Injection moulding process is a complex process involving plastic engineering materials, molds, injection molding machines, molding process and many other aspects. Appearance of defects in injection molded products is inevitable. Therefore, it is particularly important to find inherent mechanism of defects, location and types of defects that may be generated to guide project development. Today, we will talk to everyone about common appearance defects - flow marks.

 

If you want to directly explain flow marks in injection molding, it means "traces in flow process", but why there are "band patterns" or "band shape molding". European and American countries are described by “Flow Mark” and United Kingdom is defined by “Zebra Mark”. There are many types of flow marks in injection molding, and we talks about flow marks in shape of a strip groove and flow marks like surface drift.

 

As shown in figure: It was discovered by Japanese Plastic Molding Processing Association. It is mainly curing and shrinkage of melt front of flow front end, and then corrugated flow marks generated by former new melt is repeated. A large number of experiments prove that flow marks in injection molding appear at a position where melt speed is too slow or at a position where speed is slow due to a sudden change in speed. Reason why melt speed will be slow is mostly caused by a sharp change in speed when cross-sectional area of shape changes or injection moulding process is switched to pressure holding process as shown in Figure 1-1.

 

If flow of molten plastic is as shown in Figure 1-2, then surface layer of flow mark will shrink and roll on inside, molten plastic will repeat same action, so if it is case of Figure 1-1, it is better to set lifting speed before shape abrupt position. If it is case of Figure 1-2, it is necessary to reduce speed and pressure at the same time, so as to reduce rate of sharp change, speed setting at the end of injection moulding process and start of holding pressure must be same. If pressure setting of holding pressure is only reduced, natural flow under pressure control will reduce speed. But assuming that pressure drop is too low, flow will stop, and sometimes it will cause flow mark area to increase. Therefore, in entire injection moulding process, you need to fully understand relationship between pressure and speed to think about how to set a smooth speed change.

 

In process of injection molding, when viscosity of material used is very high, it is often seen that a large amount of release agent needs to be applied in order to be smoothly produced. As shown in Figure 1-3, surface layer needs to withstand shear stress generated by movement of internal melt. When shear stress is greater than friction of surface layer and mold surface, surface layer will be shredded when surface layer cannot withstand. At the same time, it will be dragged to the front of peeling. During dragging process, flow of melt is much easier, so internal pressure is reduced and shearing is reduced. Because of continuous process of flow, above action is repeated, so that drifting flow marks are produced on the surface of product.

 

Flow marks of Figures 1-4 are very similar to those of Figures 1-3, but are not tearing of surface layer, which is caused by interlacing of surface layer and sliding layer. Thus sliding portion leaves a cloud-like mark on molding surface.

 

1. Cause of flow marks product is highly dependent on wavefront velocity of flow.
2. Shape of flow marks is substantially matched to flow contour.

 

According to statistics of American Plastics Industry Association, among factors affecting quality of plastic parts, product structure design and mold design are reasonably occupied 60%, injection moulding process, plastic materials and injection molding machines account for 35%. Formation of flow marks has a certain relationship with mold, plastic materials and injection moulding process. In many cases, it is necessary to optimize and improve these three aspects in order to achieve good results, optimization and improvement of mold and product structure can achieve twice result with half effort, and it can also play a good role after replacing material with better fluidity.

 

1. How far will flow marks appear from gate? How to predict this distance?
2. Different gate sizes, flow materials, injection speeds, whether there will be flow marks in the end. These tasks need to be verified by experiments and MOLDFLOW's mold flow analysis to summarize rules.