Flow Pattern Of Injection Defect

Classification of flow lines:  

1) Snake flow pattern  — When the melt enters the mould cavity from the gate, it produces a jet effect and appears on the surface of the product like a snake, so it is called a snake flow pattern. 

2) Wave pattern  — The melt flow in the mould cavity is not stable, sometimes fast and sometimes slow, and it appears on the surface of the product like waves, so it is called a wandering pattern. 

3) Radiation pattern  —  generally only appears near the gate. When the melt enters the mould cavity, it sprays, which is radial on the surface of the product, so it is called the radiation pattern. 

4) Fluorescent pattern  — The shear stress generated by the melt flow causes the surface of the product to produce a lustre that is very similar to the body of the firefly, so it is called the fluorescent pattern.

The solution to flow lines:

1) Snake flow pattern 

When the gate depth is much smaller than the cavity entrance depth, and the filling rate is very high, when the melt flow becomes unstable jet flow, the front jet has solidified and the flowing melt fills the cavity, and the product will be Snake flow patterns appear on the surface. 

solution:

① Change the process conditions. The method of reducing the injection rate will gradually eliminate the jet effect and expand the flow of the melt flow. The expanded flow will make the product have a better surface quality; in addition, increasing the mould temperature and melt temperature will also reduce the jet effect and expand the melt flow. flow.

② Change the gate size of the mould. When the gate depth is slightly smaller than the cavity depth, the outlet expansion of the jet will fuse the melt behind and the leading edge of the jet that is not far out in front, so that the jet effect is not obvious. When the gate depth is equal to or close to the depth of the cavity, the mould filling rate is low and an expanding flow is formed.  

③Change the angle of the mould gate. Make the angle between the mould gate and the mold movable mould 4o ~ 5o, so that when the melt flows out of the gate, it will first be blocked by the cavity wall, which can prevent the appearance of snake flow patterns. 

④ Change the gate position of the mould. Set the mould gate at the closest position to the mould cavity wall (perpendicular to the gate direction). When the melt flows out of the gate, it will be blocked by the cavity wall first, and it can also prevent the jet from appearing and make it Extend the flow to avoid the appearance of snake flow patterns.

2) Wave pattern 

During the melt filling process, the new melt flow is continuously laminated from the inside, pushing the forward wave stagnation flow to move, and at the same time, the forward wave edge is continuously stretched. Due to the flow resistance, the later melt-pressure rises and then the front The newly formed corrugations are flattened and advanced, causing the stagnant flow to accumulate, thereby forming wavy patterns on the surface of the product. Especially when the injection rate is fast, the injection pressure is small, or the mould structure is unreasonable, the melt flow will stop and the PP will crystallize quickly and slowly, which will cause inconsistent crystallinity on the surface of the product and form wavy lines on the surface of the product.

solution:

① Change the process conditions. Using high-pressure and low-speed injection can maintain the stability of the melt flow and prevent the appearance of wave patterns. 

② Increase the mould temperature. As the mould temperature increases, the melt fluidity increases. For crystalline polymers, a higher temperature is beneficial to the uniformity of crystallization, thereby reducing the appearance of wave patterns.

③Change the cavity structure. The structure of the mould can also cause waves to appear on the surface of the product. For example, the edges and corners of the mould core are more prominent and the melt flow resistance is greater, which will cause the melt flow to be unstable and form wave patterns. Therefore, changing the edges and corners of the mould core to buffer the transition, keep the melt flow stable, and prevent the appearance of wavy lines.

④ Change the thickness of the product. The uneven thickness of the product will cause the melt flow resistance to be large and sometimes small, resulting in unstable melt flow. Therefore, try to design the product thickness to a uniform thickness to prevent the appearance of wavy lines.

3) Radiation pattern 

If the injection rate is too large, when the melt is sprayed, because the melt has elasticity, when the melt flows from the barrel to the mould cavity through the mould gate, the melt produces elastic recovery too fast, causing the melt to break and produce radial lines.

solution:

① Change the process conditions. The use of high-pressure and low-speed injection can increase the flow time of the elastic melt at the same flow length and increase the degree of elastic failure, thereby reducing the appearance of radial lines. 

② Change the shape of the mould gate. Enlarging the gate or changing the gate to a fan shape can restore the elasticity of the melt before the melt enters the cavity to avoid melt fracture. 

③ Extend the length of the main runner of the mould. Before the melt enters the cavity, its elasticity is invalidated, which can also prevent the melt from breaking. ④The equipment is replaced with an extended nozzle. Lengthening the flow path of the melt before the mould cavity increases the degree of elastic failure of the melt, and can also avoid the occurrence of radial lines due to melting fracture.

4) Fluorescent pattern 

When the melt flows in the mould cavity, one end of the molecular chain close to the solidified layer is fixed on the solidified layer, and the other end is stretched along the flow direction by the adjacent molecular chain.

Since the melt flow resistance close to the cavity wall is the largest and the flow rate is the smallest, the flow resistance at the centre of the cavity is the smallest and the flow rate is the largest, so that a velocity gradient is formed in the flow direction. Therefore, the injection rate is small and the injection pressure is large. Or when the product is thinner, the melt shear force near the cavity wall is the strongest and the degree of orientation is the highest. The polymer is stretched during the flow and exhibits internal stress, causing fluorescent lines to appear on the surface of the product.

solution: 

① Change the process conditions. With medium-pressure and medium-speed injection, as the injection rate increases, the cooling time of the melt in the same flow branch length decreases, the solidification of the melt per unit volume is relatively slow, the internal stress of the product is weakened, and the appearance of fluorescent lines on the surface of the product is reduced. 

②Increase the mould temperature. Compared with the mould temperature, the relaxation of macromolecules is accelerated, and the molecular orientation and internal stress are reduced, thereby reducing the appearance of fluorescent lines on the surface of the product. 

③Change the cavity structure to increase the thickness of the product. The thickness of the product is larger, the melt cooling is slower, the stress relaxation time is relatively prolonged, and the orientation stress will be reduced, thereby reducing the fluorescent pattern. 

④ Heat treatment (baking in oven or boiling in hot water). The heat treatment intensifies the movement of macromolecules, shortens the relaxation time, strengthens the de-orientation effect, and reduces the fluorescence streaks.