Many people mistakenly believe that filling speed is just a rough guideline and can be set to any value. However, this is not actually true. As shown in the effective viscosity curve in the table below, when gradually increasing the filling speed from 10% to 99%, it is found that "the higher the injection speed, the higher the shear rate, but the lower the relative viscosity." What does this indicate?
High-speed filling
During high-speed filling, the shear rate between the two liquid surfaces is high. Due to shear thinning, the plastic's viscosity decreases, reducing overall flow resistance. Localized viscous heating also thins the solidified layer. Therefore, during the flow-controlled phase, filling behavior often depends on the volume to be filled. Specifically, during this flow-controlled phase, due to high filling speeds, the shear thinning effect of the melt is often significant, while the cooling effect of thin walls is less pronounced, resulting in a rate-dependent effect.
Low speed filling
When filling at low speeds controlled by heat conduction, the shear rate is low, the local viscosity is high, and the flow resistance is large. Because the hot plastic is replenished at a slow rate and flows more slowly, the heat conduction effect is more pronounced, and heat is quickly carried away by the cold mold wall. This, combined with a small amount of viscous heating, results in a thicker solidified layer, further increasing flow resistance in areas with thinner walls.
According to the effective viscosity test results, in order to ensure that the filling speed is less affected by the viscosity, the filling speed should be selected in the lower viscosity area. For example, when the speed is 50%, the corresponding pressure should not be lower than 75Bar.
Improper feed adjustment, lack of material or excess material
Inaccurate feeding metering or abnormal operation of the feeding control system, abnormal injection cycle due to limitations of the injection molding machine, mold or operating conditions, low pre-plastic back pressure or low density of particles in the barrel may all cause material shortage. For particles with large particles and many gaps and plastics with large crystalline specific volume changes such as polyethylene, polypropylene, nylon, etc., as well as plastics with high viscosity such as ABS, a higher material amount should be adjusted, and the material amount should be increased when the material temperature is high.
When there is too much material at the end of the barrel, the screw will consume extra injection pressure to compact and push the excess material in the barrel during injection, which greatly reduces the effective injection pressure of the plastic entering the mold cavity and makes it difficult to fill the product.
Injection pressure is too low, injection time is short, plunger or screw retracts too early
Molten plastic has high viscosity and poor fluidity at relatively low operating temperatures, so it should be injected at higher pressure and speed. For example, when making colored ABS parts, the colorant's high-temperature resistance limits the barrel's heating temperature, which requires higher injection pressure and longer injection time than usual to compensate.
Slow injection speed
Injection speed is of great significance for products with complex shapes, large thickness variations, long process times, and for plastics with high viscosity such as toughened ABS. When high pressure cannot fully inject the product, high-speed injection should be considered to overcome the problem of incomplete injection.
Material temperature is too low
The temperature at the front end of the barrel is low, and the viscosity of the molten material entering the cavity rises too early to the point where it is difficult to flow due to the cooling effect of the mold, which hinders the filling of the far end; the temperature at the rear end of the barrel is low, and the plastic with high viscosity has difficulty flowing, which hinders the forward movement of the screw. As a result, it seems that the pressure displayed on the pressure gauge is sufficient, but in fact the molten material enters the cavity at low pressure and low speed;
If the nozzle temperature is low, it may be that the nozzle has been in contact with the cold mold for a long time during fixed feeding and has lost heat, or the nozzle heating ring has insufficient heat supply or poor contact, resulting in low material temperature, which may block the mold's feeding channel; if the mold does not have a cold well, use a self-locking nozzle and adopt a post-feeding procedure, the nozzle can better maintain the required temperature; when the nozzle is too cold when the machine is just started, sometimes you can use a flame gun for external heating to accelerate the nozzle heating.
