Compared with ordinary runner molds, hot runner molds have significant advantages such as time saving, material saving, high efficiency, and stable quality. With technological progress of mold industry, hot runner molding has made great progress in runner melt temperature control, structural reliability, design and manufacture of hot runner components, etc., which makes hot runner technology regain people's attention and favor. .
In design and application of hot runner molds, there are many problems worthy of consideration and attention. How well these problems are solved is directly related to success or failure of hot runner system and quality of products. Therefore, it is undoubtedly very helpful to successful application of hot runner molding technology to discuss failures of hot runner system and their causes, and to understand the matters that should be paid attention to in application of hot runner molding.

2.1.1 Main reasons
Gate structure selection is unreasonable, temperature control is improper, and there is a large residual pressure in melt in runner after injection.
2.1.2 Solutions
(1) Improvement of gate structure.
Usually, length of gate is too long, which will leave a long gate handle on the surface of plastic part, and diameter of gate is too large, which will easily lead to occurrence of drooling. When above failure occurs, it is important to consider changing gate structure. Common gate forms for hot runners include sprue gate, point gate and valve gate.
Main runner gate is characterized by a relatively large diameter of runner, so gate is not easy to condense, which can ensure smooth injection of melt of deep cavity product; it will not condense rapidly, and residual stress of plastic part is the smallest, which is suitable for forming deep-cavity products with multiple cavities in one mold. However, this gate is prone to drooling and wire drawing, gate residue is large, and even columnar material is left behind. Therefore, material temperature at gate should not be too high, and it needs to be controlled stably;
Characteristics of direct gate are basically same as those of ordinary injection molding, but residual marks on plastic parts are relatively small; characteristics of point gate are that residual stress of plastic parts is small, condensation speed is moderate, phenomenon of drooling and wire drawing is not obvious; it can be applied to most engineering plastics, it is also a type of gate type that is widely used in hot runner molding at home and abroad.
Valve gate has small residual marks and low residual stress, does not produce drooling and wire drawing, but valve port wear is obvious, and drooling phenomenon will occur with increase of fitting clearance during use. At this time, valve core and valve port body should be replaced in time.
Choice of gate form is closely related to properties of resin being molded. For low viscosity resins prone to drooling, valve gate options are available. Molding temperature range of crystalline resin is narrow, and temperature at gate should be appropriately high. For example, resins such as POM and PPEX can use a gate with a heating probe. Amorphous resins such as ABS, PS, etc. have a wide range of molding temperature. Since head of torpedo nozzle core forms a melt insulation layer, there is no heating element contact at gate, so condensation can be accelerated.
(2) Reasonable control of temperature.
If cooling water in gate area is not enough, it will cause heat concentration, resulting in salivation, dripping and wire drawing. Therefore, cooling in this area should be strengthened when above phenomena occur.
(3) Resin pressure relief.
Excessive residual pressure in runner is one of main causes of salivation. In general, injection machine should adopt a buffer circuit or buffer device to prevent salivation.

 

2.2.1 Main reasons
Improper temperature control; too small runner or gate size causes greater shear heat generation; dead spots in runner lead to excessive heating of retained material.
2.2.2 Solutions
(1) Accurate control of temperature.
In order to measure temperature fluctuations accurately and quickly, thermocouple temperature measuring head should be reliably contacted with runner plate or nozzle wall, it should be located in the center of each independent temperature control area, distance between temperature sensing point of head and runner wall should be no more than 10mm, heating elements should be distributed evenly on both sides of runner as much as possible.
Temperature control can choose intelligent fuzzy logic technology under operation of central processing unit, which has function of temperature over-limit alarm and automatic adjustment, so that melt temperature change can be controlled within required accuracy range.
(2) Correct gate size.
Dead center of runner should be avoided as much as possible, gate diameter should be appropriately increased within allowable range to prevent excessive shear heat generation. Melt of internal heating nozzle has a large temperature difference in radial direction of runner, which is more prone to coke and degradation. Therefore, it should be noted that radial size of runner should not be designed too large.

2.3.1 Main reasons
Obstacles or dead ends in runner; gate blockage; thick condensation layer in runner.
2.3.2 Solutions
(1) During design and processing of runner, arc transition of melt flow to wall at the corner should be ensured, so that the entire runner is smooth and there is no dead angle of flow.
(2) Appropriately increase material temperature without affecting quality of plastic parts to avoid premature condensation of gate.
(3) Appropriately increase temperature of hot runner to reduce thickness of condensed layer of internal heating nozzle and reduce pressure loss, which is conducive to filling cavity.

2.4.1 Main reasons
Sealing element is damaged; heating element is burnt causing uneven expansion of runner plate; center of nozzle and sprue sleeve is misaligned, or projected area of melt insulation layer determined by o-ring on nozzle is too large, causing nozzle to retreat.
2.4.2 Solutions
(1) Check whether sealing element and heating element are damaged. If there is any damage, carefully check whether it is quality problem of element, structural problem, or result of normal service life before replacement.
(2) Select an appropriate leak-stopping method. According to the thermal insulation method of the nozzle, two structures can be used to prevent leakage of material, either a leak-proof ring or a nozzle contact. Care should be taken to keep leak-proof contact parts in a reliable contact state.
Within allowable range of strength, it is necessary to ensure that projected area of melt between nozzle and sprue sleeve is as small as possible to prevent nozzle from retreating due to excessive back pressure during injection. When leak-stop method is adopted, direct contact area between nozzle and sprue sleeve should ensure that resin leakage will not occur when centers of the two are misaligned due to thermal expansion. However, contact area should not be too large, so as not to increase heat loss.

 

2.5.1 Main reasons
Distance between wire channels is not enough, which leads to wire breakage; when assembling mold, wires intersect and short-circuit, leakage and other phenomena occur.
2.5.2 Solutions
Select correct processing and installation process to ensure that all wires can be placed, use high-temperature insulating materials as required, and regularly check damage of wires.

2.6.1 Main reasons
Improper method of material change or color change; unreasonable runner design or processing leads to a lot of retained material inside.
2.6.2 Solutions
(1) Improve structural design and processing method of runner. When designing runner, dead point of runner should be avoided as much as possible, and arc transition should be sought at each corner. Within the allowable range, the size of the runner should be as small as possible, so that retained material in runner is less and flow rate of new material is larger, which is conducive to quick cleaning.
When processing runner, no matter how long runner is, it must be processed from one end. If it is processed from both ends at the same time, it is easy to cause misalignment of center of hole, which will inevitably form a retained material part. Generally, outer heating nozzle can easily clean runner because heating device does not affect flow of melt, while inner heating nozzle is likely to form a condensation layer on outer wall of runner, so it is not conducive to rapid material change.

(2) Select correct refueling method. In the process of material change and color change in hot runner system, new material is generally pushed out of all retained materials in runner, and then retained material on the wall of runner is moved forward as a whole, so cleaning is easier. On the contrary, if viscosity of new material is low, it is easy to enter center of stagnant material, and stagnant material is separated layer by layer, which is more troublesome to clean. If viscosities of old and new materials are similar, rapid material change can be achieved by increasing injection speed of new material. If viscosity of retentate is sensitive to temperature, viscosity can be reduced by increasing temperature of material appropriately to speed up refueling process.

 

In order to eliminate or reduce failures in use as much as possible, following matters should be paid attention to when selecting and applying hot runner system.

(1) Internal heating method. Internal heating nozzle has a complex structure, high cost, difficult replacement of parts, and high requirements for electric heating elements. Placing heater in the middle of runner will generate annular flow, increase friction area of volume, and pressure drop may be three times that of externally heated nozzle.
However, since heating element for internal heating is set in torpedo body in nozzle, all heat is supplied to material, so heat loss is small and electric energy can be saved. If a point gate is used, tip of torpedo body is kept in the center of gate, which is convenient for gate to be cut off after injection, and residual stress of plastic part is lower due to late condensation of gate.

Use of externally heated nozzles can eliminate cold film and reduce pressure loss. At the same time, due to its simple structure, convenient processing, and advantages of thermocouple being installed in the middle of nozzle to ensure accurate temperature control, it has been widely used in production at present. However, heat loss of external heating nozzle is large, and it is not as energy-saving as internal heating nozzle. 3.2 Selection of gate form
Design and selection of gates directly affect quality of plastic parts. When applying hot runner system, appropriate gate form should be selected according to resin flow properties, molding temperature and product quality requirements to prevent drooling, dripping, material leakage and poor color change.

When gate form is determined, control of melt temperature fluctuation will play a key role in quality of plastic parts. In many cases, phenomenon of coke, degradation or runner blockage is caused by improper temperature control, especially for heat-sensitive plastics, which often require rapid and accurate response to temperature fluctuations.
For this reason, heating element should be reasonably set to prevent local overheating, ensure matching gap between heating element and runner plate or nozzle, and minimize heat loss.

(1) Temperature and pressure balance calculation of each branch channel. Purpose of hot runner system is to pass hot plastic injected from injection molding machine nozzle through hot runner at same temperature and distribute melt to each gate of mold with a balanced pressure, so temperature distribution of heating zone of each runner and pressure of melt flowing into each gate should be calculated.
(2) Calculation of center offset of nozzle and sprue sleeve due to thermal expansion. That is, centerlines of hot (expanded) nozzle and cold (non-expanded) sprue sleeve should be properly positioned and aligned.
(3) Heat loss calculation. Internally heated runner is surrounded and supported by cooled die sleeve, so amount of heat loss due to heat radiation and direct contact (conduction) should be calculated as accurately as possible, otherwise, actual runner diameter will be reduced due to thickening of condensation layer on runner wall.

 

Two aspects of thermal insulation and injection pressure should be fully considered. Spacers and supports are usually arranged between runner plate and mold plate, which can withstand injection pressure on the one hand, so as to prevent runner plate from deforming and cause leakage, and on the other hand, it can also reduce heat loss.

For hot runner molds, it is very important to regularly perform preventive maintenance of hot runner components during use. This work includes electrical testing, inspection of sealing components and connecting wires, and cleaning of component dirt.