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As the material flow enters the runner system of a mold from the sprue, it has already started to form a skin of colder material all around itself. As the material pushes towards the gate, the skin on the end continues to thicken as it cools. If the gate is positioned directly at the end of the runner, the incoming material must break through the thickened skin to fill and pack the part. This causes many cosmetic problems, some of which are shear splay, gate blush, and in some cases, excessive gas burns caused by inadequate removal of gas before it enters the mold.
By positioning the gates at a right angle to the runner, the thicker portion of the skin formed, and most of the gases, pass beyond the gate into the cold slug area. It is important that the ends of the runners are well vented to ensure adequate gas
removal.
-S. Hatfield, Accent Plastics, Corona, CA, (909) 273-7777.
There are three main reasons for placing the gate at right angles to the runner: (1)the more homogeneous the temperature of the plastic material that's in the cavity, the less residual internal stress. Internal stress can cause warping and part life span reduction. When the material flows through the cold runner, it cools the initial front of the material significantly. Removing this by extending the runner beyond the gate greatly helps homogenization; (2) air in the runner should always be vented. Venting runner air through the mold is poor practice. Gating at right angles improves runner venting; (3) it contributes to the best cosmetic
results.
-I. Rubin, consultant, Brooklyn, NY, (718) 258-7363.
A gate located at right angles to the runner is useful when molding certain materials. Letting some material flow beyond the gate lets the runner have an extra cold slug, which can prevent snaking, gate blush, and some other cosmetic problems. I have used the "bypass runner" method when molding ABS and HIPS in cosmetic
applications.
-T.Lynn, GE Appliances, Louisville, KY, (502)452-4193, tom.lynn@appl.ge.com
I feel compelled to correct some misconceptions on right angle gate placement. When shear-sensitive thermoplastic resins flow down a runner, the tip of the flow front is actually hotter than the sides. Thus, the "cold slug" is actually a hot slug. If the runner in a multicavity mold must gates located at right angles, all gates should be located at right angles with "hot slugs" at the ends. This will give more even filling because the tip of the flow front meets a blind alley instead of another cavity.
If, on the other hand, the runner can be designed without 90º angles (straight in), the cavities will fill better. Also, the runner should be vented at various points down its length, not just at the
tip.
-F. French, Amitima Inc., Manchester, MO.
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