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What are the reasons for the poor appearance of the surface of the hot runner mold?
Jun 08, 2018

Hot Runner Mold, Residues at gates, or drooling, poor surface appearance


1. 1 The main reason

Unreasonable gate structure selection, improper temperature control, and large residual pressure in the melt in the flow path after injection.


1. 2 Countermeasures

(1) Improvement of the gate structure. Usually, if the length of the gate is too long, a long gate material will be left on the surface of the plastic part. If the gate diameter is too large, drooling phenomenon 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 mainstream gate, which is characterized by a large diameter flow channel, so the gate is not easy to condense, can guarantee the smooth injection of deep cavity products; will not quickly condense, the minimum residual stress of plastic parts, suitable for molding a multi-cavity Deep-cavity products, but this kind of gate is more likely to produce runny and wire drawing phenomenon, and the gate marks are large, even leaving the column material, so the material temperature at the gate can not be too high, and the need for stable control; At the same time, the residual marks on the plastic parts are relatively small; the characteristic is that the residual stress of the plastic parts is small, the condensing speed is moderate, and the flowing and drawing phenomenon are not obvious; it can be applied to most engineering plastics, and is also the current domestic and international hot runners. A type of gate with more moldings is used, the quality of plastic parts is higher, and only traces are left on the surface; there are small traces and low residual stress, and no runny or wire drawing occurs, but the valve port wear is more Obviously, in the course of use, there will be flow phenomenon with the increase of the fit clearance. At this time, the spool 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, selectable valve gate. Crystal resin molding temperature range is narrow, the temperature at the gate should be appropriately high, such as POM, PPEX and other resins can be used with a heated probe gate form. Amorphous resins such as ABS, PS, etc. have a wide range of molding temperatures. Since the head of the torpedo core forms a melt insulation layer, there is no heating element contact at the gate, so the condensation can be accelerated.


(2) Reasonable temperature control. If the amount of cooling water in the gate area is insufficient, 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 circuit or buffer device to prevent flow.

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