringing, and Sprue Sticking
Drooling from the hot drop tip or sprue is usually a result of a lack of cooling or
lack of bearing surface. Tip designs over the years have incorporated insulator
gaps and minimal contact with the tip to prevent freeze off and heat transfer to the
cavity blocks. In many cases and with some materials this is a good thing. But with
some materials an increased contact surface with the cavity steel along with cool-
ing lines around the hot drop is necessary to prevent drooling, stringing, and
sprues from sticking.
11.6 Freeze Off
Freeze off in the tip area is usually caused by a lack of heat or the orifice size being
too small. With some materials, especially semi-crystalline polymers such as
nylon, temperature control at the tip is critical. This is when you want minimal
contact surface with the tip and the cavity steel. Many hot runner suppliers use
different tip designs to increase temperature control at the orifice. Low vestige tips
have a pointed insert, typically called a spreader tip, that is designed to maintain
temperature control at the orifice to prevent freeze off. The location of this spreader
tip in relation to the orifice is critical and manufacturer specifications should be
followed.
11.7 Orifice Size
The orifice size will depend on the material, wall stock, and tip style being used. It
is important to keep an open mind with orifice sizes when addressing issues with
pressures, high gates, drooling, freeze off, and scrap.
