Polymer Injection Molding Modeling. Mold Filling

The injection molding process can be used to produce complex-shaped parts, ranging in weight from a gram (or even less in the case of micro-injection) to several kilograms, at high speed, with great regularity and automatically. Similar techniques are used to inject thermoplastic polymers, thermosetting polymers and elastomers. One important difference concerns temperatures: "hot" thermoplastic polymers (usually between 200°C and 300°C) are injected into a "cold" mold (between 20°C and 80°C), while "cold" thermosets and elastomers (20°C to 80°C) are injected into a "hot" mold (150°C to 200°C), which activates the polymerization or vulcanization reaction. This article focuses on thermoplastic polymers, but for the cavity-filling phase, the calculations can be transposed to reactive materials where polymerization is not activated during cavity filling.

Let's recall the different phases of the thermoplastic injection cycle:

• the polymer is melted in a screw-head plasticizing system, by rotating and retracting the screw. A volume of material is accumulated at the screw head. This is the dosing phase;

• this polymer is injected into the cavity at a high flow rate, imposing the translational speed of the screw-piston: this is the filling phase;

• after switching over, which is very close to the end of filling, the press is regulated in terms of the pressure imposed on the screw, and an additional quantity of material is injected to compensate for the shrinkage caused by cooling. This is the compaction-maintenance phase, the modeling of which is described in the article [AM 3 696] ;

• the part continues to cool in the mold, then, after ejection from the mold, it enters the cooling phase. During cooling in the mold, a new dosing phase takes place.

Calculations for the injection process answer the following questions:

• Will the cavity be completely filled before the polymer solidifies?

• Will the filling be balanced, and will it be completed at the same time in the different regions of the footprint?

• Where should any vents be located?

• Will any welding lines, sources of mechanical weakness and appearance defects, be placed in areas where they are least troublesome?

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