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Residues at the sprue protrude or drool drip and poor surface appearance
Jun 04, 2018

Problem: Residues at runners are drooling or drooling, and surface appearance is poor

The main reason is that the structure of the gate is unreasonable and the temperature is not properly controlled. There is a large residual pressure in the melt in the flow path after injection.


    (1) Improvement of the gate structure. Usually, if the length of the gate is too long, it will leave a long sprue on the surface of the plastic part. If the gate diameter is too large, drooling may easily occur. When the above failure occurs, the gate structure can be changed with emphasis. Hot runners Common sprue types include sprue, spot gate, and valve gate.

       The main gate, which is characterized by a large diameter of the runner, is not easy to condense at the gate, can guarantee the smooth injection of the melt of the deep cavity products; will not condense quickly, the residual stress of the plastic parts is minimum, and it is suitable for molding a multi-cavity Deep-cavity products, but such gates are more likely to cause turbulence and wire drawing, and the gate marks are larger, and even the pillar-shaped material is left, so the material temperature at the gate cannot be too high, and it needs to be controlled stably; The mouth features are basically the same as ordinary injection molding, but the marks on the plastic parts are relatively small; the feature of the spot gate is that the residual stress of the plastic parts is small, the condensation speed is moderate, and the flow and wire drawing phenomena are not obvious and can be applied to most Engineering plastics are also a type of gates used in hot runner molding at home and abroad. The quality of plastic parts is high and the surface has only minimal traces. Valve gates have small marks and low residual stress. The phenomenon of turbulence and wire drawing will occur, but the valve port wear will be more obvious. In the course of use, turbulence may occur with the increase of the fit clearance. At this time, the valve plug and valve body should be replaced in time.

    The choice of gate form is closely related to the properties of the resin being molded. Low-viscosity resin that flows easily, and valve gate can be selected. Crystal resin molding temperature range is narrow, the gate temperature should be appropriately higher, such as POM, PPEX resin can be used with a heated probe gate form. Amorphous resins such as ABS and PS have a wide range of molding temperatures. Due to the formation of a melt insulation layer on the tip of the torpedo, there is no heating element at the gate, which accelerates condensation.

    (2) Reasonable temperature control. If the amount of cooling water in the gate area is not sufficient, heat concentration will occur, resulting in flow, drooling, and wire drawing. Therefore, the cooling of the area should be strengthened when the above phenomenon occurs.

    (3) Resin pressure release. Excessive residual pressure in the flow path is one of the main causes of flow.

     Under normal circumstances, the injection machine should take a buffer back loop or buffer device to prevent the flow.

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