How do you prevent bubbles in plastic injection molding?

Bubbles in injection

Although it is true that sometimes it is not possible to avoid thickness variations, it is very important that it be as regular as possible, without abrupt changes. An irregular thickness with excesses or defects can cause problems such as deformation due to differences in shrinkage or shrinkage on the opposite side.

Avoid threads with too fine a pitch. In the case of female threads, prevent the thread from reaching the bottom of the part, thus avoiding any breakage. In the case of male threads demolded by sliding, they can be interrupted at the height of the joint plane, avoiding as far as possible the problem of wear on the seats. As mentioned above, avoid sharp angles that have poor mechanical properties, therefore soften them in this type of parts.

The insert should not embrittle the material surrounding it, therefore the optimum thickness of the material around the insert should be verified. The shrinkage of the plastic on the metal will cause an overflow of the insert. This overflowing will be suppressed by practicing a uniform and crossed grooving of the insert. From experience, it is necessary to heat the insert prior to injection.

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Defects in injection molded parts

Processing at the absolute maximum melt temperature may require special precautions and reduced holding times. Refer to the resin manufacturer’s processing specifications for more detailed recommendations on processing temperatures.

The cylinder separates from contact with the sprue injection bushing and the injection piston moves forward, forcing the molten plastic to flow from the nozzle. If the melt is trapped in a metal container as it flows out of the nozzle, you can use a pyrometer probe to check the temperature. This method is one of the most accurate measurements of melt temperature.

To prevent air traps from forming, change the location of the injection point or the thickness of the part. Provide vents at the location where air entrapment occurs.

Anisotropic shrinkage is present in filled materials due to shrinkage restriction along the fiber length, which usually occurs in the flow direction. This type of shrinkage also occurs in unfilled materials when the molecules do not have enough time to relax from viscous elongation due to shear during flow.

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Table of plastic injection defects

Occasionally, the use of high compaction pressures produces acceptable sink marks by reducing volumetric shrinkage although these cannot be completely eliminated. This is because the volumetric change of plastic from melt to solid is approximately 25% and the compressibility of plastics at typical injection molding pressure is only 15%, which means that it is impossible to compact the molten plastic sufficiently to compensate for cooling shrinkage.

Incomplete filling occurs when an injection molded part lacks material to properly generate its geometry. This occurs when the molten polymer cannot fill the entire cavity (or cavities) of the injection mold, usually the thinner sections where the molten polymer cools before completely filling the mold. Any factor that increases the flow front resistance of the polymer melt can cause incomplete filling. Some of these factors are:

Bubbles in intramuscular injection

The process of obtaining a plastic part by injection follows an order of operations that is repeated for each part. This order, known as injection cycle, can be divided into the following stages

The cycle begins with the closing of the mold, preparing it to receive the injection of the molten material. In this phase, the clamping nut is applied, which is the one made by the machine to keep the mold closed during injection. It depends on the projected surface of the part and the actual pressure (specific pressure) in the mold cavity.

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Once the mold is closed and the clamping force is applied, the mold filling (injection) phase begins. The screw of the injection unit injects the molten material into the mold at a high pressure; while injecting, the screw advances without rotation. The duration of this stage can be from tenths of a second to several seconds, depending on the quantity of material to be injected and the characteristics of the process.