Temperature Profile in Rubber Injection Molding: Application of a Recently Developed Testing Method to Improve the Process Simulation and Calculation of Curing Kinetics

Abstract

Injection molding of rubber compounds is an easily conducted yet sophisticated method for rubber processing. Simulation software is used to examine the optimal process conditions, identify failure scenarios, and save resources. Due to the complexity of the entire process, various aspects have to be considered in the numerical approach. This contribution focused on a comparison of process simulations with various definitions of the material’s inlet temperature, ranging from a stepwise increase, but constant temperature, to an exact axial mass temperature profile prior to injection. The latter was obtained with a specially designed, unique test stand consisting of a plasticizing cylinder equipped with pressure sensors, a throttle valve for pressure adjustments, and a measurement bar with thermocouples for the determination of the actual state of the mass temperature. For the verification of the theoretical calculations, practical experiments were conducted on a rubber injection molding machine equipped with the mold used in the simulation. The moldings, obtained at different vulcanization time, were characterized mechanically and the results were normalized to a relative degree of cure in order to enable comparison of the real process and the simulation. Considering the actual state of the mass temperature, the simulation showed an excellent correlation of the measured and calculated mass temperatures in the cold runner. Additionally, the relative degree of cure was closer to reality when the mass temperature profile after dosing was applied in the simulation.

Peroxide-Based Crosslinking of Solid Silicone Rubber, Part I: Insights into the Influence of Dicumylperoxide Concentration on the Curing Kinetics and Thermodynamics Determined by a Rheological Approach

“…With the advent of numerical simulation, polymer processing has become a precise and highly technological industrial step. Before actual rubber injection moulding, for example, the process can be computer-aided simulated, in order to establish the most suitable processing conditions for a specific application [ 8 , 9 ]. During simulation, several models are applied to describe the injection moulding process, from the filling phase to the crosslinking phase of the product [ 10 , 11 ].…”

Investigation on strategies for optimizing process definition in rubber processing: A study on mechanical and chemical properties of vulcanizates as basis for the development of a new calculation model for quality prediction

“…For this reason, the process conditions inducing the curing reaction must be chosen with utmost care in order to secure maintenance of the part characteristics despite the expectable occurrence of batch variations. 11 First and foremost, measuring the reaction kinetics with proper devices, for example, a rubber process analyzer (RPA), is certainly the common method that is used to derive the definitions required. Thereby, measurements are conducted at various temperatures to obtain the respective vulcanization isotherms, which are then normalized to display the temperature-dependent progress of the degree of cure over time.…”

“…Anyway, these variations may display significant differences in the performance of the cured rubber parts, which, in the worst‐case scenario, can cause failure of the quality requirements. For this reason, the process conditions inducing the curing reaction must be chosen with utmost care in order to secure maintenance of the part characteristics despite the expectable occurrence of batch variations 11 …”

Monitoring of shear heating effects during injection molding of rubber to improve the process control

“…Therefore, an optimal rubber temperature during the injection process must ensure a balance between relatively low viscosity (therefore low injection time) and a scorch safety condition (curing starts after complete mold filling) [2][3][4][5]. In spite of its importance, the rubber temperature during flow cannot be directly controlled, because it is raised not only by the heating system of the machine and by the curing reaction (if by mistake it occurs during flow) but also by shear heating, i.e., temperature increase due to viscous heat dissipation [2][3][4][5][6][7][8][9][10][11][12][13][14]. For most rubber compounds, characterized by a viscosity higher than plastics, the latter phenomenon is predominant and it is very difficult to be replicated in the laboratory.…”