Compared with ordinary flow channel molds, hot runner mould have significant advantages such as saving time and materials, high efficiency, and stable quality. However, its wide application has been affected by their failure in use. With technological advancement of mold industry, hot runner mould has made great progress in flow channel melt temperature control, structural reliability, hot runner component design and manufacture, which makes hot runner technology regain people's attention and favor.

 

Gate structure selection is unreasonable, temperature control is improper, and there is a large residual pressure in the melt of flow channel after injection.

1) Improvement of gate structure. Usually length of gate is too long, which will leave a long gate material on the surface of plastic part. If gate diameter is too large, it will easily lead to phenomenon of drooling. When above failure occurs, it is important to consider changing gate structure. Common gates for hot runners are direct gate, point gate and valve gate .
2) Reasonable control of temperature. If amount of cooling water in gate area is insufficient, it will cause heat to concentrate, causing runny, dripping and drawing. Therefore, cooling of this area should be strengthened when above phenomenon occurs.
3) Resin pressure release. Excessive residual pressure in flow path is one of main causes of runny. In general, injection machine should adopt a buffer circuit or device to prevent runny.

 

Improper temperature control; size of runner or gate is too small to cause large shear heat generation; dead point in runner causes retaining material to be heated for too long.

1) Accurate control of temperature. In order to accurately and quickly measure temperature fluctuation, thermocouple temperature measuring head should be reliably contacted with flow channel plate or nozzle wall, placed at the center of each independent temperature control zone. Distance between head temperature sensing point and flow channel wall should be no more than 10mm, and heating elements should be evenly distributed on both sides of flow channel.
(2) Correct gate size. Runner point should be avoided as much as possible, and gate diameter should be appropriately increased within allowable range to prevent excessive shear heat generation. Melt of inner hot nozzle has a large temperature difference in radial direction of flow passage, which is more likely to cause phenomenon of coke and degradation. Therefore, it should be noted that radial dimension design of flow passage should not be too large.

Obstructions or dead spots appear in flow path; gate is blocked; a thicker condensation layer appears in flow path.

1) When designing and processing runner, transition of melt to arc of wall at the corner should be ensured, so that entire runner is smooth without flow dead angle.
2) Appropriately increase material temperature without affecting quality of plastic parts, and avoid premature condensation of gate.
3) Appropriately increase hot runner temperature to reduce thickness of condensation layer of internal heat nozzle and reduce pressure loss, thereby facilitating filling of cavity.

Sealing element is damaged; heating element is burnt and causes flow plate to expand unevenly; nozzle and center of sprue sleeve are misaligned, or projected area of melt insulating layer on nozzle determined by leakage preventing ring is too large, causing nozzle to retreat.

1) Check sealing and heating element for damage. In case of damage, careful inspection before replacement is a component quality problem, a structural problem, or a result of normal service life.
2) Choose appropriate leak stop method. Depending on the way nozzle is insulated, leakage can be prevented by a leak stop ring or a nozzle contact. Care should be taken to maintain a reliable contact between leak-proof contact areas.

Wire channel spacing is not enough, causing wire to break; when mold is assembled, wire intersects with short circuit and leakage.

Choose correct processing and installation process, ensure that all wires can be placed, use high-temperature insulation materials as required, and regularly detect damage of wires.

 

Improper method of refueling or color change; unreasonable design or processing of runner leads to more stagnant material inside.

1) Improve structural design and processing of runner. When designing flow channel, runner dead point should be avoided as much as possible, stress at each corner should be circular. Within scope of permit, size of runner is as small as possible, so that less material is retained in runner and flow rate of new material is larger, which is beneficial for quick cleaning. When processing flow path, no matter how long flow path is, it must be processed from one end. If it is processed from both ends at the same time, it is easy to cause center of hole to not coincide, thus retentate portion is inevitably formed. Generally, external heating nozzle can clean flow passage relatively easily because heating device does not affect melt flow, inner heating nozzle easily forms a condensation layer on outer wall of flow passage, which is disadvantageous for quick refueling.
2) Choose correct refueling method. Hot runner system refueling and color changing process generally pushes out all retentive materials in flow channel directly from new material, and then moves retaining material on flow channel wall as a whole. Therefore, cleaning is relatively easy. On the contrary, if viscosity of new material is low, it is easy to enter center of retentate, retentate is separated layer by layer, which is troublesome to clean. If viscosity of new and old materials are similar, fast refueling can be achieved by speeding up injection rate of new material. If viscosity of retentate is sensitive to temperature, temperature of material can be appropriately increased to lower viscosity to speed up refueling process.