Gate Optimization and Weld Line Prediction Using Injection Molding Simulation

In injection molding, the part quality is affected by the gate location, which influence the plastic flows into the mold cavity. Therefore, the part quality can be improved by determining the optimum gate location, type of gate and size. In this analysis, we approached different gate location and studied the different weld line position which is acceptable level for the product and molding process requirements. This analysis is performed only for filling & packing. Also confirming the robustness of gate design and the manufacturing feasibility.

Injection molding is the most commonly used manufacturing process for the plastic parts. The quality of the injection molded parts is a function of plastic material, part geometry, mold concept and process condition. The placement of a gate in an injection mold is one of the most important variables of the total mold design.
However, the computer aided engineering (CAE) simulation requires the mold designer to run the simulation, perform the design evaluation by post processing the results, and redesign molds, until the satisfactory design is obtained. A balance filling process within the cavity gives an evenly distributed pressure and temperature but due to parts nature & functional requirements (due to thickness change), often this is not achievable. The gate design & optimum location should meet the complex product requirement and with manufacturing feasible.
The optimization is performed for gate location based on mold filling pressure, filling pattern, filling temperature differences and potential weld line during the molding process. In this paper we discussed the gate optimization for automotive front panel part.
Analysis Approach
The part design and material was finalized by product design team and the product requirement was single cavity and weld line should not be visible in aesthetic area. (As shown in Figure 1). The overall size of the part is 374.4 x 176.8 x 110.6 mm, (Approx.) and the part thickness variation minimum of 0.6 to maximum 2.7 mm observed.

Part with direct visible zoneFigure 1.Part with direct visible zone

The mold design layout was developed with seven side cores as shown in Figure 2.The cold feed system with banana gate design is suggested for the concept.

Part with side coresFigure 2.Part with side cores

The proposal was given with different gate scenarios one, two and four gates as shown in Figure 3, 4 & 5. The mold material used is P6. The melt temperature is 235°C and the mold temperature is 40°C used for all gate scenarios. And also ideal cooling condition is assumed.

Part with single gate designFigure 3.Part with single gate design

Part with two gate designFigure 4.Part with two gate design

Part with four gate designFigure 5.Part with four gate design

From the filling and packing results, the average fill time observed for 3 proposal was 1.86s for 100% fill.
The filling pressure in single gate is 85 MPa with feed system as shown in Figure 6. High filling pressure is required to fill the cavity. The temperature drops observed in end of fill and also observed the shear rate in gate area is exceeding the limit. Visibility of weld line in aesthetic area for single gate is less as shown in Figure 7.

Single gate_ filling pressureFigure 6.Single gate_ filling pressure

Single gate_ Weld lineFigure 7.Single gate_ Weld line

Two gate_ filling pressureFigure 8.Two gate_ filling pressure

Two gate_ Weld lineFigure 9.Two gate_ Weld line

In two gate design, the observed flow balance is not uniform because of product features & feed direction. The filling pressure required is 53.3 MPa as shown in Figure 8. The pressure is less compare to single gate and the temperature drop with in the EOF limit. Shear rate in gate area is in acceptable limit and visibility of weld line in aesthetic area for two gate is more compare to single gate as shown in Figure 9.

In four gate design, the filling hesitation observed in the flow front. The filling pressure required is 60.4 MPa as shown in Figure 10. Filling volume difference observed in gate contribution. The temperature rise & drops observed inside the cavity and Shear rate in gate area are within the limit. The weld line in aesthetic area for four gate is more compare to single & double gate as shown in Figure 11.

Four gate_ filling pressureFigure 10.Four gate_ filling pressure

Four gate_ Weld lineFigure 11.Four gate_ Weld line

Two gate_ Predicted Weld lineFigure 12.Two gate_ Predicted Weld line

Results and Discussion
In all the gate scenarios, cooling considered as ideal condition. In actual cooling condition, the single gate may create a filling issue due to cooling turbulent flow. The pressure in the feed system dominates the flow rate, minor change in runner system (multi-gates) gives a major change in filling pattern.
The Following table shows analysis and results in following parameters such as, Filling Time, Cavity Pressure, and Filling pressure with feed system.


In the results for all the proposed gate design, the part filling is 100%. While comparing cavity pressure, two gate has less pressure compare to single & four gate.

The weld line is less in single gate when compare to two gates. If we consider four gate option, the wastage of material is high.

From this study, it is suggested that two gate gives a better molding results & the possibility of moving the potential weld line to non-aesthetic area by changing the runner & gate dimensions, as shown in Figure 12.


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