Process control during injection molding

Temperature must be controlled throughout the injection molding process. The temperature of the plastic is increased in the barrel to melt the plastic. The melting temperature and glass transition temperature of 

FIGURE A.2 Process control variables for injection molding. 

The temperature is increased in the compaction zone where the solid plastic is melted from shear heating and from the increased temperature. The temperature is increased in the metering zone and the nozzle. 

TABLE A.1 Plastic Material Melting and Glass Transition Temperatures  

Plastics can be either amorphous or crystalline. Pure crystallinity is rare among polymers. Most polymers have crystalline and amorphous regions and are considered semi-crystalline. Amorphous polymers have low crystallinity or irregular crystalline regions. A listing of amorphous and crystalline polymers is found in Table A.2 (Belofsky 1995a). 

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The next category of issues is processing issues. Most often, we think of a processing issue as a problem that was caused by improper process control during injection molding but this category includes all of the processes that the material undergoes as it is transformed into a finished part. The key here is to confirm that the material in the finished part has the desired properties and performance. 

Control of the mold temperature is a key component for plastics quality. Table A.3 lists the typical mold temperatures for common plastics during injection molding (http //www.matweb.com/). The mold temperature can influence the quality of the plastic part. Warmer mold temperatures can help relieve stresses imposed on the plastic part during injection molding process and aid material flow of thin wall sections. The stresses imposed on the plastic part can cause warpage in the plastic part after it is removed from the mold. Cooler mold temperatures can help cool the plastic part faster and cause a reduction in cycle time. 

The extrusion process has fewer process control problems with TPs than does injection molding but has greater problems in dimensional control and shape. During... 

In the injection blow molding process, the parison is formed by injection molding of the preshaped parison onto a steel rod, as shown in Fig. 14.18. The rod with the molded thread already completed is moved to the blowing station, where the article is inflated free of scrap. The parison thickness distribution is determined in the injection mold without the need of further control. Some axial orientation is introduced during injection, resulting in an article with partial biaxial orientation. 

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