Study On Saddle Surface Formation And Process Of Stepless Biaxially Oriented Polypropylene Film

The rapid development of the economy promotes the expansion of plastic film market.And the comprehensive properties of ordinary film materials can’t meet the use requirement of people.Due to the excellent comprehensive properties,the application of biaxially oriented film is more and more widely.At present,the two main methods of producing the biaxially oriented film are flat film processing method and tube film processing method.Tube film processing method has the following disadvantages: film thickness is difficult to control,blowing process is not stable,production efficiency is low,and so on;Flat film forming method has the following disadvantages: complex structure,high cost,difficult to control,easy to take off the membrane,and so on.Jinping Qu,the professor of South China University of Technology,has put forward and invented a method and device for realizing film continuously strap-on biaxially stretching by the synergistic effect of double chevron wheels and bundling band.From the working principle to the mechanical structure,this new technology is different from the previously described biaxially oriented film processing method.This method has the advantages of simple structure,easy control and low cost.It is of great scientific significance to study the process of this new technology.In this paper,the stepless biaxially oriented film testing machine which was designed by the lab independently was the research object.According to the film stretching process,the physical model was established.Under the basic assumptions,the mathematical model was established by using the continuity equation,the motion equation and the constitutive equation.Combined with the shape of the saddle surface which was formed during the experiment,a reasonable assumption was made for the surface equation.According to the assumptions and boundary conditions,the velocity field and the saddle surface equation were solved and the influence of the key processing parameters on the shape of the saddle surface during the stretching process was revealed.In the paper,PP film was used to stretching experiments.The effect of processing parameters on the size of saddle surface was studied.The result showed that it was consistent with theoretical analysis,and the rationality of approximate solution of saddle surface equation was verified.The effect of processing parameters on tensile mechanical properties and thermal properties were studied by tensile test and DSC test.The results showed that with the increase of stretching temperature,transverse tensile strength and crystallinity of film first increase then decreases;with the increase of transverse tensile ratio,transverse tensile strength and crystallinity of film increased;When heating rate was constant,the effect of the film conveying rate on tensile mechanical properties and thermal properties was similar to the effect of stretching temperature,and the essence of this effect was changed the stretching temperature.Through the theoretical and experimental analysis,it was proved that the formation of saddle surface with reasonable size and reasonable processing parameters were critical to the biaxial stretching process.The relationship between the process parameters,the saddle shape and the film properties was analyzed,and a new idea of real-time measurement of the saddle surface size to monitor the stability of the film processing was proposed.The above research achievements enrich the theory and practice of biaxially oriented film processing technology.The research laid an important theoretical for the further research and application of film stretching technology and the popularization of this new technology.