To this end, we will explore the relationship between injection conditions and flow resistance one by one. Before understanding the flow resistance, let us first understand three important concepts, which are "Newtonian fluid", "cooling consolidated monolayer" and "flow resistance" impact on injection molding.
Is the flow of water the same as the flow of plastic?
The answer is: different.
Water belongs to "Newtonian fluid" and plastic belongs to "non-Newtonian fluid". The so-called "Newtonian fluid" characteristic means that when you stir with an agitator's arm, a downward whirlpool of water will be generated. However, non-Newtonian fluid will produce the Weissenberg effect, and non-Newtonian fluid is divided into two characteristics," shear-thickening "and" shear-thinning ". That is, plastic belongs to shear-thinning, meanings the faster the agitation, the thinner the viscosity of the plastic.
What is the cooling consolidated monolayer when plastic flows?
For PP materials, the temperature of PP must be at least 180 degrees or more. Assuming the mold temperature is 30 degrees, when doing injection molding, the melted high-temperature PP material contacts the relatively cold mold, a layer will be generated instantly, and the consolidated monolayer will become thicker and thicker with time. As the consolidated monolayer gets thicker and thicker, space, where plastic can flow, becomes narrower, and the "flow resistance" will increase, that is, it will become more and more difficult for the plastic to move forward.
What effect does a larger flow resistance impact on the stability of injection molding?
To fill plastic into a mold, its "injection pressure" must be greater than "flow resistance", and its ability to injection pressure depends on the horsepower of the injection machine. Suppose that the injection was originally set at a pressure of 80 Bar, but because of the "flow resistance" is large, the injection pressure must be raised to a limit value of 140 Bar. In the long run, it will negatively affect the life of the mold and the machine. For example, when riding a motorcycle, riding at speed 70 for 2 hours is usually not a problem, but if you continue riding at speed 120 for 2 hours, it may cause damage to the motorcycle engine. Therefore, if you can reduce the flow resistance, you can avoid testing the performance of the machine, and it can be smoother and more stable during production.
The quality of injection molding is closely related to the "flow resistance" at the time of injection. The reasons for the increase in "flow resistance" can be divided into 6 categories, we will introduce you in the future. See you next time.