Polymer Engineering Center > Research > Thin-Wall Injection Molding Processes

Thin-wall injection molding has become increasingly important due to the explosive growth of wireless telecommunication and portable electronic devices that require thinner and lighter plastic housings. This process is also the key technology for mass-producing complex medical, optical, electronic, automotive and biotechnological devices, some of which exhibit micro-scale features. However, thin-wall injection molding presents several technical challenges. First of all, process physics and material behavior under the extreme processing conditions (with pressure and shear rate exceeding 150 MPa and 20,000 sec^-1, respectively) are not fully understood. Secondly, as a result of that, existing commercial computer simulation tools, whose solutions are based on questionable input material properties, fail to duplicate the experimental observations for thin-wall parts. Thirdly, thin-wall parts have a more restrictive flow path compared to conventional injection molded parts. This leads to a much narrower processing window and poorer production yield. Finally, thermal degradation and material degradation from shear also become significant during thin-wall molding as evidenced by the change of failure mechanism for the same PC/ABS blend.

In this research, systematic experimental and computational work will be conducted to improve the solution of CAE analysis with major CAE suppliers and to further the understanding of the process physics, material behavior, and the relationships between the process, the material, and the resulting product. Since the processing pressure and shear rates in thin-wall injection molding substantially exceed the capability of any existing commercial rheometers, we will develop an innovative technique for characterizing the material behavior under the high pressure and shear stress. In specific, we will employ an injection-molding machine with a special slit-die rheometer for studying the rheological properties of polymers at extreme conditions. By measuring the pressure drop, volumetric flow rate, and temperature variation of material flowing through a channel of known geometry, we will be able to derive information about the material rheology. In addition, we will investigate material degradation and melt instability during processing. Results from these studies will help to improve the accuracy of computer simulation and its usefulness.

In this study, we will also perform a series of thin-wall molding experiments with various processing condition combinations, such as melt temperature, mold-wall temperature, injection speed, and residence time prior to injection. Given the molded parts, we will then test the viscosity reduction and impact performance of the molded part in terms of failure strain to study the effect of processing conditions on the material degradation.

Anticipated benefits:

• More accurate material behavior at extremely high pressure and shear rate to provide much reliable input and material behavior model for computer simulation.
  

• Better understand of the thermal and shear-induced material degradation during processing to identify the permissible processing window.