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Analysis of common faults in hot runner system

Date:2021-06-08View:413

One,The residue at the gate is prominent or drooling and the appearance is poor


  1. Main reason


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


  2. Solutions


  1) Improvement of gate structure. Generally, the length of the gate is too long, it will leave a longer gate material on the surface of the plastic part, and the gate diameter is too large, it is easy to cause the phenomenon of drooling. When the above problems occur, you can focus on changing the gate structure. The common gate methods of hot runners are straight gate, point gate and valve gate.


   2) Reasonable temperature control. If the amount of cooling water in the gate area is not enough, it will cause heat concentration, resulting in drooling, dripping and wire drawing. Therefore, the cooling of this area should be enhanced when the above phenomenon occurs.


   3) Relief pressure by resin. Excessive residual pressure in the runner is one of the main reasons for the formation of salivation. Under normal conditions, the injection machine should adopt a buffer circuit or buffer installation to avoid salivation.


   2. The material is discolored, burnt or degraded


   1. Main reasons


   Improper temperature control; too small runner or gate size causes greater shear and heat generation; dead spots in the runner cause the retained material to be heated for too long.


  2. Countermeasures


   1) Precise temperature control. In order to accurately and quickly measure temperature fluctuations, the thermocouple temperature measuring head must be firmly contacted with the runner plate or nozzle wall, and be located at the center of each independent temperature control zone, the temperature sensing point of the head and the runner wall The interval should not be greater than 10mm, and the heating elements should be evenly distributed on both sides of the flow channel.


   (2) Correct the gate size. Try to prevent the dead point of the runner, increase the gate diameter appropriately within the allowable range, and avoid excessive shearing and heat generation. The internally heated nozzle has a large temperature difference in the radial direction of the flow channel, which is more likely to cause scorching and degradation. Therefore, it is necessary to pay attention to the design of the radial size of the flow channel not to be too large.


   Three, the injection volume is short or out of no material


   1. Main reasons


   There are obstacles or dead corners in the runner; the gate is blocked; there is a thick condensation layer in the runner.


  2. Countermeasures


  1) When designing and processing the flow channel, ensure that the melt flows to the arc transition of the wall at the corner, so that the entire flow channel is smooth and there is no flow dead angle.


   2) Without affecting the quality of the plastic parts, appropriately increase the material temperature to prevent premature condensation of the gate.


  3) Appropriately increase the temperature of the hot runner to reduce the thickness of the condensate layer of the internally heated nozzle and reduce the pressure loss, thereby facilitating the filling of the cavity.


  Four, serious leakage


   1. Main reasons


  The sealing element is damaged; the heating element is burnt to cause uneven shrinkage of the runner plate; the nozzle is misaligned with the center of the sprue sleeve, or the projection area of the melt insulation layer determined by the stop ring on the nozzle is too large, causing the nozzle to retreat.


  2. Countermeasures


   1) Check whether the sealing element and heating element are damaged. If there is damage, carefully check whether it is a component quality problem, structural problem, or the result of normal service life before replacement.


   2) Choose an appropriate way to stop leakage. According to the heat insulation method of the nozzle, two structures can be adopted to prevent leakage of the material, the leak-proof ring or the nozzle contact. Attention should be paid to keep the leak-proof contact part in a firm contact state.


   5. The hot runner cannot be heated up normally or the heating time is too long


   1. Main reasons


  The distance between the wire channels is not enough, causing the wire to break; when the mold is assembled, the wires intersect and cause short-circuit, leakage and other phenomena.


  2. Countermeasures


  Choose the correct processing and device technology to ensure that all wires can be placed, and use high-temperature insulation materials as required, and regularly check the damage of the wires.


   Six, poor material or color change


   1. Main reasons


   Improper material or color changing methods; unreasonable runner design or processing leads to more retained material inside.


  2. Countermeasures


  1) Improve the structural design and processing method of the runner. When designing the runner, try to prevent the dead center of the runner and the arc transition of the stress diagram at each corner. Within the permitted range, the size of the flow channel should be as small as possible, so that there is less retained material in the flow channel and the new material flow rate is large, which is conducive to rapid cleaning and cleaning. When processing the flow channel, no matter how long the flow channel is, the processing must be stopped from one end. If the two ends are processed at the same time, it is easy to form a non-overlapping hole center, which will inevitably constitute a retained material position. Ordinary external heating nozzles can easily clean the flow channel because the heating installation does not affect the movement of the melt, while the internal heating nozzle is easy to form a condensation layer on the outer wall of the flow channel, which is not conducive to rapid material change.


   2) Choose the correct refueling method. The hot runner system material change and color change process generally push all the retained materials in the runner directly from the new material, and then move the retained materials on the wall of the runner forward as a whole, so the cleaning is easier to stop. On the contrary, if the viscosity of the new material is low, it is easy to enter the center of the retained material, separate the retained material layer by layer, and it will be more troublesome to clean. If the viscosities of the new and old materials are similar, quick material change can be accomplished by increasing the injection speed of the new material. If the viscosity of the retained material is more sensitive to temperature, the material temperature can be increased appropriately to reduce the viscosity to speed up the refueling process.