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What are the reasons for burrs in plastic injection molding products?

Burr (commonly known as that peak) is a common problem encountered in injection mol. When the pressure of the plastic in the mold cavity is too high, the parting force generated is greater than the clamping force, thus forcing the mold to open, so that the plastic is extruded out and forms a peak on the surface of the plastic part. However, there may be many reasons for this phenomenon, such as plastic problems, or damage to the injection machine, or related to the timing adjustment.

The following summarizes several common causes of burrs in plastic injection molding:

Because the viscosity of the plastic affects its flow rate and pressure loss, too high or too low viscosity can cause burrs. If the viscosity of the plastic is too low, its fluidity is high, and it is easy to flow into the tiny gap between the mold surfaces, increasing the parting force until the other peak appears.

In general, plastic temperature has the greatest effect on viscosity, while pressure and shear rate also have an effect on viscosity. If the temperature of the plastic is raised, its viscosity decreases, and if the temperature is lowered, its viscosity increases. Another problem with plastics is their drying condition and whether they are mixed with impurities.Some plastics, such as nylon and ABS, have properties that affect plastics. As for polycarbonate, although it does not absorb water, its properties are also sensitive to its surface moisture. Therefore, when molded, many plastics must be dried before their properties can be properly controlled. Of course, it is more difficult to predict the performance of plastics if they are mixed with impurities or different kinds of plastics.

plastic injection molding
plastic injection molding

The pressure of plastic in the mold cavity will change with the filling of the mold cavity. Before the mold 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 that at other positions in the mold cavity. However, when the mold cavity is completely filled, the pressure loss during the flow of plastic no longer exists, and the pressure in the whole mold cavity becomes the same static pressure. Therefore, the force to force the mold open will be greatly increased, causing the generation of burrs.

To avoid this, the injection pressure must be adjusted to a lower holding pressure as soon as the cavity is filled. In addition to adjusting the pressure control system of the injection machine correctly, another auxiliary way is to reduce the injection speed first. In this way, the plastic at the front end of the melt is cooled and partially solidified, thereby avoiding the generation of burrs. Since too slow an injection speed can slow down production, the best adjustment of the injection speed must also be matched to the clamping force used.Otherwise, burrs may also occur.

If there is a problem in the structure of the injection mechanism, the complexity will be greater and it will be more difficult to find the cause of the problem. For example, there is a deviation in the parallelism between the mold plates, or the force distribution of the pull rod of the mold plate is not uniform, which will also cause the imbalance of the clamping force between the molds, so that the other peak will appear at the position where the clamping force is weak.On the other hand, if the wear of the screw or the melting cylinder is large, the melt may slide and flow back between the outer diameter of the thread and the melting cylinder, resulting in incorrect pressure switching points, resulting in local burrs and insufficient injection.

In addition to the above factors, if there is a problem with the mold, it will also produce burrs. For example, if the mold is used for a long time, some positions will be worn, and it is easy to have burrs. Even minor defects, such as a clogged vent, can cause the cavity pressure to rise, and too high a pressure can cause burrs. In some multi-cavity dies, if the runner design is underbalanced, the flow of plastic is asymmetric, and in order to avoid insufficient pressure in individual cavities, other cavities may have burrs.

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