Injection moulding defects

Injection moulding defects are classified into seventeen sections ... 

The surface finish of injection-moulded foamed articles are not ideal because of external defects, burst bubbles, etc. This can be avoided by sandwich moulding. This is accomplished by initially injecting a slug of polymer melt into the mould followed immediately by a polymer containing a blowing agent, which chases the initial polymer into all the interstices. 

In summary, therefore, processing, and in particular injection moulding, can introduce limited chemical degradation, local polymer orientation, orientation of short fibre reinforcements, internal stresses, warpage, shrinkage and defects such as weld lines and voids. 

M. Bulters and A. Schepens, The Origin of the Surface Defect Slip-stick on Injection Moulded Products, Proc. Annu. Meet., American Institute of Chemical Engineering, Los Angeles, CA, 2000. 

The influence of injection moulding conditions on the occurrence of surface defects on PVC pipe fittings was studied experimentally and by finite element analysis. Comparison of the experimental results with theoretical predictions allowed optimisation of the processing conditions for the production of parts having the required surface quality. 3 refs. 

Scanning electron micrographs of surface defects on (a) The inner surface of a blow-moulded HOPE bottle (b) a polyethylene injection moulding. In both micrographs the ridges run at 90° to the flow direction. 

Before discnssing moulds and machines, it is necessary to understand the factors that inflnence the qnality of an injection moulding. These factors affect the ability of the mannfactnrer to produce mouldings without defects and at minimum cost to the customer. The first to be considered is scorch. 

Typical conditions for the injection moulding of a TPU are given in Table 9.4. The causes of possible defects in parts injection-moulded from a TPU are given in Fig. 9.2. 

Fig. 9.2. Some causes of typical moulding defects in injection - moulded TPU.

Compression moulding, a primitive form of injection moulding, is widely used for elastomers and some thermosets. However, it suffers from the primitive control of melt flow, although tools are simple and production rates high. The features that may become defects are similar to those of injection moulding. Perhaps the most sophisticated method is used in tyre building, where temperature control is crucial to achieving the best properties of the many different parts of the product. 

Moreover, there were signs of defects on the plastic inside the metal tube, at a point below the subcritical cracks. They comprised a series of flutes (Fig. 10.26) and a curious wavy line on the inner surface (Fig. 10.27). This combination of features observed on the plastic insert indicated material degradation, because this part should have had a smooth blemish-free surface. Being an injection moulding, the surface had been in contact with the steel core. If the polymer had flowed, then the viscosity must have dropped to allow the material to flow when the product was removed from the tool, showing that the molecular weight was too low. 

Tables 11.1-11.8 gives only those defects which could be caused by the HR system. A wider discussion of the defects and causes of their occurrence is contained in the specialist literature on injection moulding technology [1].

An inspection by Daimler-Benz of elastomer components for use in car construction showed that the percentage of defective components was much higher than that for plastics or metal parts. Reasons for this situation are discussed and ways and methods (failure mode effects analysis and statistical process control) of improving the quality of rubber injection mouldings outlined. It is concluded that quality should be achieved by quality planning and development rather than by quality control. 13 refs. 

Injection moulded parts with incompletely developed outer profiles are called incompletely filled parts (or short shots). An example is shown in Figure 9.28. This kind of defect often appears far from the gate if there are long flow distances, or on thin walls (e.g., ribs as shown in Figure 9.29). Due to insufficient mould venting, this defect can also occur in other areas. 

Much work continues to address these problems and much debate centres around the origin of defects which appear after binder removal. Thomas has shown that under certain circumstances such defects can be traced back to the solidification stage of injection moulding. Carefully designed experiments are needed to isolate the causes of the many defects that beset the ceramic injection moulding process. 

Hunt, K.M., Evans, J.R.G. and Moodthorpe, J., Computer Modelling of the Origin of Defects in Ceramic Injection Moulding. To be published. 

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