1. Drooling and Poor Surface Quality or Obvious Residue at the Gate
Main Reasons: Improper temperature control, a large residual pressure in the melt in the runner after injection, and an unreasonable choice of the gate structure.
Solutions: Enhancing the gate structure
The diameter of the gate is typically too big, which can easily result in drooling, and the length of the gate is typically too long, leaving a lengthy gate handle on the surface of the plastic portion. It is crucial to consider the gate structure when the aforementioned failure occurs.
Main Runner Gate: It has a large runner diameter, which makes the gate difficult to condense and allows for smooth injection of the plastic melt into a deep cavity part. Because the gate won¡¯t condense quickly, the plastic part¡¯s residual stress will be the least, making it ideal for molding a mold with several cavities, or plastic pieces with deep cavities. The material temperature at the gate should not be too high and must be tightly regulated since this type of gate is more prone to wire drawing and drooling, leaves a huge gate residue, and even leaves a columnar material handle.
Spot Gate: Its characteristics include minimal plastic part residual stress, a moderate rate of condensation, and a lack of wire drawing and visible drooling. The majority of engineering polymers may be used with it.
Needle Valve Gates: Both domestically and internationally, Hot Runner Injection Molding frequently employs this kind of gate. The plastic parts are of excellent quality, and there are only very minor surface traces. Drooling and wire drawing is not a problem, and there are no significant residual marks or stresses. However, valve port wear is evident. In use, the phenomenon of drooling will happen when the matching gap widens. The valve core and valve body should now be promptly replaced.
The resin qualities have a direct impact on the gate form selection. To prevent drooling in low-viscosity resins, needle valve gates can be used. The molding temperature range for the crystalline resin is limited, so the gate temperature needs to be raised properly. A gate with a heating probe can be utilized with resins like POM and PPE. Amorphous resins have a wide variety of molding temperatures, including ABS, PS, etc. There is no heating element contact at the gate because the head of the torpedo nozzle core generates a melt-insulating layer, allowing the condensation to proceed more quickly.
Reasonably Controlled Temperature In Hot Runner Injection Molding:
If there is insufficient cooling water in the gate region, heat concentration will occur, leading to salivation, dripping, and wire pulling. As a result, the cooling in this area needs to be strengthened when the aforementioned occurrences occur.
Resin Pressure Relief Drooling: A circuit or buffer to prevent drooling.
One of the primary triggers for salivation is an excessive amount of residual pressure in the flow channel. To stop salivation, the injection molding machine should generally incorporate a buffer circuit or a buffer device.
2. Material Discoloration, Charring, or Degradation
Main Reasons: The retained material is heated for an excessively long period for several causes, chief among them improper temperature control, tiny runners or gates, excessive cutting heat, and dead spots in the runners.
Solutions: Reliable Temperature Regulation in Hot Runner Injection Molding
The thermocouple temperature measuring head should be positioned in the middle of each independent temperature control area to measure temperature swings rapidly and precisely. It should also be reliably in touch with the flow channel plate or the nozzle wall. The flow channel wall and the head¡¯s temperature sensor. The space should not be greater than 10mm, and the heating components should be placed as evenly as possible on both sides of the flow channel. The central processing unit can be used to control the temperature using intelligent fuzzy logic technology, which can automatically adjust and alarm when the temperature exceeds the set limit. This technology can also control melt temperature changes within the required accuracy range.
Make the Gate Size Proper:
To prevent excessive shear heat generation, the dead point of the runner should be avoided as much as possible, and the gate diameter should be appropriately raised within the permitted range. The radial direction of the runner, which is more prone to searing and deterioration, experiences a significant temperature variation in the melt from the internal heating nozzle. As a result, it should be emphasized that the runner¡¯s radial size shouldn¡¯t be too large.
To prevent the material stored in the runner from zooming and burning black, the temperature of the hot runner injection molding, should be set to the heat preservation state if production is momentarily interrupted for a period longer than 20 minutes.
3. Lack of Material Injection, Insufficient Injection Volume, or Imbalanced Feed
Main Reasons: A heavy condensation layer forms in the runner, the gate is obstructed, obstacles or dead ends arise, and the gate temperature fluctuates.
Solutions
To ensure that the runner is smooth throughout and has no dead angles of flow, it should be ensured that the melt flows to the arc transition of the wall at the corner during the design and processing phases.
Properly raise the material temperature to prevent premature condensation of the gate without compromising the quality of the plastic parts.
Properly raise the hot runner¡¯s temperature to lessen the thickness of the condensation layer that forms on the internal heating nozzle and to lower pressure loss, which helps the cavity to fill.
Normally, there are some discrepancies in the actual temperature of each gate and the way the heating wire and mold heat dissipation are coordinated. The temperature balance can be changed if there are no specific restrictions.
4. Significant Glue in Hot Runner Injection Molding
Main Reasons: The runner plate expands unevenly due to the burnt heating element and damaged sealing element. The middle of the sprue sleeve and the nozzle are not lined up correctly. The nozzle retracts because the projected area is too huge.
solutions
Check whether the sealing element and the heating element are damaged. If there is any damage, carefully check whether it is the quality problem of the element, the structural problem, or the result of the normal service life before replacement.
Choose a suitable leak-stopping technique. The leakage prevention ring or nozzle contact structure can be utilized by the thermal insulation method of the nozzle to stop material leakage. It is important to maintain a reliable contact condition for the leak-proof contact parts. To avoid the nozzle retreating as a result of high back pressure during injection, it is essential to make sure that the projected area of the melt between the nozzle and the sprue sleeve is as small as possible within the permissible range of strength. The direct contact area between the nozzle and the sprue sleeve, when the leak-stop method is used, should prevent resin leaking from happening when the centers of the two are misaligned as a result of thermal expansion. To avoid increasing heat loss, the contact area should be as small as possible.
The majority of glue leakage is caused by the system not operating by the design parameters, not by a bad system design. The seal between the hot nozzle and the manifold plate is where leaks typically start. The height of the hot nozzle assembly must be less than the actual groove depth on the hot runner injection molding plate, therefore per typical hot runner design specifications, there is a stiff edge at the hot nozzle. This dimensional difference, often known as the ¡°cold gap¡± is intended to prevent component damage brought on by thermal expansion when the system is heated to its operational temperature.
For instance, from room temperature to operating temperature (230¡ãC), a 60mm thick manifold plate and a 40mm hot nozzle assembly (100mm total height) will expand by 0.26mm. Thermal expansion can harm the hot nozzle¡¯s edge if there isn¡¯t a cold gap. When there is insufficient effective sealing under cooling circumstances, hot runner injection molding leakage happens. Heat the system to working temperature, which creates a force strong enough to oppose the injection pressure, preventing the injection pressure from pulling the two parts apart, to ensure the sealing of the system (hot nozzle and manifold plate).
*Leak prevention strategies:
Making sure that the runner plate and hot nozzle are loaded is crucial. To properly stop the system from leaking glue, the dimensions and tolerances specified by the hot runner injection molding provider must be strictly adhered to when the mold is created.
Start the injection when the machine is turned on, even if you don¡¯t wait for the temperature to reach the working level or even neglect to switch on the heating system. The injection pressure will cause the glue to seep from the hot runner injection molding since it cannot achieve its operational temperature because of the cold gap.
Overheating is another factor that might cause it. When the system is overheated and then brought down to operating temperature, the sealing pressure produced by it cannot stop leakage due to the influence of rigid deformation because heat nozzles with rigid edges do not adapt well to thermal expansion. Due to the enormous pressure in this situation, in addition to producing glue leakage, the hot nozzle will also sustain permanent damage and must be replaced.
*How to spot leaking glue?
a. Despite being injected, the plastic melt does not enter the mold cavity.
For instance, if the melt channel can hold three shots (each shot is equal to the capacity of the melt channel/mold cavity), then after three shots, plastic melt should already be present in the mold cavity. If not, the melt has most likely seeped into the manifold slot.
b. The hollow or plastic component is only partially filled.
This is because an inadequate amount of plastic parts are injected. After all, some of the injected molten material seeps into the runner plate groove. This circumstance is displayed on the injection molding machine¡¯s control interface as an abrupt modification of the process parameters. The operator should immediately stop the injection molding machine and wait for the system to cool before assessing if there is a leak. All components should be thoroughly inspected after the system has been cleaned and the source of the leak has been found, as overheated temperatures or the cleaning procedure may harm components.
5. The Main Reasons for the Blockage of Ordinary Gates
Too little heat is present at the gate. Reduce the cooling rate at the gate, verify that the mold insulation cavity is large enough, and determine whether the hot crystalline plastic nozzle type is adequate.
An insulating gap must be created at the front end of the hot nozzle because the temperature there is too low, decreasing the area that can be used for sealing between the hot nozzle¡¯s sealing mouth and the mold.
The gate doesn¡¯t have adequate heat. Shorten the tip¡¯s length, increase the insulating cavity¡¯s diameter, and examine the tip¡¯s concentricity at the gate.
The temperature of the nozzle is too low. Boost the heating coil¡¯s power and verify the hot runner injection molding thermostat¡¯s temperature.
Unwanted material is blocked. To remove foreign objects, separate the hot runner injection molding system.
6. Causes of Needle Valve Gates being Plugged
A defective or too-short valve needle. Examine or replace
If the gate is broken, determine whether the valve needle¡¯s length is too long.
A cylinder or cylinders leaking air or oil. Examine the cylinder seal and the cylinder interface.
Lengthen the valve pin and reduce the mold temperature at the gate because the contact between the valve pin and the gate is improper.
Inappropriate cylinder/cylinder pressure. pressure is raised or lowered.
The holding period is excessive. Reduce the holding period.
Foreign metal particles are incorporated into the recycled materials, blocking the gate.
7. The Hot Runner can¡¯t be Heated Normally or It Takes too Long to Heat Up
Main Reasons: When the wires collide, short circuits and leakage happen during the assembly of the mold because there is not enough space between the wire channels, causing the wires to break.
Solutions
Select the correct processing and installation process to ensure that all wires can be placed, and use all high-temperature insulating materials by regulations
Regularly check for wire breakage
8. Material Change or Color Change is Not Clean
Main Reason: A lot of residual material occurs inside because of improper material or color-changing techniques, as well as unrealistic runner design or processing.
Solutions
Upgrade the runner¡¯s structural design and manufacturing process. The dead point of the runner should be avoided wherever possible when constructing it, and the arc transition should be sought at each corner. The flow channel¡¯s size should be as small as feasible within the permitted range to reduce retained material in the channel and increase the flow rate of fresh material, which promotes quick cleaning. The runner must always be processed from one end, When treated simultaneously from both ends, it is simple to misalign the hole¡¯s center, which will surely result in a significant section being retained.
Pick the appropriate refill technique. The new material is typically pushed out of all the retained materials in the flow channel during the process of material change and color change in the hot runner injection molding system. The retained material on the flow channel wall is then transported forward as a whole, making cleaning easier. In contrast, if the viscosity of the new substance is low, it is simple to penetrate the retained material¡¯s center and the retained material separates into layers, making cleaning more difficult. Rapid material transition can be accomplished if the old and new materials¡¯ viscosities are in the same phase and the new material¡¯s injection speed is increased. If the retentate¡¯s viscosity is temperature-sensitive, the temperature of the substance can be suitably raised to lower the viscosity and hasten the refueling process.
