The invention and application of hot runners has brought more opportunities for mold manufacturers; from general plastics such as polypropylene and polyethylene, to engineering plastics such as polycarbonate and glass fiber reinforced nylon, many different materials can pass. The hot runner system is processed. However, this technology and equipment often bring some troubles to mold manufacturers, and leakage is one of the problems.
Prerequisites for successful application of hot runner systems
To successfully use the hot runner system, the key factor for Zui is that mold manufacturers need to choose the right hot runner product according to their own needs.
One of the important differences between various hot runner systems is the way in which the melt is heated, which can be divided into internal and external heating. As the name implies, the internal heating system installs the heater directly in the melt tunnel and heats it from the inside; the external heating system heats the material externally, allowing the material to flow through the passage without obstacles. The external heating system solves the dead point problem in the melt channel and makes the shear curve in the melt channel more reasonable.
The hot runner system is an extension of the nozzle of the injection molding machine. To uniformly transfer the raw material to each injection port, the effective method is to install a balanced manifold to ensure the length of the melt flow from the injection point to each membrane cavity. Same as the runner size. This mechanically balanced design ensures balanced filling of individual products while avoiding flash and short shots.
Different melt channel sizes directly affect the performance of the hot runner. Inappropriate melt channel size can cause plastic degradation, or unevenness in injection molding or incomplete filling, and slow color change. Moldmakers must consider factors such as pressure drop, residence time, temperature rise, shear rate, and color change frequency when determining the size of the melt.
After determining the type of system, you will then choose the appropriate gate type and its fill speed. There are a number of gate types to consider, including the allowed gate marks, gate locations, and the type of material being injected. Regardless of whether it is amorphous or crystalline plastic, different gate types have limitations on specific materials. Most hot runner systems can match different sizes of hot nozzles, each offering a specific fill speed. Too small a gate will limit plastic filling and may result in excessive shearing leading to material degradation.
Analysis of the causes of leakage and countermeasures
One of the problems that Zui has made the mold maker complain is that the melt in the hot runner system will leak into the manifold. This is also one of the main causes of mold damage and deactivation.
Most of the leakage is not due to poor system design, but not due to design parameters. Leakage typically occurs at the seal between the hot nozzle and the manifold. According to the general hot runner design specification, the hot nozzle has a steel edge to ensure that the height of the nozzle assembly is less than the actual groove depth on the hot runner plate. The purpose of designing this dimensional difference (commonly referred to as a cold gap) is to prevent component damage from thermal expansion when the system is at operating temperature.
For example, a 60mm thick manifold and a 40mm nozzle assembly (total height of 100mm) will expand by 0.26mm from room temperature to operating temperature (230°C). If there is no cold gap, thermal expansion can cause damage to the edges of the hot nozzle. Hot runner leakage is the case where there is a lack of effective sealing under cooling conditions. In order to protect the system's seal (hot nozzle and manifold), the system must be heated to the operating temperature, and the resulting force (eg, 20,000 lbs) is sufficient to offset the injection pressure and prevent the injection pressure from pushing the two components apart. Inexperienced system operators may not wait until the temperature rises to the operating level or even forget to turn on the heating system. The surface pressure generated by injection molding does not prevent leakage before the hot runner with cold gap does not reach its operating temperature.
Leakage can also occur with excessive heating. Since the hot mouth with the steel edge has poor adaptability to thermal expansion, when the system is heated to the operating temperature after being heated, the sealing pressure generated by the steel deformation cannot prevent leakage. In this case, in addition to the damage caused by the leakage, the irreversible damage to the hot nozzle due to excessive pressure may require replacement of the hot nozzle.
It is important to ensure that the load on the hot nozzle and manifold is critical, and the mold manufacturer must strictly adhere to the dimensions and tolerances provided by the hot runner supplier to effectively prevent system leakage.
How to find leaks
It is quite difficult to find out if the hot runner is leaking because it is not visible from the outside of the mold. However, the system operator can observe the occurrence of leakage from some subtleties.
First, the plastic is injected but does not reach the cavity. When setting up a new hot runner, the operator should know how many injections can be included in the melt. For example, if the melt channel can contain 3 shots (each shot is the capacity of the melt/cavity), there should already be melt in the cavity after 3 injections. If not, it means that the melt is likely to have leaked into the manifold.
Another indication of leakage during operation is that some of the mold cavity or product charge is incomplete because some of the injected melt leaks into the manifold plate, resulting in insufficient product injection molding. On the control interface of the injection molding machine, this situation is shown as a sudden change in process parameters.
If the operator suspects that a leak has occurred, the injection molding machine should be shut down immediately before the system is cooled down for inspection. After cleaning the system and identifying the cause of the leak, all parts should be inspected carefully, as the temperature may be overheated or the cleaning process may damage the parts.
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