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● Classification of flow pattern: 1) Snake flow pattern—When melt enters the moldcavity from the gate, it produces jet effect, which is like a snake on the surface of the product, so it is called snake flow pattern. 2) Wave pattern—The melt flows unsteadily in the mold cavity, which is fast and slow, and it is just like waves on the surface of products, so it is called wandering pattern.
3) Radiative striation—generally it only appears near the gate, and when the melt enters the mold cavity, it produces jet, which is radial on the surface of the product, so it is called radial striation. 4) Fluorescent streak—The shear stress caused by melt flow makes the surface of the product produce luster very similar to that of firefly body, so it is called fluorescent streak.
● The solution to the flow pattern
1) snake streak
● When the gate depth is much smaller than the entrance depth of the mold cavity, and the mold filling rate is very high, and the melt flow becomes unstable jet flow, the flowing melt after the front jet has solidified will fill the mold cavity, and then snake streak will appear on the surface of the product.
● Solutions:
① Change the process conditions. By reducing the injection rate, the jet effect will be gradually eliminated, and the melt flow mode will expand, which will make the product have better surface quality In addition, increasing the mold temperature and melt temperature will also reduce the jet effect and make the melt flow expand.
② Change the mold gate size. When the gate depth is slightly smaller than the cavity depth, the outlet expansion of the jet makes the melt at the back merge with the jet front not far from the front, which makes the jet effect not obvious. When the gate depth is equal to or close to the cavity depth, the mold filling rate is low, forming an expanding flow.
③ Change the angle of mold gate. The angle between the mold gate and the moving mold is 4o～5o ~ 5 o, so that when the melt flows out from the gate, it will be blocked by the cavity wall at first, which can prevent the occurrence of snake streak.
④ Change the mold gate position. Setting the mold gate at the nearest position to the cavity wall (perpendicular to the gate direction) will prevent the melt from flowing out of the gate from the cavity wall at first, and can also prevent the jet from appearing, making it an expanding flow, thus avoiding the occurrence of snake streak.
2) wavy lines
● In the process of melt filling, the new melt flow is continuously laminated from the inner layer, which pushes the front wave to move, while the front wave edge is continuously stretched. Because of the flow resistance, the melt pressure rises later and the ripples just formed in front are flattened and moved forward, resulting in stagnation accumulation, thus forming wavy lines on the product surface. Especially in the case of high injection rate, low injection pressure or unreasonable mold structure, the melt flows up and down, while PP crystallizes quickly and slowly, which is more likely to cause inconsistent crystallinity on the product surface and form wavy lines on the product surface.
● Solutions:
① Change the process conditions. By using high pressure and low speed injection, the stability of melt flow can be maintained, thus preventing the occurrence of wavy lines.
② Increase die temperature. With the increase of mold temperature, melt fluidity increases. For crystalline polymer, higher temperature is beneficial to crystallization uniformity, thus reducing the occurrence of wavy lines.
③ Change the cavity structure. The structure of the mold can also cause wavy lines on the surface of the product. If the edges and corners of the mold core are prominent, the melt flow resistance is large, which will cause the melt flow instability and form wavy lines. Therefore, changing the edges and corners of the mold core, making it buffer transition, keeping the melt flow stable, can prevent the occurrence of wavy lines.
④ Change the thickness of products. Uneven product thickness will make melt flow resistance increase and decrease, resulting in unstable melt flow. Therefore, the product thickness should be designed as uniform as possible, which can also prevent the occurrence of wavy lines.
3) Radiation pattern
● When the injection rate is too high and the melt is sprayed, because the melt has elasticity, when the melt flows rapidly from the charging barrel to the mold cavity through the mold gate, the elastic recovery of the melt is too fast, causing the melt to break and produce radiation lines.
● Solutions:
① Change the process conditions. By using high pressure and low speed injection, the flow time of elastic melt at the same flow length can be increased, and the degree of elastic failure can be increased, thus reducing the occurrence of radiation streaks.
② Change the mold gate shape. By increasing the gate or changing the gate into a fan shape, the elasticity of the melt can be recovered slightly before it enters the mold cavity, and the melt can be prevented from cracking.
③ lengthen the main runner length of the mold. Before the melt enters the mold cavity, it can make its elasticity fail, which can also avoid the melt fracture. ④ The equipment is replaced with extended nozzle. Lengthening the flow path of the melt before the mold cavity increases the degree of elastic failure of the melt, and can also avoid radial streaks caused by melt fracture.
4) Fluorescent pattern
When the melt flows in the mold cavity, one end of the molecular chain near the solidification layer is fixed on the solidification layer, while the other end is stretched by the adjacent molecular chain along the flow direction. The flow resistance and flow rate of the melt near the cavity wall are the largest and the flow resistance and flow rate at the center of the cavity are the largest, thus forming a velocity gradient in the flow direction. Therefore, when the injection rate is small, the injection pressure is high or the product thickness is thin, the shear force and orientation degree of the melt near the cavity wall are the strongest, and the polymer is stretched to show internal stress in the flow, resulting in fluorescent streaks on the product surface.
● Solutions:
① Change the process conditions. With medium-pressure and medium-speed injection, with the increase of injection rate, the cooling time of melt in the same flow branch length is reduced, the solidification of melt per unit volume is relatively slow, the internal stress of the product is weakened, and the appearance of fluorescence lines on the surface of the product is reduced.
② Increase the mold temperature. Compared with the mold temperature, the relaxation of macromolecules can be accelerated, and the molecular orientation and internal stress can be reduced, thus reducing the appearance of fluorescent lines on the surface of products.
③ Change the cavity structure and increase the product thickness. The thicker the product, the slower the melt cooling, the longer the stress relaxation time, and the smaller the orientation stress, thus reducing the fluorescent streaks.
④ Heat treatment (baking in oven or boiling in hot water). Heat treatment intensifies the movement of macromolecules, shortens the relaxation time, and strengthens the de-orientation effect, thus reducing the fluorescence streaks.
