The amount of plastic used in automobile has become an important symbol to measure the level of automobile design and manufacturing technology. At present, more than 10% of the domestic automotive parts are made from plastic including exterior trims such as bumper, skirts and fenders, and exterior trims such as instrument panels, panels and pillars, also engine peripheral parts such as GOP, covers and inlet manifolds. Most of the plastic automotive parts are injection molded because the process has unbeatable advantages such as short molding lead time, high production efficiency and low manufacturing cost is low. However, the coin always have two sides. There are also some plastic defects such as Tiger Line that we could not overlook
What is Tiger Flow Line then?
Tiger lines are streaks, or wave lines – look like patterns on tiger skin, with stripes about perpendicular to the direction of melt flow,which is a kind of flow mark. It usually shows up on automotive plastic molding parts with large area such as bumper, dashboard, door board and post
What are the causes and how to fix it?
The flow line defect could be contributed to plastic injection mold, plastic material and molding process.
Cause: Polypropylene (PP), commonly used in automotive injection molding parts, needs to be added with a toughened material such as elastomer due to the requirement of high impact resistance . The toughened material is subjected to tensile and shear action in the injection molding process, resulting in micro deformation and unstable flow of melt. Thus “tiger” flow mark shows up on the part surface. resulting in tiger skin grain. The more toughened composition in the material, the more likely the tiger flow mark appears.
Remedy：A / reduce the content of elastomer (but this is often impossible due to the high impact strength required by many automotive parts);
B / improve the flow of the material (in practice, it is found that the material melting finger can effectively improve the flow line as long as it reaches above 25g/10min).
C / the occurrence of tiger flow mark can be effectively reduced by using materials with wide molecular weight distribution.
Cause:The thinner the wall thickness of the injection parts is, the farther the filling flow path is, or the larger the flow length ratio is, the more likely that the molten plastic flow is unstale during the molding process and tiger flow line occurs.
a）Reduce the shooting rate (the aim is to allow more time for the elastomer composition in the melt to achieve elastic recovery and to avoid sudden mold expansion resulting in pressure fluctuations and flow velocity changes at the front of the flow)
b /Increase plastic processing temperature and mold temperature (the purpose is to reduce the thickness of plastic frozen layer, reduce flow resistance, and improve the replication effect of melt and mold surface) ）
c/ Shortening the filling path of a single gate can reduce the mold filling resistance of the melt and ensure the stability of the melt flow
dAppropriate increase in the wall thickness of injection molded parts, but in the automotive lightweight requirements are getting higher and higher today, thin-walled is the development trend of automotive injection molded parts, so it is not realistic to eliminate tiger flow lines by increasing the wall thickness.
Cause: Poor design of runner and injection gate, and poor venting Remedy:
A/to expand the section thickness and width of the injection gate and increase the diameter of the runner, the gate thickness must meet the requirment 0.8 ≤ 0.9 times of the wall thickness of the part (in order to reduce the resistance in the runner and thus reduce the expansion effect of the plastic injectin mold
B/increase the number of gates and minimize the flow length ratio (in the process of adjusting the molding parameter, we found that there are no tiger flow lines near the gate, and they will show up where the flow exceeds a certain distance
C/Try to use direct gate, side gate and fan gate whose size and cross section area gradually enlarge. Avoid using sub-gate and point gate with decreasing cross section area. In practice, it is found that tiger flow mark could appear on the parts with these type of gates.
D/Mold vent is as good as possible, which can help to achieve high speed injection and avoid appearance quality defects caused by stagnant flow at the front caused by trapped gas.
At as a injection molder with over 20 years of experience, HS Mold knows how to help you avoid defects in the injection molding process!