How to solve the problem of plastic leakage in hot runner system?
The invention and application of hot runner brings more development opportunities for mould manufacturers.From common plastics such as polypropylene and polyethylene, to engineering plastics such as polycarbonate and glass fiber reinforced nylon, many different materials can be processed through the hot runner system.However, this technology and equipment also often bring some trouble to mold manufacturers, leakage is one of the problems.
(1) the prerequisites for successful application of the hot runner system.
The most important factor for successful application of the hot runner system is that the mold manufacturer should select the appropriate hot runner products according to their own needs.
One of the important differences between various hot runner systems is the heating mode of the molten material, which can be divided into internal and external heating.As the name implies, the internal heating system is to install the heater directly in the fuse, heating from the inside.The external heating system is the external heating raw material, so that the raw material flows through the channel without obstacles.The external heating system solves the dead point problem in the melt channel and makes the shear curve more reasonable.
Hot runner system is an extension of the sprue injection molding, evenly to deliver material to each note mouth, the most effective way is to install a balanced distribution board, to ensure that from the injection point to the length of each cavity of the melt flow and flow channel size is equal.This kind of mechanical balance design can ensure the balanced filling of each product, while avoiding the occurrence of flying side and short shot.
Different weld size can directly affect the performance of the hot runner.The improper size of the fuse leads to the degradation of the plastic, or the inhomogeneity of the injection molding parts or the incomplete filling, and the slow change of color.The mold manufacturer should consider the factors such as pressure drop, dwell time, temperature rise, shear rate and color changing frequency when determining the size of the weld.
After determining the system type, select the appropriate gate type and its filling speed.The factors to be considered in gate type include the allowed gate mark, gate position and the material type of injection molding.No matter whether it is crystallization or crystalline plastic, different gate types have restrictions on specific materials.Most hot runner systems can match different sizes of hot mouths, each with a specific filling speed.The gate too small will limit the plastic filling, and may produce too much shear leading to material degradation.
(2) analysis and countermeasures of leakage reasons.
One of the most difficult problems for the mold manufacturers is the flow of molten material in the hot runner system to the shunt plate trough, which is one of the main reasons for mold damage and discontinuation.
Most of the leakage is not due to poor system design, but because it is not operated according to the design parameters.The leakage usually occurs in the seal between the hot mouth and the shunt plate.According to the design specification of the general heat flow path, the hot mouth has a steel edge to ensure the height of the hot mouth component is less than the actual groove depth on the hot runner plate.The purpose of designing this difference (usually called a cold gap) is to avoid thermal expansion leading to component damage when the system is operating temperature.
For example, a 60 mm thick shunt plate and a 40 mm hot mouth components (total height is 100 mm) from room temperature to operating temperature (230 ° C), can expand the 0.26 mm.If there is no cold space, thermal expansion can cause damage to the edge of the hot mouth.The leakage of hot runner is the result of the lack of effective sealing under the condition of cooling.In order to ensure the sealing of the system (hot mouth and shunt plate), the system must be heated to the operating temperature, and the force produced is enough to offset the injection pressure.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 produced by the injection is unable to prevent the leakage of the hot runner with the cold clearance before it reaches its operating temperature.
The leakage may also occur under excessive heating.Because with the edge of the hot mouth to thermal expansion of rigidity poor adaptability, when after points heating system to reduce the operating temperature, based on the rigid deformation, the influence of the sealed pressure to prevent leakage.In this case, in addition to the leakage will cause damage, it will also cause unrecoverable damage to the hot mouth due to excessive pressure, so the hot mouth needs to be replaced.
It is very important to ensure the load of hot mouth and shunt plate. The mould manufacturer must strictly comply with the size and tolerance provided by the hot runner supplier to prevent the leakage of the system.
It is difficult to find out whether the hot runner is leaking because it is not visible outside the mold.However, the system operator can observe the occurrence of the leakage from a few fine points.
First, the plastic injector, but not the mold cavity.When setting up a new hot runner, the operator should be aware of how many times of injection can be contained in the fuse.For example, if the fuse can contain three injection molds (each injection is the capacity of the fuse/mold cavity), there should be molten material in the mold cavity after 3 injection.If not, it is likely that the molten material has leaked into the shunt plate slot.
In the process of operation, another indication of leakage material is part of the cavity filling material or product is not completely, this is because the part of the injection liquid leakage to the shunt board slot, caused the product injection is not sufficient.In the control interface of the injection molding machine, this situation is shown as a sudden change in process parameters.
If the operator suspects that there is a leak, the injection molding machine should be closed immediately after cooling.After cleaning up the system and finding out the reason for the leak, all the parts should be carefully checked because the overheating temperature or cleaning process may cause damage to the parts.
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