Injection Molding Residual Stress Prediction in Variable Wall Geometry
Polycarbonate parts with 2.0mm to 4.5mm wall steps generate 34 MPa thermal stresses; accurate 3D viscoelastic simulation prevents expensive re-cuts.
Viscoelastic Constitutive Model
Meaning
Mathematical equations describing the time-dependent and temperature-dependent flow behavior of polymer melts represent the dual viscous and elastic nature of these materials during processing. Applying a viscoelastic constitutive model allows engineers to simulate complex extrusion and injection moulding flows with high accuracy. These models relate the stress tensor to the deformation history of the melt, capturing phenomena like die swell, shear thinning, and stress relaxation.
This modeling approach is necessary for designing complex dies and predicting product defects like melt fracture or warpage.
Rheological Simulation
Non-linear equations are used to model the extreme deformations that polymer melts experience in narrow die gaps and gates. Integrating a viscoelastic constitutive model into computational fluid dynamics software enables the calculation of localized stresses and flow orientation. These simulations show how the polymer molecules stretch and align under shear and elongational forces.
This information is used to optimize the geometry of flow channels in extrusion dies.
Processing Prediction
Stress relaxation after the melt exits the die dictates the final dimensions and internal stresses of the extruded profile. A viscoelastic constitutive model predicts the amount of die swell that will occur, which helps toolmakers adjust the die opening size to match the target part dimensions. It also estimates the residual stresses that cause moulded parts to warp as they cool.
This prediction reduces the need for expensive trial-and-error modifications to the metal tooling.
Parameter Calibration
Fitting the model to experimental rheological data requires measuring the polymer’s behavior across a wide range of shear rates and temperatures. Engineers use capillary and rotational rheometers to collect the necessary viscosity and normal stress data. These measurements are then used to calibrate the parameters of the model, which may include the relaxation spectrum and the non-linear flow coefficients.
Accurate calibration is necessary to ensure that the simulation results match the actual performance of the polymer on the production floor. By establishing a precise mathematical representation of the melt flow, manufacturers can test different processing scenarios digitally before running physical trials.