Weld line fractures

Hadinata C, Zheng R, Kennedy PK, Zhu P, Edward G, Lee Wo D, Tanner RI (2008) The effects of additives on material properties and shrinkage characteristics. Keynote Lecture at 24th Annual Conference of the Polymer Processing Society, Salerno Hagerman EM (1973) Weld-line fracture in molded parts. Plast Eng 29 67-69 Hand GL (1962) A thecny of anisotropic fluids. J Fluid Mech 13 33-46 Harris JB, Pittma TFT (1975) Equivalent ellipsoidal axis ratios of slender tod-Uke particles. J Coll Interf Sci 50 280-282... 

Hagerman, E.M., Weld-line Fracture in Molded Parts, Plastics Engineering, 29 10,67-69 (1973). 

Moulded plastics will also have crack initiation sites created by moulding defects such as weld lines, gates, etc and by filler particles such as pigments, stabilisers, etc. And, of course, stress concentrations caused by sharp geometrical discontinuities will be a major source of fatigue cracks. Fig. 2.72 shows a typical fatigue fracture in which the crack has propagated from a surface flaw. 

However, a plate that deforms to this degree is not in danger of brittle fracture at low impact energy. Hence, the value of the test is to screen injection mouldings for brittle failure, at forces well below fmax, due to orientation, weld lines or a weak surface structure. 

Make a collection of broken plastics products, then assess from the fracture appearance (i) Whether fracture was due to yielding, fatigue or environmental stress cracking (ii) whether the major load system was tension, bending or torsion (iii) whether there were processing features involved such as weld lines and orientation (iv) whether design faults could have been remedied. 

Failure analysis can be related to potential crack growth behavior to prevent fracture. Fracture is a crack-dominated failure mode. For fracture to occur, a crack must somehow be created, then initiate, and finally propagates. The prevention of any of these events will prevent fracture. Cracks can be considered elastic discontinuities that can come from a variety of sources such as internal voids or dirt, and/or surface scratch, embrittlement, or weld line. Cracks can be consequences of faulty design, poor processing, and/or poor handling of raw material, assuming material arrived clean (Figure 7.6). 

Selden and co-workers [59] measured weld line tensile strength of injection moulded glass fibre reinforced PA 6 and talc filled PP. Fracture surfaces were also examined using SEM. 

FIGURE 14.3 Typical fracture surface of the low-molecular-weight PVC blend showing the trough of the weld line where fracture initiated and magnifications of the initiation site for (a) Fracture surface (b)PVCA4. 67,000 (c)PVC 83,000 and (d) PVC 141,000 (Jarusa et al., 2000)... 

The elongation of the PVC phase at the weld line was found to be the cause of weak weld lines. The domain shape at the fracture initiation site was used in conjunction with a modified Nielsen approach to predict the ductile to brittle transition at the weld line. For the composition studied, a critical aspect ratio of the PV C phase of 1.24 was determined. Elongation of the PVC domains above this critical value... 

Special Crack Causes. Apart from crack formation due primarily to loading and material, and which has just been discussed above, in fracture analysis there are always fracture characteristics that are attributable to design, processing deficiencies, or both. However, it is often not possible to definitively assign, for example, failing weld lines and voids to defects in design or processing. 

Weld Lines. In unreinforced polymers, smooth fracture surfaces may be a sign of weld-line failure (Fig. 13a). This is a case of adhesive failure the melt fronts were imable to join together in such a way that cohesive failure and the corresponding deformation in the fracture surface or different fracture planes, shown in Figure 13b, could occur. 

If blends are involved, the poor miscibility of most polymers with each other generally leads to a situation in which one component forms a matrix in which the other is dispersed. It is then possible that only the matrix component is foimd in the weld line. Proof of this can be established, where the blend composition is appropriate, by conducting comparative etching at the fractured weld-line surface and a reference fracture through the blend. 

Fig. 13. Part made from recycled PE. (a) Fracture responsible for damage in weld line and (b) fracture due to base material.

Seldon [10] measured the weld line strength and impact, and flexural and tensile properties of injection-molded specimens of glass fiber-reinforced polyamide 6 and talc-filled polypropylene (PP). Further information was obtained by examination of fracture surfaces by scanning electron microscopy. 

However, the mouldings tend to be highly anisotropic, have low weld line strength and low toughness with a fracture behaviour resembling that of wood. As polyesters, TLCP are susceptible to hydrolysis by acids and bases. 

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