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What is the reason for the burrs in the production process of the injection molding machine?

writer:LI SONG Share

Flash is a common problem encountered in the production process of injection molding machines. When the pressure of the plastic in the mold cavity of the injection molding machine is too large, the splitting force generated is greater than the clamping force, so the mold is forced to squeeze out and form a peak on the surface of the plastic part. However, there may be many reasons for this phenomenon, such as plastic problems, damage to the injection molding machine, or timing adjustments.
 
Generally speaking, plastic temperature has the greatest effect on viscosity, while pressure and shear rate also have an effect on viscosity. If the injection molding machine raises the temperature of the plastic, its viscosity will decrease, and if its temperature is lowered, its viscosity will increase. Another problem with plastic is its dry condition and whether it is mixed with impurities. Some plastics, such as nylon and ABS, have properties that affect plastics. As for polycarbonate, although it has no water absorption, its properties are also sensitive to moisture on its surface. Therefore, many plastics must be dried before molding. In order to properly control its performance. If impurities are mixed in the plastic, or different types of plastic are mixed, it is of course more difficult to predict the change in the properties of the plastic.

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The pressure of the plastic in the cavity of the injection molding machine will change with the filling of the cavity. Before the cavity is filled, the pressure at the front end of the melt is almost equal to zero, and the pressure at the nozzle is higher than the pressure at other positions in the cavity, but when the cavity is completely filled, the pressure loss when the plastic flows is No longer exists, the pressure in the entire mold cavity becomes the same static pressure, so the force to push the mold open will increase greatly, causing burrs. To avoid this situation, once the cavity is filled, the injection pressure must be adjusted to a lower holding pressure immediately. In addition to correctly adjusting the pressure control system of the injection molding machine, another auxiliary method is to first reduce the injection speed of the injection molding machine. In this way, the plastic at the front end of the melt is cooled and partially solidified, thus avoiding the generation of burrs. Since the injection speed is too slow, it will slow down the production, the best injection speed adjustment must also be matched with the clamping force used. Otherwise, burrs may also occur.

If there is a problem with the structure of the injection molding machine, its complexity will be greater, and it will be difficult to find the cause of the problem. For example, the deviation of the parallelism between the templates, or the uneven distribution of the force of the template tie rods, will also cause the clamping force between the molds to be unbalanced, so that the plastic part will appear at the position where the clamping force is weak. On the other hand, if the screw or melt barrel wears a lot, the melt may slide and flow backwards between the outer diameter of the thread and the melt barrel. As a result, the pressure switching position is incorrect, causing local burrs and shots. Insufficient glue.

In addition to the above-mentioned factors, if there is a problem with the mold, flash will also occur. For example, if the mold is used for a long time, some parts are worn out, and it is easy to have burrs. Even some minor faults, such as blocked vents, will cause the cavity pressure to rise, and if the pressure is too high, there will be burrs. In some multi-cavity molds, if the runner design is underbalanced, the plastic flow will be asymmetrical. In order to avoid insufficient pressure in individual cavities, other cavities may have burrs.

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