Molding defects during injection

Molding defects. Figure 1 indicates molding defects during injection molding... 

Molding defects can occur during the injection molding of plastic parts. Some common molding defects and possible causes are listed in Table A.6 (Belofsky 1995b). 

Molded part defects. Figure 2 indicates the molded part defects occur during injection molding. 

Several case studies are presented in the next sections that show some common root causes of contamination in injection-molded parts. In these case studies, the problem is presented in a manner that the troubleshooter would encounter during a trial or information-gathering session. In each case study, the modifications required to fix the process are detailed along with supporting fundamental information. Two of the case studies used (ET) screws to eliminate the defects. ET screws and other high-performance screws will be discussed in Chapter 14. 

Factors related to the fabrication parameters and/or end use conditions affect the dominant failure mechanism by altering the specimen itself. For example, (a) specimens manufactured by injection molding often manifest some anisotropy which affects their mechanical properties, (b) defects incorporated in specimens during processing and/or use can act as critical flaws that can cause or accelerate fracture, and (c) exposure to harsh environments (such as oxidizing molecules) during use can cause or accelerate fracture. 

Because of PMMA s brittle nature, fines can easily be produced during regrinding and should be removed before injection molding. Fines should be sieved from the material for two reasons - fines, or very small particles, are extremely susceptible to moisture absorption and, the problem of poor plasticization. These problems result in defective moldings being produced. 

The molding parameters have a great influence on part quality after the thermo-mechanical properties of mold material [45]. An unsuitable process parameter setting can cause many product defects (e.g., a long lead time, a large amount of scrap material, etc.) and unstable product quality during the injection molding process. 

After finishing the assembly of these two groups, the spot-grinding of the two mold halves on spotting presses follows (Figure 4.26). Here, the two mold halves have to be closely spotted to each other, so that during the subsequent injection process, the material remains bound in the mold cavity. Leakage of material is a defect and needs to be dealt with later. 

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