Research On Related Defects Of Residual Stress In The Rib Reinforcement Area Of Injection Mold Products

Plastic products are widely used in aerospace,automotive,home appliances,daily necessities etc,which makes people have higher requirements on the surface quality of products.Among them,the residual stress of the products may cause defects,which are more obvious in crystalline polymer products such as polypropylene,including warpage,sink marks,stress whitening and cracking,Scholars mainly focus on the research of warpage,rarely involve shrinkage and stress whitening,especially quantitative analyses of shrinkage and stress whitening are even less from the point of view of residual stress.So far,enterprises generally solve the aforementioned defects by repeated trials,which cost time and labor,but it has not achieved satisfactory results.In this paper,the sink marks and stress whitening of multi-ribbed PP products are studied,a quantitative analysis method is proposed to provide theoretical guidance and technical means for improving the surface quality of product.Firstly,the laws of stress state inside the plastic products are gained by Moldflow.The area of the whitish area will be decreased as the decrease of residual stress in core layer;the sink marks also increased as the residual stress of the subsurface.Secondly,the high-low temperature tensile test is carried on the annealed samples with similar crystallinity at the defect.And the stress state of stress whitening and plastic deformation is obtained.The residual stress at the defect obtained in the simulation is compared with the experiment to determine whether the defect is generated or not.The prediction of sink marks and stress whitening is realized in product design stage,which provides a technical support to reduce unfounded test patterns.Finally,a variety of characterization methods were used to study the stress whitening region.It was found that the stress whitening is caused by the accumulation of massive micropores in a certain region,which causes the optical properties of the products to change.The stress whitening will reduce the crystallinity of the products and affects the crystal form by XRD experiments.The characterization experiment showed that the stress whitening only appeared in the core of the products for verifying the accuracy of the whitening criterion.In addition,the accuracy of sink marks determination was verified based on the sink marks on the surface of the products.The sink marks were improved by external gas assisted injection molding.The effect of stress whitening on the depth of sink marks was explained.In summary,the stress-strain properties of polypropylene at different temperatures in this paper were studied.The Moldflow was used to simulate the residual stress distribution at different locations of the products,and the inner connection between residual stress,sink marks and stress whitening was studied respectively.At a certain temperature,whether the sink marks and stress whitening are determined by the combination of finite element simulation and experiment,which is used to predict the formation of sink marks and stress whitening.Generally,this criterion can achieve product defect avoidance and guide the process of production.