The basic instructions presented in this section are intended to develop a training program in steps conducive to easy learning, which over time will result in full knowledge of the molding operation. The program provides instruction that can be made to fit any time span, in order to suit individual abilities to absorb information while actively engaged in learning by doing. 

           Suggestions are included for the substitution of calculated values for those obtained by the trial and error method, in the interest of conserving time of personnel and minimizing the loss of material. The main object of the instructions is to give each worker in the Injection Molding operation a good understanding of every element that goes into the operation; the worker, in turn, having gained the needed knowledge, should take full advantage of such information by putting it to constructive and productive use.

            Under  practical  operating  conditions, learning the Injection Molding process takes place in stages: 

             • The first stage covers the running of an in- jection molding machine. 

             • The second stage involves setting molding conditions on a prescribed set of param- eters for a specific plastic material and a specific mold that will produce acceptable parts. 

             • The final stage is devoted to problem solving and fine-tuning of the operation, which will lead to high  productivity and part quality. 

             Table  2-7 provides  general  information and is a guide for Injection Molding settings. Specific information on all machine settings and plastic properties is acquired initially from the plastic supplier’s data sheet on the material to be used. Initial setting information can also be obtained from workers in the molding plant who have experience in processing the same material. 

             In general, once the mold is filledinitially, additional material is added to themold by the injection pressure to compen-sate for thermal shrinkage as the materialcools. This  process  is  called packing.  Toomuch packing will result in highly stressedparts and may cause ejection problems. In-sufficient packing  causes short shots, poorsurface, sink marks, welds, and other defects.The proper amount of packing is determinedby trial and error or with the assistance of computerized process simulation. The mate-rial will continue to flow into the mold  aslong as there is injection pressure, providedthat the gate is not sealed. When no morematerial enters the mold, contraction of thecooling material results in a rapid decrease inthe pressure in the mold. The residual pres-sure caused by the original deformation of the steel of the mold and the adhesion of theplastic to the steel must be overcome by theknockout system to eject the parts.