Discussion on testing methods and problems of fric

2022-08-12
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Discussion on the testing methods and problems of the friction coefficient of packaging materials

recently, the development of the flexible packaging industry has advanced by leaps and bounds. At the same time, the accompanying quality problems have also become prominent, and the complaints of users about the breakage, slippage and disconnection of the packaging production line are increasing. Many flexible packaging material factories and flexible packaging printing factories have caused huge losses. In fact, the root of this phenomenon lies in the friction properties of flexible packaging materials. Therefore, the friction performance of flexible packaging materials is very important for production. The above problems can be effectively solved by testing the friction coefficient of packaging materials, and the size of the friction coefficient must be determined by experiments

equipment and method for measuring friction coefficient

1 Test equipment and test standards

① test equipment: mc-600 friction coefficient instrument of Beijing landmark packaging equipment company: vmcpp.25 μ m. Heat sealing interview sample

② the test standard refers to GB

2. Test method

① take three 8cm × 20cm sample. The required sample of the test surface shall be flat and free of wrinkles and scars that may change the friction properties. The edge of the sample should be smooth. The test surface of the sample shall be free of dust. Fingerprints and any foreign matter that may change the surface properties. Fortunately, tragedies such as the grenfelta fire are extremely rare substances

② the state adjustment of the sample and the standard environment for the test. In the standard environment with temperature of 23 ± 2, because people only test the hardness of materials more and less, and the strength of materials less, and the relative humidity of 45% - 55%, adjust the sample state for at least 16 hours, and then carry out the test in the same environment

③ after adjusting the level of the equipment, fix the test surface of a sample upward and smoothly on the horizontal test bench. The length direction of the sample and the test bench should be parallel. Put the test surface of another sample downward, wrap the slider, and fix the sample on both sides of the slider with adhesive tape

④ put the slider fixed with the sample on the first place without impact. Don't slack off in the center of one sample, and make the test direction of the two samples parallel to the sliding direction, and the force measuring system just doesn't bear force

⑤ set the parameters, and then start the test. After the two samples are kept in contact for 15s, the two samples move relatively, and the first peak of the force is static friction --fs. The average value of the force within 6cm of the relative movement of the two samples (excluding static friction) is dynamic friction --fd. This process is automatically controlled, and the curve is displayed in real time during the test process. The friction coefficient is automatically calculated at the end of the experiment

after the average of the three groups of experiments μ S=0.543, μ D=0.478, from the experimental results, the three groups of experimental data are relatively stable, and the static friction coefficient is greater than the dynamic friction coefficient

due to the function of drawing friction coefficient curve, the uniformity of materials can be understood according to the drawn curve. According to the analysis of relevant data, the friction coefficient of flexible packaging materials is generally less than 0.3, which is more suitable for production practice. Based on this, the data is relatively large. In this way, the friction coefficient can be controlled by selecting the type and content of opening agent and smoothing agent in the resin, so as to meet the requirements of users

so, what is the appropriate friction coefficient? The author believes that the best friction coefficient parameters should be determined through continuous experiments according to the on-site use of materials

several problems in friction coefficient test: 1 During the communication with some enterprises, we learned that the measured friction coefficient is greater than 1, and the data provided, such as CPP sample μ S=1.189, μ D=0.591, and others are larger. Therefore, the author consulted a lot of data. Only when one of the two objects to be measured is placed obliquely as an inclined plane, and the other is placed on the inclined plane to slide down along it, and gradually reduce the inclined angle of the inclined plane to a certain degree, the friction coefficient is greater than or equal to 1. According to the conventional test method, the author has done many experiments with different materials repeatedly, and there is no phenomenon that the friction coefficient is greater than 1

① from the relevant data of friction, we know that the friction in static friction is called static friction. When the tangential external force gradually increases but the two objects remain relatively static, the static friction increases with the increase of the tangential external force, but the increase of the static friction can only reach a certain maximum value. When the tangential external force is greater than this maximum value, the two objects will move from relative static to relative sliding. This maximum value of static friction is called "maximum static friction". This limit friction is expressed in Fmax. The maximum static friction is proportional to the positive pressure n between the contact surfaces of two objects, that is Fmax= μ nN。 Fmax is the maximum static friction, n is the positive pressure, where the proportional constant μ N is called the static friction coefficient, which is a value without unit. μ 0 is related to the material, smoothness and roughness of the contact surface, dry and wet conditions and other factors, but has nothing to do with the size of the contact surface. From the above introduction, the static friction coefficient μ 0 It should not be that the spherical structure can carry drugs greater than 1 around the body

② dynamic friction. The friction appearing in dynamic friction is called "dynamic friction". When the principal force applied to an object is greater than the maximum static friction, the object begins to move. After moving, if the applied force is reduced, the object will stop moving again. This situation shows that after the object moves, there is also a force to prevent the object from moving, that is, there is friction resistance. The friction generated by the movement of this object is called dynamic friction. The ratio of the magnitude of dynamic friction to the magnitude of positive pressure is called "dynamic friction coefficient". In the formula μ Is the proportional coefficient, whose value is related to the material and surface roughness of the two objects in contact, as well as the relative sliding speed between the two objects in contact. In most cases, the dynamic friction coefficient will slightly decrease with the increase of the relative sliding speed. From the definition, the dynamic friction coefficient must be less than the static friction coefficient, and it will not be greater than 1

if the friction coefficient is greater than 1 in the user's actual test, it is not the true reflection of the material. The reason should be found from the aspects of operation and whether the surface properties of the material have been artificially changed

2. The dynamic friction coefficient measured during use is greater than the static friction coefficient. From the above analysis, it can be seen that the dynamic friction coefficient is certainly smaller than the static friction coefficient. This result should be caused by the abnormality of the test material surface at the rear end (such as pinholes, foreign objects, etc.), or the external force during the test process. Abnormal mutations can be found here by observing the friction curve. (end)

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