The ripple problem of injection molding products and corresponding solutions
Wire harnesses are usually processed by injection molding. During the molding process, due to factors such as molding performance, types of raw materials and equipment working conditions, mold cavity structure, material rheology, internal and external quality of the material, the product may sometimes be defective. Based on many years of injection molding experience, we know that common appearance defects include: shrinkage, flashing, dark spots, flow marks, welds, bright lines, lack of glue, bubbles and flowers. Although the appearance defects will not affect the performance of connectors and cable assemblies, the high-quality appearance can reflect the strict production attitude of enterprises. Let us introduce the ripple problem.
The generation of ripples is related to the parameters of the injection molding, the mold and the material of the injection molding. There are several types of flow modes: snake flow mode, radial mode, wave mode, and fluorescence mode.
1, snake flow. When the gate depth is much smaller than the cavity inlet depth and the mold filling rate is very high, the melt flow becomes an unstable jet. The previous jet has solidified and the flowing melt fills the cavity. Serpentine ripples appear on the surface.
There are several measures to solve the problem of serpentine flow:
Change the process conditions. The method of reducing the injection rate will gradually eliminate the jet effect, thereby expanding the melt flow pattern. Prolonged flow will give the product a better surface quality. In addition, increasing the mold temperature and melt temperature will also reduce jet effects and increase melt flow. flow.
Change the size of the mold gate. When the depth of the gate is slightly smaller than the depth of the cavity, the expansion effect of the jet outlet makes the melt flowing out at the rear and leading edges of the jet, so the jet effect is not obvious. When the gate depth is equal to or close to the cavity depth, the mold filling rate is low and an expanded flow is formed.
Change the mold gate angle. The angle between the mold gate and the mold moving mold is 4o ~ 5o, so that when the melt flows out of the gate, the melt will be blocked by the cavity wall first, which can prevent the appearance of serpentine ripples.
Change the position of the mold gate. The gate of the mold is placed closest to the cavity wall of the mold (the direction perpendicular to the gate). When the melt flows out of the gate, it will first be blocked by the cavity wall. It also prevents the emergence of jets, allowing them to enter the extended stream, avoiding serpentine ripples.
2, in the radial mode, the spray rate is too large. When the melt is sprayed, because the melt has elasticity, when the melt flows from the barrel through the mold gate to the cavity, the melt elasticity recovers. Melt fracture is quickly caused to produce radial streaks.
There are several measures to solve the problem of radial patterns:
Change the process conditions. The use of high-pressure low-speed injection can increase the flow time of the elastic melt over the same flow length and increase the degree of elastic damage, thereby reducing the occurrence of radial streaks.
Change the shape of the gate. Enlarging the gate or changing the gate to a fan shape can slightly restore the melt’s elasticity before the melt enters the mold cavity to avoid melt fracture.
Increase the length of the main channel of the mold. Before the melt enters the cavity, its elasticity fails, which also prevents the melt from breaking.
Replace the equipment with an extension nozzle. Prolonging the flow path of the melt before entering the mold cavity increases the degree of melt elastic failure and also avoids radial lines due to melt fracture.
3, the wave pattern. During the melt filling process, new melt flows are continuously stacked from the inside, pushing forward waves to stagnate, and the forward wave edges are continuously stretched. Due to the flow resistance, the subsequent melt pressure rises again. The newly formed corrugations flatten and advance, causing stagnation and accumulation, creating a wavy pattern on the surface of the product. Especially when the injection speed is fast, the injection pressure is small, or the mold structure is unreasonable, the melt flow will advance and stop, and PP will slowly and slowly crystallize, which is likely to cause inconsistent crystallinity on the product surface. A wavy pattern is formed on the surface of the product.
There are several measures to solve the wave chart problem:
Change the process conditions. The use of high-pressure and low-speed injection can maintain the stability of the melt flow and prevent the occurrence of wave waves.
Increase the mold temperature. As the mold temperature increases, the melt fluidity increases. For crystalline polymers, higher temperatures are conducive to the uniformity of crystallization, thereby reducing the appearance of wave patterns.
Change the cavity structure. The structure of the mold may also cause the product surface to undulate. If the edge of the core is more prominent, the melt flow resistance is larger, which will cause the melt flow to be unstable, thereby forming a waveform. Therefore, changing the angle of the core to cushion the transition, keep the melt flow stable, and prevent waves from appearing.
Change the thickness of the product. The uneven thickness of the product will increase the melt flow resistance, which will cause the melt flow to be unstable. Therefore, the thickness of the product should be designed to be as uniform as possible to prevent wavy patterns.
4, fluorescent patterns. When the melt flows in the cavity, one end of the molecular chain near the solidified layer is fixed on the solidified layer, and the other end is stretched by the adjacent molecular chain in the flow direction. Because the wall near the mold cavity has the highest melt flow resistance and the smallest flow velocity, the minimum flow resistance and the highest flow velocity in the center of the cavity. This creates a velocity gradient in the direction of flow. Therefore, the injection rate is small, the injection pressure is large, or the product thickness is large. In the case of thinness, the melt near the cavity wall has the strongest shear force and the largest degree of orientation, and the polymer is stretched during the flow process to show internal stress, thereby generating fluorescent stripes on the surface of the product.
There are several measures to solve the problem of fluorescent patterns:
Change the process conditions. For medium-pressure and medium-speed injection, as the injection rate increases, the melt cooling time decreases at the same branch length, and the solidification of the melt per unit volume is relatively slow, the internal stress of the product is weakened, and the product surface is reduced. Appears on a fluorescent marker.
Increase the mold temperature. Higher mold temperature can accelerate the relaxation of macromolecules, reduce molecular orientation and internal stress, and thus reduce the appearance of fluorescent stripes on the surface of the product.
Change the cavity structure and increase product thickness. The thickness of the product is large, the melt is slowly cooled, the stress relaxation time is relatively prolonged, and the orientation stress will be reduced, thereby reducing the fluorescent stripes.
heat treatment (baking in the oven or boiling in hot water). Heat treatment enhances the movement of macromolecules, shortens the relaxation time, enhances the de-orientation effect, and reduces fluorescent stripes.