![[IMAGE]](images/torotb1g.gif)
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.
![[IMAGE]](images/torotb2j.gif)
![[IMAGE]](images/torotb3j.gif)
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.
Conclusions
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: info@robustmolding.com Tel:(513) 860-4585.