Analysis and solution of flow marks on the surface of injection molded products

Flow marks are common surface defects of plastic parts near the gate, which appear as a wavy mark and present a concentric circle centered on the gate. Flow marks are usually caused by uneven temperature distribution of plastic parts during the molding process or too fast plastic solidification. In addition, during the melt flow process, turbulence and cold material may be formed, or insufficient plastic may not be added during the pressure holding stage. These factors may lead to the appearance of flow marks.

Main causes of flow marks

The reasons for flow marks include:

1. The temperature of the plastic raw material and the mold temperature are too low.

2. Insufficient injection speed and injection pressure.

3. Improper cross-sectional design of the runner and gate causes excessive flow resistance.

Through visual observation of the injection molding mold, it can be seen that the front of the melt cooling on the cavity wall interacts with the subsequent tumbling and cooling effects of the melt, resulting in the generation of flow marks.

Analysis and countermeasures of flow mark defects

1. Annual ring-shaped wave flow marks:

– Cause: The melt has poor fluidity. The low-temperature and high-viscosity melt is semi-solidified when injected into the cavity, flows along the surface of the cavity and is squeezed by the subsequent melt, resulting in local backflow and stagnation, thus forming annual ring-shaped wave flow marks.
– Solution:
– Increase the temperature of the mold and nozzle.
– Increase the injection rate and filling speed.
– Increase the injection pressure and holding time.
– Add a heater at the gate to increase the local temperature.
– Expand the cross-sectional area of ​​the gate and runner, and give priority to the use of a circular cross-section to promote the best filling effect. If the gate is set in the weak area of ​​the plastic part, a square cross-section should be used instead.
– A large cold slug hole can be set at the bottom of the injection port and the end of the runner to ensure good flow of the melt. The cold slug hole should be placed at the end of the melt flow direction.
– For the case where the annual ring-shaped wave flow marks are caused by insufficient resin performance, a low-viscosity resin can be selected.

2. Spiral wave flow marks:

– Cause: The melt does not flow smoothly in the runner, the runner section is narrow or the gloss is poor, which easily causes turbulence and produces spiral wave flow marks.
– Solution:
– Appropriately reduce the injection speed, or adopt a slow, fast, slow graded injection control method.
– Set the gate in the thick wall area or the wall side, and preferably use a handle-type, fan-shaped or diaphragm gate design.
– Expand the cross-sectional area of ​​the runner and gate to reduce flow resistance.
– Adjust the flow rate of cooling water in the mold to keep the mold temperature at an appropriate level.
– Within the process operating temperature range, appropriately increase the temperature of the barrel and nozzle to improve the fluidity of the melt.

3. Cloudy wave flow marks:

– Cause: When using ABS or other copolymer resins, if the processing temperature is too high, the volatile gases released by the resin and lubricant will cause cloud-like wave flow marks on the surface of the plastic part.
– Solution:
– Reduce the processing temperature of the mold and barrel.
– Improve the exhaust conditions of the mold.
– Reduce material temperature and mold filling speed.
– Expand gate cross section.
– Consider changing lubricant type or reducing lubricant dosage.

ABS plastic molding manufacturers can significantly improve the quality and technology of plastic molding products by analyzing the causes of flow marks and taking appropriate solutions.