This is an answer to a netizen who asked about the quality control of injection molding materials. Can MI be used only to control the quality of plastic pellets? When MI is increased, does it represent the degradation of plastic properties? Because many people think that the cracking of plastic products should have no direct relationship with MI value.
About this question, I think that if you want to control the molding quality of plastic, logically, it is indeed necessary to control the mechanical properties of plastic, such as Tensile Strength, Flexural Strength, Impact Strength…
And other direct indicators. Because these characteristics really directly affect the characteristics of the use of plastic, we generally consider these characteristics when designing plastic parts before choosing plastic raw materials.
But these indicators directly, if at the time of each incoming inspection, all need to redo a test and inspection, every time is about the need to spend about 0.5 tons (the real data we’ll need to verify) plastic raw materials, but also have to develop a set of mold, with each test also need to occupy a plastic injection molding machine, for general corporate, This is not a small number, so few companies are willing to lose this kind of incoming inspection.
Therefore, our company took the next step and used the indirect index MI value of plastic to measure whether the plastic was improperly added with regrind resin or excessively remixed chain particles, resulting in the possibility of degradation of plastic characteristics. Because MI testing is so easy and cheap, only a few grams of testing is enough, and it does not require the same trouble as testing mechanical properties. Reference: Plastic MFI/MI/MFR (melt index/melt index) inspection method.
Our experience shows that its MI value will increase rapidly if the plasticine is improperly added with too much secondary material or excessively remixed, stained, and extracted. Detailed I already have a space before the MI value if after processing, such as lead screw, the originally long molecular chains will be cut for short-chain, so after the melting flow rate increases, which is the MI value will rise, but also can reduce the strength of the plastic, because can intertwine long molecular chains into the short-chain is not wrapped.
Therefore, in consideration of economic factors and practicality, the use of MI value to do a preliminary plasticine quality inspection can effectively control its quality, reduce the cost of quality control, and speed up the inspection speed; When found in doubt, then consider the measured Tensile Strength, Flexural Strength, Impact Strength… And other direct indicators.
Q: In addition, if the plastic can break itself (like the broken lines in the windshield when hit by a rock), is there any suggestion to monitor the plastic properties? Hardness? Toughness? Or something else?
A: This problem must first understand plastic cracking is caused by internal stress or external stress. If it is external stress, it is necessary to control the impact strength of plastic (impact strength is divided into two kinds of incision and no incision, must depend on demand, or both); If it is caused by internal stress, it is necessary to check the plastic mold design and plastic forming conditions, to see how to eliminate internal stress. For example, suppose a stone hit the windshield in the broken lines. In that case, this should be caused by external cracking, so to control the impact strength, and is notched and notched impact strength should be considered, with no notched impact strength to measure how much force, plastic may crack; The impact strength of the incision is then used to measure, once there is a rupture, and then how much force will be the whole cracking.
