One of the Injection Defects - Flow Lines

  • Jan 21,2022

Classification of flow lines:


1) Snake flow line —When the material enters the plastic injection mold cavity from the gate, creating a jet effect, which is displayed on the surface of the product like a snake, so it is called a snake flow line.

2) Wavy line —The material in the cavity flow unevenly, sometimes fast and sometimes slow, which is like a wave on the surface of the product, so it is called a wavy line.

3) Radial line —Generally only appears near the gate. When the material enters the mold cavity, it is sprayed, which is radial on the surface of the product, so it is called a radial line.

4) Fluorescent line —The shear stress generated by the flow of the material makes the surface of the product produce a luster that is very similar to the firefly, so it is called a fluorescent line.



The solutions for the flow lines as below.


-- Snake flow line

When the gate depth is much smaller than the cavity entrance depth, and the filling rate is very high, the material flow becomes an unstable jet flow, and the front jet has solidified and the flow material behind fills the cavity, which will cause the product snake flow line appears on the surface.


Solutions

① Change the process conditions. Using the method of reducing the injection rate will gradually eliminate the jet effect and expand the flow of the material, which will make the product have better surface quality; in addition, increasing the mold temperature and material temperature will also reduce the jet effect and expand the material flow.

② Change the plastic injection mold gate size. When the gate depth is slightly smaller than the cavity depth, the outlet expansion of the jet causes the material behind to merge with the leading edge of the jet that flows out not far ahead, so that the jet effect is not obvious. When the gate depth is equal to or close to the cavity depth, the filling rate is low and an expanding flow is formed.

③ Change the gate angle of the mold. The angle between the gate of the plastic injection mold and the core of the mold is 4o ~ 5o, so that when the material flows out of the gate, it will be blocked by the wall of the mold cavity firstly, which can prevent the appearance of snake flow line.

④ Change the mold gate position. Set the gate at the closest position to the mold cavity wall (perpendicular to the gate direction). When the material flows out of the gate, it will be blocked by the plastic injetion mold cavity wall firstly, and it can also prevent the jet, making it a expand flow to avoid the appearance of snake flow line.


-- Wavy line

During the process of material filling, new material flow is continuously extruded from the inner layer, pushing forward wave stagnation to move, and at the same time, the front wave edge is continuously stretched. Due to the flow resistance make the later material pressure rises, and pushes the front newly formed corrugations forward flatly, resulting in accumulation of stagnant flow, thus forming wave lines on the surface of the product. Especially when the injection rate is fast, the injection pressure is small or the mold structure is unreasonable, the material flows sometimes, and stops sometimes, and the PP crystallizes fast sometimes and slow sometimes. It is more likely to cause inconsistent crystallinity on the surface of the product, forming a wave lines on the surface of the product.


Solutions

① Change the process conditions. The use of high pressure and low speed injection can maintain the stability of the material flow, thereby preventing the appearance of wavy lines.

② Increase the mold temperature. As the mold temperature increases, the material fluidity increases. For crystalline polymers, higher temperatures are beneficial to the uniformity of crystallization, thereby reducing the appearance of wavy lines.

③ Change the cavity structure. The structure of the mold can also cause wavy lines on the surface of the product. For example, the edges and corners of the mold core are more prominent, and the material flow resistance is large, which will cause the material flow to be unstable, resulting in the formation of wavy lines. Therefore, change the edges and corners of the mold core to buffer the transition, keep the material 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 material flow resistance to be large sometimes and small sometimes, resulting in unstable material flow. Therefore, the thickness of the product should be designed as a uniform thickness, which can also prevent the appearance of wavy lines.


-- Radial line

When the injection rate is too large, and the material is sprayed, due to the elasticity of the material, when the material flows rapidly from the barrel through the mold gate to the mold cavity, the elastic recovery of the material is too fast, resulting in material rupture and creating radial lines.


Solutions

① Change the process conditions. Using high pressure and low speed injection can increase the flow time of the elastic material 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 mold gate. Enlarging the gate or changing the gate to a fan shape can restore the elasticity of the material slightly before it enters the mold cavity to avoid material fracture.

③ Lengthen the length of the main runner of the mold. Breaking the material can also be avoided by disabling the elasticity of the material before it enters the mold cavity.

④ The equipment is replaced with an extension nozzle. Lengthening the flow path of the material before the mold cavity, it can increase the degree of elastic failure of the material, and also avoids radial lines due to material fracture.


-- Fluorescent line

When the material flows in the cavity, one end of the molecular chain close to the solidified layer is fixed on the solidified layer, while the other end is stretched along the flow direction by the adjacent molecular chain. Since the material flow resistance near the cavity wall is the largest and the flow rate is the smallest, while the flow resistance at the center of the cavity is the smallest and the flow rate is the largest, a velocity gradient is formed in the flow direction, so when the injection rate is small and the injection pressure is large, or the thickness of the product is thin, the material near the cavity wall has the strongest shear force and the largest degree of orientation, and the polymer is stretched in the flow to show internal stress, resulting in fluorescent lines on the surface of the product.


Solutions

① Change the process conditions. Using medium pressure and medium speed injection, with the increase of injection rate, the cooling time of the material at the same flow branch length decreases, the solidification of the material 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.

② Increase the mold temperature. Higher mold temperature can accelerate the relaxation of macromolecules, reduce molecular orientation and internal stress, thereby reducing the appearance of fluorescent lines on the surface of the product.

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

④ Heat treatment. Heat treatment intensifies the movement of macromolecules, shortens the relaxation time, and strengthens the de-orientation effect, thereby reducing the fluorescence lines.







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