Compared with ordinary runner molds, hot runner molds have the significant advantages of saving time and material, high efficiency, and stable quality, but once due to the use of easy-to-produce failure affects its widespread use.
With the technical progress of the mold industry, hot runner molds in the runner melt temperature control, structural reliability, and hot runner component design and manufacturing have made great progress, which makes the hot runner technology regains people’s attention and favor.
Many issues deserve consideration and attention in the design and application of hot runner molds. These issues are solved well and directly related to the success or failure of the hot runner system and product quality.
Therefore, the hot runner system failure and its causes are discussed, and the matters that should be paid attention to in the application of hot runner molding are undoubtedly very helpful to the successful application of hot runner molding technology.
2. Defects and troubleshooting of the common failure of hot runner injection molding system
2.1. Hot runner injection molding has residue at the gate or saliva dripping and poor surface appearance
Main reasons
The reasons are the little choice of the hot runner injection molding gate structure, improper injection molding temperature control, and large residual pressure of the melt in the runner after injection.
Solutions
(1) Improvement of the hot runner injection molding gate structure.
Usually, if the length of the gate is too long, it will leave a long sprue on the surface of the plastic part, and if the diameter of the gate is too large, it will easily lead to the occurrence of salivation and dripping material. When these failures occur, it is important to consider changing the gate structure. The common hot runner gates are straight, point, and valve gates.
A thicker runner diameter characterizes the main hot runner injection molding gate, so it is not easy to condense at the gate, which can ensure smooth injection of melt into deep cavity products;
It will not condense quickly, and the residual stress of plastic parts is minimized, so it is suitable for molding deep cavity products with multiple cavities in one mold.
But this kind of gate is easy to produce salivation and pulling phenomenon. The residual marks at the gate are large, even leaving a column-shaped material handle, so the material temperature at the gate should not be too high and needs stable control.
The characteristics of a straight gate are the same as common injection molding, but the residual marks on the molded part are relatively small; the point gate is characterized by low residual stress, moderate condensation speed, salivation, and wire pulling; it can be used in most engineering plastics, and it is the most used type of gate in hot runner molding at home and abroad.
The valve gate has small residual marks and low residual stress and does not produce salivation and drawing phenomenon. Still, the valve gate wear is obvious, and the salivation phenomenon will appear again in use with the clearance increase, so the spool and the valve gate body should be replaced in time.
The choice of hot runner injection molding gate form is closely related to the resin molding performance. Val valve gates can be selected for low-viscosity resins that are susceptible to salivation.
Crystalline resins have a narrow molding temperature range, so the temperature at the gate should be appropriately high, e.g., POM, PPEX, etc., can use a gate with a heating probe. Amorphous resins, such as ABS, PS, etc., have a wide range of molding temperatures.
Since the melt insulation layer is formed at the head of the torpedo nozzle core, there is no heating element in contact with the gate so the condensation can be accelerated.
(2) The reasonable control of hot runner injection molding temperature.
Suppose the cooling water in the gate area is not enough. In that case, it will cause heat concentration, resulting in salivation, dripping material, and drawing phenomenon, so the area’s cooling should be strengthened when the above phenomenon occurs.
(3) Resin pressure release.
Too much residual pressure in the hot runner injection molding flow channel is one of the main causes of salivation.
The injection machine should generally take a buffer circuit or buffer device to prevent salivation.
2.2. Hot runner injection molding material discoloration, scorching, or degradation
Main
causes
Improper hot runner injection molding temperature control; small runner or gate size causes large shear heat generation; dead spot in the runner leads to long heat time for stagnant material.
Solutions
(1) Accurate control of hot runner injection molding temperature.
To accurately and quickly determine the temperature fluctuations to make the thermocouple temperature measurement head reliable contact with the runner plate or nozzle wall, and make it located in the center of each independent temperature control area.
The head temperature sensing point and the runner wall distance should be no more than 10mm is appropriate. It would be best if you tried to make the heating element in the runner on both sides of the even distribution.
The temperature control can use intelligent fuzzy logic technology under the operation of the central processor, which has the function of temperature over-limit alarm and automatic adjustment and can make the melt temperature change control within the required precision range.
(2) Fix the hot runner injection molding gate size.
Avoid the dead spot of the runner as much as possible, and increase the gate diameter within the permitted range to prevent excessive shear heat generation.
The endothermic nozzle of the melt in the runner radial temperature difference is more likely to occur scorched material, and degradation phenomenon, so pay attention to the radial runner size design should not be too large.
2.3. Injection volume shortage or no material injection
Main
reasons
The reasons are obstacles or dead space in the hot runner injection molding, blockage of the hot runner injection molding gate, and thick condensation layer in the hot runner injection molding.
Solutions
(1) Flow channel design and processing should ensure that the melt flow to the corner of the wall of the arc the transition so that the entire flow channel is smooth and does not exist in the dead corner of the flow.
(2) In the case of not affecting the quality of plastic parts, appropriately increase the material temperature to avoid premature condensation at the gate.
(3) Properly increase the hot runner injection molding temperature to reduce the thickness of the condensation layer of the endothermic nozzle, and reduce the pressure loss to facilitate the filling of the cavity.
2.4. Serious material leakage
Main
causes
Seal element damage; heating element burned caused by uneven expansion of the hot runner injection molding plate; nozzle and gate sleeve center misalignment, or stop leak ring to determine the melt insulation layer on the nozzle projection area is too large, resulting in the nozzle backward.
Solutions
(1) Check the sealing and heating elements for damage. If there is damage, check carefully before the replacement whether it results from component quality problems, structural problems, or normal service life.
(2) Select the appropriate way to stop leakage. According to the nozzle insulation, preventing material leakage can be used to stop the leak ring or nozzle contact between two structures. You should pay attention to making the contact part of the stop leak to maintain a reliable contact state.
The melt projection area between the nozzle and the gate sleeve should be kept as small as possible to prevent the nozzle from retreating due to excessive back pressure during injection within the range of strength allowed.
The direct contact area between the nozzle and the gate sleeve should be such that the resin will not leak, even if the center of the nozzle is misaligned due to thermal expansion. But the contact area should not be too large so as not to cause increased heat loss.
2.5. The hot runner injection molding cannot be warmed up normally or it takes too long to warm up
Main
reasons
Wire channel spacing is not enough, resulting in wire breakage; assembly mold when the wire intersection of short circuit, leakage, and other phenomena.
Solutions
Select the correct processing and installation process to ensure that all the wires can be placed, and the use of high-temperature insulating materials in accordance with the regulations, and regular detection of wire breakage.
2.6. Poor change of material or color
Main
causes
Improper method of changing material or color; unreasonable design or processing of hot runner injection molding leads to more stagnant material.
Solutions
(1) Improve the hot runner system structure design and processing of the hot runner injection molding. When designing the hot runner, injection molding should try to avoid the dead point of the runner, and the corners should strive to round the transition.
Within the scope of permission, the hot runner injection molding size is as small as possible so that the runner in the stagnant material is less, and the new material flow rate is larger, conducive to rapid cleaning clean.
Processing runner, no matter how long the runner must be processed from one end if processed from both ends at the same time, easy to cause the hole center not to overlap, which will inevitably form stagnant material parts.
General external heating nozzle because the heating device does not affect the flow of melt, so it can be easier to clean the runner, while the internal heating nozzle is easy to form a condensation layer on the outer wall of the runner, so it is not conducive to rapid material change.
(2) Select the correct method of material change. Hot runner system for material, the color change process generally by the new material directly launched into the runner, all the stagnant material, and then the runner wall stagnant material to move forward as a whole, so the cleaning is easier to carry out.
On the contrary, if the viscosity of the new material is low. It is easy to enter the center of the lagging material and separate the lagging material layer by layer, so it is more troublesome to clean.
If the viscosity of the old and new material is similar. The new material can be injected faster to achieve a quick change of material. If the viscosity of the lagging material is sensitive to temperature, the material temperature can be increased to reduce the viscosity to speed up the process of material change.
3. Selection and application of hot runner injection molding considerations
In order to eliminate or reduce as much as possible in the use of failure, the selection and application of the hot runner system should pay attention to the following matters.
3.1. The choice of hot runner injection molding heating method
(1) Internal hot runner injection molding heating method.
Internal hot runner injection molding heating nozzle structure is more complex, more expensive, more difficult to replace parts, the electrical heating element requirements are higher.
The hot runner injection molding heater is placed in the middle of the runner, which will produce a circular flow,
increasing the friction area of the capacitor. The pressure drop may have an external heating nozzle 3 times as much.
But because the internal heating element is located in the torpedo body inside the nozzle, all the heat is
supplied to the material, so the heat loss is small and can save electricity.
If a pointed gate is used, the tip of the torpedo body is kept in the center of the gate, which facilitates the gate cut
off after injection and lowers the residual stress of the plastic part because of the late condensation of the gate.
(2) External hot runner injection molding heating method.
The external hot runner injection molding heating nozzle can eliminate the cold film and reduce pressure loss. At the same time, due to its simple structure, easy processing, and thermocouple installed in the middle of the nozzle so that the temperature control is accurate and other advantages, currently in production has been commonly used.
But the external heat nozzle heat loss is larger, and not as energy-saving as the
internal heat nozzle.
3.2. Selection of hot runner injection molding gate form
The design and selection of gates directly affect the quality of plastic parts.
In the application of the hot runner system, the suitable gate form should be selected according to the flowability of resin, molding temperature and product quality requirements, in order to prevent salivation, dripping, leakage, and poor color change.
3.3. Hot runner injection molding temperature control method
After the hot runner injection molding gate form is determined, the control of melt temperature fluctuation will play a key role in the quality of plastic parts.
Many times the scorching, degradation, or blockage of the runner is caused by improper temperature control, especially for heat-sensitive plastics, which often require a rapid and accurate response to temperature fluctuations.
For this reason, the heating element should be reasonably set to prevent local overheating, to ensure that the gap between the heating element and the runner plate or nozzle minimizes heat loss, and to choose the more advanced electronic temperature controller to meet the temperature control requirements.
3.4. Hot runner system structure to determine the calculation content
(1) The temperature and pressure balance calculation of each manifold.
The purpose of the hot runner system is to inject the hot plastic from the injection molding machine nozzle, pass through the hot runner with the same temperature and distribute the melt to each gate of the mold with balanced pressure, so the temperature distribution in the heating area of each runner and the pressure of the melt flowing into each gate should be calculated.
(2) Calculation of nozzle and gate sleeve center offset due to thermal expansion.
That is to ensure that the centerline of the hot (expanding) nozzle and cold (not expanding) gate sleeve can be accurately positioned and aligned.
(3) Hot runner injection molding heat loss calculation.
The internally heated runner is surrounded and supported by the cooled mold sleeve, so the heat loss due to heat radiation and direct contact (conduction) should be calculated as accurately as possible, otherwise, the actual runner diameter will be smaller due to the thickening of the condensation layer on the runner wall.
3.5. Installation of the hot runner injection molding plate
Both the thermal insulation and the injection pressure should be fully considered. This can withstand the injection pressure to avoid the deformation of the hot runner injection molding plate and the leakage of material, and reduce the heat loss.
3.6. Maintenance of hot runner system
For hot runner molds, it is important to perform regular preventive maintenance of the hot runner components during use, including electrical testing, an inspection of sealing components and connecting wires, and cleaning of dirty components.
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