Compared with ordinary runner molds, hot runner molds have significant advantages such as time and material saving, high efficiency, and stable quality. However, they have been widely used due to their ease of use. With the technological progress of the mold industry, hot runner molding has made great progress in melt temperature control, structural reliability, and hot runner component design and manufacturing in the runner. This has made the hot runner technology regain the attention and favor of people. .
First, the residue at the sprue protrudes or drools and the surface appearance is poor
1. The main reason
Unreasonable gate structure selection, improper temperature control, and large residual pressure in the melt in the flow path after injection.
2. Solution
(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.
(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, when the above phenomenon occurs, the cooling in the area should be strengthened.
(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.
Second, the material discoloration, char or degradation
1. The main reason
Improper temperature control; too small flow channel or gate size results in greater shear heat generation; dead spots in the flow path cause the retentate to heat for too long.
2. Solution
(1) Accurate temperature control. In order to accurately and quickly determine the temperature fluctuations, the thermocouple temperature probe must be reliably contacted with the flow plate or the nozzle wall and placed in the center of each independent temperature control zone. The distance between the temperature point of the head and the flow channel wall is It should be no more than 10mm. The heating element should be as uniform as possible on both sides of the flow channel.
(2) Correct the gate size. The dead point of the flow path should be avoided as much as possible, and the diameter of the gate should be appropriately increased within the allowable range to prevent excessive shear heat generation. The radial temperature difference of the melt of the internal thermal nozzle in the flow path is large, and the phenomenon of coke material and degradation is more likely to occur. Therefore, it should be noted that the radial dimension of the flow channel should not be designed too large.
Third, the shortage of injection or no material injection
1. The main reason
Obstructions or dead spots occurred in the flow path; gates were blocked; thicker condensing layers appeared in the flow channels.
2. Solution
(1) When designing and processing the flow channel, it is necessary to ensure that the melt flows to the circular arc of the wall at the corner of the curve, so that the entire flow channel is smooth and there is no flow dead angle.
(2) Under the condition of not affecting the quality of plastic parts, the material temperature should be properly increased to avoid premature condensation of the gates.
(3) Appropriately increase the temperature of the hot runner to reduce the thickness of the condensing layer of the inner thermal nozzle and reduce the pressure loss, thereby facilitating the filling of the cavity.
Fourth, leakage is serious
1. The main reason
The sealing element is damaged; the heating element burns and the runner plate is unevenly expanded; the center of the nozzle and the gate sleeve are misaligned, or the projected area of ​​the melt insulation layer on the nozzle determined by the leak-proof ring is too large, causing the nozzle to retreat.
2. Solution
(1) Check the sealing element and heating element for damage. If there is damage, careful inspection before replacement is the result of component quality problems, structural problems, or normal service life.
(2) Select the appropriate leak stop method. According to the thermal insulation mode of the nozzle, the leakage prevention material can adopt two structures, namely, a leak-proof ring or a nozzle contact. Care should be taken to maintain the leak-proof contact area in a reliable contact state.
5. The hot runner cannot heat up or heat up too long
1. The main reason
Insufficient spacing of the lead channel leads to wire breakage; short circuit, leakage, etc. occur when the wires intersect when assembling the mold.
2. Solution
Select the correct processing and installation process to ensure that all the wires can be placed, and use high-temperature insulating materials according to regulations, and regularly check the damage of the wires.
Six, refueling or bad color change
1. The main reason
The method of refueling or changing the color is not proper; the unreasonable design or processing of the flow channel leads to the existence of more stagnant material inside.
2. Solution
(1) Improve the flow channel structure design and processing methods. When designing the flow path, the dead point of the flow path should be avoided as much as possible, and the transition of the circular arc should be made at each corner. Within the allowable range, the size of the flow channel is as small as possible, so that less material is retained in the flow channel and the flow rate of the new material is larger, which is favorable for rapid cleaning. When processing the flow channel, no matter how long the flow channel is, it must be processed from one end. If it is processed from both ends at the same time, the center of the hole will not be coincident, and thus the part of the material to be retained will be formed. Generally, the external heating nozzle can clean the flow passage easily because the heating device does not affect the flow of the melt, and the internal heating nozzle easily forms a condensation layer on the outer wall of the flow passage, which is not conducive to rapid refueling.
(2) Select the correct refueling method. Hot runner system refueling, color change process is generally directly from the new material to push out all the remaining material in the flow channel, and then the flow channel wall remnants of the overall forward movement, therefore, easier to clean. On the contrary, if the new material has a low viscosity, it is easy to enter the center of the retentate, and the retained material is separated layer by layer, which makes cleaning more troublesome. If the viscosity of the old and new materials is similar, rapid refueling can be achieved by accelerating the injection rate of the new material. If the viscosity of the retentate is more sensitive to temperature, the material temperature can be appropriately increased to reduce the viscosity to speed up the reloading process.
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