Analysis of Common Defects in PC Injection Molding and Its Solutions Part Three


Fingerprints and turbulence marks appearing on products

Because of the high viscosity and poor fluidity of PC melt, PC products are more prone to fingerprints and turbulence marks. These two phenomena are more common in the processing of switch sockets and general electrical device panels, and these two phenomena are sometimes difficult to distinguish due to the similar shapes. In fact, the causes of these two phenomena are not the same, and their solutions are also different.
(1) Fingerprints are named, because the shapes resemble the fingerprints of human hands. Sometimes they are also called ripples, vibration patterns or shock patterns, which means that their patterns are like ripples formed by stones falling on calm water. The main reason for this is that the viscosity of the PC melt is too great. When the injection pressure and injection rate are small, the melt fills the mold with a stagnant flow. When the melt at the front contacts the cold mold surface, it quickly condenses and shrinks. The hot melt in the back is pushed by the pressure to expand and shrink the cold material and continue to move forward. The alternate process of this process will form a vertical wave line on the material flow and inlet.
The solutions are as the following:
Mainly increase temperatures of the nozzle, front end of the barrel and mold, especially the temperature where the ripples are generated.
This is to reduce the melt viscosity of PC and improve melt fluidity. And if the product is more precise and has strict requirements on appearance, it is necessary to add a mold temperature controller to accurately control the mold temperature at about 120°C.
Increasing the injection rate and pressure are mainly to increase the melt flow rate at the fingerprint part and to prevent the melt from flowing in the form of stagnation. If the fingerprint is generated in the center of the product or far away from the gate, multi-stage injection must be used to adjust the injection rate step by step.
Modification of the mold is mainly to reduce resistance of the melt during the filling process, such as increasing the size of the runner and gate, paying attention to polishing the nozzle hole and runner, increasing the exhaust ditch and groove, setting inserts and ejector air-entraining device, improving mold exhaust and setting a large enough cold material trap to reduce the blocking effect of the cold material at the front end.

(2) Turbulent flow marks refer to irregular flow lines on PC products centering on the gate. Unlike the fingerprint line, turbulence marks appear along directions of material flows instead of being perpendicular to directions of material flows. The reason may be that the molten material injected into the mold cavity is subject to a great impact, resulting in sticking and slipping on the cold mold.

The solutions are as the following:
Increase the melt temperature to reduce the premature cooling of the melt. Increase the mold temperature, especially temperature of the turbulent flow mark, to prevent the melt from cooling and sliding in the cavity prematurely. Adopt multi-stage injection. Appropriately reduce the injection rate and pressure at the flow mark part. Change the gate position to alter the flow mode of the melt. Enlarge the cold material trap to prevent the cold material from sliding in the mold. Adopt materials with good fluidity to make the melt filling smooth.

4. Cold material spots occurring on the product

The cold material spot is one of the common defects in the gates of PC products. The phenomenon is that the products have foggy or bright spots near the gate, or the curved scars which look like an earthworm from the gate sticking to the surface of the product. The reason for its formation is mainly propulsion of the front cold material of molten materials entering the mold cavity or the cold material that is later squeezed into the mold cavity due to excessive pressure holding. The heat of the front material is transferred by the nozzle being in contact with the cold template or the cooling of the runner, and it is pushed by the hot melt when entering the mold cavity, forming cold spots. Cold material spots will be spread out on thinner products and become smog or paste like turbid spots, while a scar that is curved and shaped like an earthworm will be left on thick walled products with free flows. As for the cold material spots formed by the excessive holding pressure, the holding time is too long, and the cold material on the runner and gate continues to be squeezed into the product when the holding pressure is too high. Cold material spots often make a small area near the gate form a round bright spot.
Another is that the melt is quickly squeezed into a small gate and the melt is broken around the gate, or the interference of the gas in the mold causes a cloud like or ray like bright spot at the gate. Cold material spots not only damage apparent quality of the product, but also affect the effect of subsequent processes such as spraying and electroplating, and also reduce the mechanical strength of the product to varying degrees.
The measures that can be taken to resolve this defect are as follows:
1. Increase the temperature of the barrel and nozzle, and increase the temperature of the mold to reduce the impact of cold materials.
2. Slow down the injection rate and increase the injection pressure to avoid melt fracture or interference from the gas in the mold.
3. Adjust the injection time and pressure holding time to avoid overfilling.

4. Reasonable mold gate design can reduce or avoid the formation of cold material spots in advance, which is a traditional and effective method.

5. The cold material trap is set at the end of the runner, so that the front material can be trapped in the trap and not enter the mold cavity. In addition to the cold material trap, the rationality of forms, sizes and locations of gates need to be considered for some molds.

6. Improve mold discharge; remove contaminants in materials; enhance the drying effect of materials; reduce or replace lubricants and use mold release agents as little as possible.