Mold flow and Stress analysis of Toro Company's 20" Blower Tube part and mold

Toro Co. was interested in proactively optimizing the design and injection molding of their 20" blower tube product.  The objective of the analysis/optimization work was to create a robust design/molding system, which complies with quality and productivity requirements the first time and every time.

Initial Conditions

The material selected for the product was a rubber modified styrenic polymer;  the planned 4-cavity, valve gated, hot runner mold had to operate flawlessly in a 600 US ton injection molding press.  The estimated location of the single gate per cavity was at the large end of the product to ensure the filling of two snap tabs with sufficiently hot melt.  The part contained two heavy side bands for aesthetic reasons.


Design Review based on Flow Analysis

The side bands pointed toward the possibility of air entrapment at the smaller end of the part. In order to clarify this issue and determine the moldability window and the optimum settings, 3D Flow Analysis was carried out.  Complete thermodynamic and rheological characterization of the polymer preceded this task.  Based on the flow computation the temperature distribution of the polymer during filling is displayed in the picture above.  The ranges of nine additional conditions of cavity filling, including pressure, temperature, flow pattern, frozen skin, shear stress and shear rate, were determined through numerous iterations.  The analysis confirmed the anticipated presence of air entrapment.  Toro's designers began immediately to core out the heavy bands based on our recommendations.  The flow analysis indicated an additional problem:  the pressure drop throughout the machine nozzle, hot runner system, gate and part was 20,100 melt psi.  This pressure value, while it was well within the capability of the machine, prohibited precise V/P Transfer to a reduced, 4,500 melt psi Holding pressure, necessary to prevent parting line flash formation.

Suggested Solutions

After considering various options, such as multiple gating, rotation of the parts in the mold to minimize the total projected area, we recommended the placing of the single gate at the middle of the part to reduce the required filling pressure.  We also realized that this approach would bend the core significantly.  Finite Element stress analysis was carried out to determine the magnitude of core deflection.  The following two pictures show the stress levels and exaggerated deformation of the core steel under the computed filling pressure distribution.



The bending problem with the core was alleviated by employing hydraulically retractable support pin.  This solution reduced the computed deflection from 0.016 to 0.003 inch.  The wall thickness of the part was increased locally, under and opposite the gate, the temperature of the material was raised to the allowable maximum, considering the calculated residence time, to minimize the filling pressure.  As a result of these changes the pressure drop decreased from 20,100 to 10,900 melt psi.  The latter value now permitted a precise V/P Transfer to 4,500 melt psi holding pressure, which called for an acceptable clamping force requirement of 580 US tons.  The hot runner system was also evaluated for pressure drop, gate shear and residence time.


The complex molding problem was resolved by using comprehensive, expert CAE, which included Finite Element Flow and Stress analysis.  The part was molded successfully right the first time based on robust process and molding system design recommendations.  Toro Co. did not request the available additional CAE solutions: Tolerance/Shrinkage, Cooling, Cycle, Warpage and part Stress analysis for this project.  The analysis/evaluation was completed by APD/Advanced Plastics Design, Inc.  Our company, in addition to the expert design/analysis work, carries out injection molding DOE and provides training for management, process engineers and setup personnel (Robust process, DOE and SPC).  E-mail:  Tel:(513) 860-4585.

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