Analysis and Countermeasures of Common Faults in Hot Runner System of Injection Mold

1. Introduction
Compared with the common runner mold, the hot runner mold has the obvious advantages of saving time and materials, high efficiency, stable quality, etc., but it was once easy to cause failures in use and affected its wide application. With the technological progress of die industry, hot runner molding has made great progress in the aspects of runner melt temperature control, structural reliability and hot runner component design and manufacturing, which makes hot runner technology regain people's attention and favor.
In the design and application of hot runner mold, there are many problems worth considering and paying attention to. The solution of these problems is directly related to the success of hot runner system and product quality. Therefore, it is undoubtedly very helpful for the successful application of hot runner molding technology to discuss the faults and causes of hot runner system and understand the matters needing attention in the application of hot runner molding.
2. Analysis and countermeasures of common faults of hot runner system of injection mold
2.1 The residue at the gate is protruding or salivating, and the surface appearance is poor
2.1.1 Main reasons
The selection of gate structure is unreasonable, the temperature is not controlled properly, and there is a large residual pressure in the melt in the runner after injection.
2.1.2 Solutions
(1) Improvement of gate structure.
Generally, if the length of the gate is too long, it will leave a long gate handle on the surface of the plastic part, and if the gate diameter is too large, it will easily lead to salivation and material dropping. When the above faults occur, we can focus on changing the gate structure. The common gate forms of hot runner include straight gate, point gate and valve gate.
The main runner gate is characterized by large runner diameter, so it is not easy to coagulate at the gate, which can ensure the smooth injection of the melt of deep cavity products; No rapid condensation, minimal residual stress of plastic parts, suitable for forming deep cavity products with the first mock examination and multiple cavities, but this kind of gate is prone to salivation and wire drawing, and the gate residue is large, even leaving a cylindrical handle, so the material temperature at the gate cannot be too high, and needs to be controlled stably;
The characteristics of the straight gate are basically the same as that of the ordinary injection molding, but the residual marks on the plastic parts are relatively small; The characteristics of the point gate are that the residual stress of the plastic part is small, the condensation speed is moderate, and the salivation and wire drawing are not obvious; It can be applied to most engineering plastics, and is also a kind of gate form that is widely used in hot runner molding at home and abroad at present. The quality of plastic parts is high, and there is only a small trace on the surface;
The valve gate has small residual mark, low residual stress, and will not produce salivation and wire drawing. However, the valve gate is obviously worn, and salivation will occur with the increase of fitting clearance in use. At this time, the valve core and valve body should be replaced in time.
The choice of gate form is closely related to the properties of the resin to be molded. For low viscosity resin prone to salivation, valve gate can be selected. The molding temperature range of crystalline resin is narrow, and the temperature at the gate should be appropriately high. For example, POM, PPEX and other resins can adopt the gate form with heating probe. The molding temperature range of amorphous resins, such as ABS and PS, is wide. Because the melt insulation layer is formed at the head of the torpedo nozzle core, and there is no heating element contact at the gate, it can accelerate the condensation.
(2) Reasonable control of temperature.
If the cooling water in the gate area is not enough, it will cause heat concentration, resulting in salivation, material dripping and wire drawing. Therefore, the cooling in this area should be strengthened when the above phenomena occur.
(3) Resin pressure relief.
Excessive residual pressure in the channel is one of the main reasons for salivation. Generally, the injection machine should adopt a buffer circuit or buffer device to prevent salivation. 2.2 Material discoloration, coke or degradation
2.2.1 Main reasons
Improper temperature control; Too small runner or gate size causes large shear heat generation; The dead point in the flow passage causes the retained material to be heated for a long time.
2.2.2 Solutions
(1) Accurate control of temperature.
In order to measure the temperature fluctuation accurately and quickly, it is necessary to make the thermocouple temperature measuring head reliably contact the runner plate or nozzle wall, and make it at the center of each independent temperature control area. The distance between the head temperature sensing point and the runner wall should be no more than 10mm, and the heating element should be evenly distributed on both sides of the runner as far as possible.
The intelligent fuzzy logic technology operated by the central processor can be selected for temperature control. It has the function of temperature overrun alarm and automatic adjustment, which can control the melt temperature change within the required accuracy range.
(2) Correct the gate size.
The dead point of the runner should be avoided as much as possible, and the gate diameter should be appropriately increased within the allowable range to prevent excessive shear heat generation. The melt of the internal-heated nozzle has a large radial temperature difference in the flow channel, which is more prone to coke and degradation. Therefore, it should be noted that the radial size of the flow channel should not be too large.
2.3 Injection quantity shortage or no material injection
2.3.1 Main reasons
Obstacles or dead corners appear in the flow channel; The gate is blocked; A thicker condensate layer appears in the flow channel.
2.3.2 Solutions
(1) During the design and processing of the flow channel, the arc transition of the melt flow to the wall at the corner shall be ensured, so that the whole flow channel is smooth and there is no flow dead angle.
(2) Under the condition that the quality of plastic parts is not affected, the material temperature shall be properly increased to avoid premature solidification of the gate.
(3) Properly increase the temperature of hot runner to reduce the thickness of the condensation layer of the internal-heated nozzle and reduce the pressure loss, thus facilitating the filling of the cavity.
2.4 Serious material leakage
2.4.1 Main reasons
The sealing element is damaged; The heating element is burnt, causing uneven expansion of the runner plate; The nozzle is misaligned with the center of the sprue sleeve, or the projected area of the melt insulation layer determined by the leakage ring on the nozzle is too large, resulting in the nozzle retreat.
2.4.2 Solutions
(1) Check whether the sealing element and heating element are damaged. If there is any damage, carefully check whether it is caused by the quality problem, structural problem or normal service life of the element before replacement.
(2) Select an appropriate leak stop method. According to the thermal insulation mode of the nozzle, two structures can be used to prevent leakage, namely, the leakage ring or the nozzle contact. Attention shall be paid to keep the leakage-proof contact parts in a reliable contact state.
Within the allowable range of strength, it is necessary to ensure that the projected area of the melt between the nozzle and the sprue sleeve is as small as possible to prevent excessive back pressure from causing the nozzle to retreat during injection. When the leakage-proof method is adopted, the direct contact area between the nozzle and the sprue sleeve shall ensure that the resin leakage will not occur when the two centers are misaligned due to thermal expansion. However, the contact area should not be too large to avoid increasing heat loss.
2.5 The hot runner cannot be heated normally or the heating time is too long
2.5.1 Main reasons
The space between the wire channels is not enough, which leads to the wire breaking; Short circuit, electric leakage and other phenomena occur when the wires intersect during mold assembly.
2.5.2 Solutions
Select the correct processing and installation process to ensure that all wires can be placed, and use high-temperature insulating materials as required to regularly detect the wire damage.
2.6 Poor material change or color change
2.6.1 Main reasons
Improper method of changing material or color; The unreasonable design or processing of the runner leads to a lot of retained material inside.
2.6.2 Solutions
(1) Improve the structural design and processing mode of the runner. When designing the flow channel, the dead point of the flow channel should be avoided as much as possible, and the arc transition should be made at each corner. Within the permitted range, the size of the flow channel should be as small as possible, so that the flow rate of the retained material in the flow channel is small and the flow rate of the new material is large, which is conducive to rapid cleaning.
When processing the runner, no matter how long the 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 the misalignment of the hole center, which will inevitably lead to the formation of the material retention position. Generally, the external heating nozzle can easily clean the flow channel because the heating device does not affect the melt flow, 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 refueling.
(2) Select the correct refueling method. During the refueling and color change process of the hot runner system, the new material is generally directly pushed out of all the retained materials in the runner, and then the retained materials on the runner wall are moved forward as a whole, so the cleaning is relatively easy. On the contrary, if the viscosity of the new material is low, it is easy to enter the center of the retained material and separate the retained material layer by layer, which is more troublesome to clean. If the viscosity of the new and old materials is similar, the new material injection speed can be accelerated to achieve rapid refueling. If the viscosity of the retained material is sensitive to temperature, the viscosity can be reduced by properly increasing the material temperature to speed up the refueling process.
3. Precautions for selection and application of hot runner
In order to eliminate or reduce the faults in use as much as possible, the following items should be noted when selecting and applying the hot runner system.
3.1 Selection of heating mode
(1) Internal heating mode. Internally heated nozzle has complex structure, high cost, difficult parts replacement, and high requirements for electric heating elements. Placing the heater in the middle of the flow channel will produce annular flow, increase the friction area of the volume, and the pressure drop may be three times as much as that of the externally heated nozzle.
However, because the heating element for internal heating is located in the torpedo body in the nozzle, and the heat is supplied to the material, the heat loss is small, and the electric energy can be saved. If a point gate is used, the tip of the torpedo body is kept at the gate center, which is conducive to the gate cutting after injection and the residual stress of the plastic part is low due to the late solidification of the gate.
(2) External heating mode.
External heating nozzle can eliminate cold film and reduce pressure loss. At the same time, because of its simple structure, convenient processing, and the thermocouple installed in the middle of the nozzle to ensure accurate temperature control, it has been widely used in production. However, the heat loss of the externally heated nozzle is relatively large, which is not as energy-saving as that of the internally heated nozzle. 3.2 Selection of gate form
The design and selection of gate directly affect the quality of plastic parts. When applying the hot runner system, the appropriate gate form should be selected according to the resin flow performance, molding temperature and product quality requirements to prevent salivation, dripping, leakage and poor color change.
3.3 Temperature control mode
When the gate form is determined, the control of melt temperature fluctuation will play a key role in the quality of plastic parts. In many cases, coke, degradation or channel blockage are mostly caused by improper temperature control, especially for thermosensitive plastics, which often require rapid and accurate response to temperature fluctuations.
Therefore, heating elements should be reasonably set to prevent local overheating, ensure the fit clearance between heating elements and runner plates or nozzles, and minimize heat loss. At the same time, more advanced electronic temperature controllers should be selected as far as possible to meet the temperature control requirements.
3.4 Calculation content after determining the structure of hot runner system
(1) Calculation of temperature and pressure balance of each runner. The purpose of the hot runner system is to pass the hot plastic injected from the injection molding machine nozzle through the hot runner at the same temperature and distribute the melt to each gate of the mold at a balanced pressure. Therefore, the temperature distribution in the heating zone of each runner and the melt pressure flowing into each gate should be calculated.
(2) Calculation of nozzle and sprue sleeve center offset caused by thermal expansion. That is, the centerline of the hot (expanded) nozzle and the cold (non-expanded) sprue sleeve shall be accurately positioned and aligned.
(3) Heat loss calculation. The internally heated runner is surrounded and supported by the cooled mold sleeve, so the heat loss caused by thermal radiation and direct contact (conduction) should be calculated as accurately as possible, otherwise, the actual runner diameter will become smaller due to the thickening of the condensate layer on the runner wall.
3.5 Installation of runner plate
Both thermal insulation and bearing injection pressure should be fully considered. Cushion blocks and supports are usually set between the runner plate and the formwork. On the one hand, they can bear the injection pressure to avoid material leakage due to the deformation of the runner plate, and on the other hand, they can also reduce the heat loss.
3.6 Maintenance of hot runner system
For hot runner molds, it is very important to carry out preventive maintenance of hot runner components regularly during use, including electrical testing, inspection of sealing elements and connecting wires, and cleaning of component dirt.