Surface defects flow related

The inability to analyze flow-related material defects is a serious drawback shear and normal stresses associated with the foaming process can be responsible for a variety of defects such as cell structure irregularities, large voids, excessive curvature or even splitting of the foam-free surface, local density gradients and leakages from molds. 

Injection molding Basically the cause can be with the material, the machine, the operator, the operating conditions, the mold or the part design. To check on the material, try material from another supplier to check the machine, use same material, conditions, mold on another machine if the trouble is random, then it is probably the machine try a different operator on the machine trouble appears same location in the product, then flow conditions and look for problems from the front of the piston to the gate. The symptom-cause information is presented as issues related to appearance (color, surface finish and transparency), strength and shape defects and operation and symptoms. 

The above defects are due to different reasons. In the article (41), the stick-slip effect is related to self-excited oscillations initiated by the dependence of static friction-stress between the billet and the die at the time when they are in contact. The latter is connected with the viscoelastic properties of the rough billet surface and by lubricant squeezing out from the region of contact. Ward and co-workers (1) explain the stick-slip effect by the heating of billet dining the deformation. The pulsatory flow is assumed to be due to a competition between the viscosity and high elasticity of polymers (42). 

Experimental results regarding flow across the fiber tows are presented in Refs. 28 and 79. Parnas et al. [28] present experimental results for flow along the fiber tows. In their work, the formation of voids was not related to the dual-scale nature of the porous media, but to defects on the fiber tow surface. Other experimental results are also presented in some of the references from the previous paragraph. Usually, satisfactory agreement of the theoretical predictions with the experimental results is concluded by the authors, suggesting that capillarity might be neglected. 

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