Using Performance Simulation and Analysis
Development of a new PET packaging system can be a high-risk, technologically challenging and costly proposition. When considering the broad application of PET packaging, the landscape of challenges is huge. Expansion of this material into new applications such as wide-mouth hot-fill food products, cleaning products and innovative shapes for pressurized packaging, offers technical challenges that will tax any development organization.
It is no longer necessary to rely on purely empirical, trial-and-error methods of development. Popularized more recently in advertisements by auto manufacturers describing virtual prototyping as a means of predicting the performance of a new automobile via computer-based simulation before ever manufacturing one, virtual prototyping and predictive analysis are changing the development processes for new polyester packages as well. Many of the most critical technical design, processing, and stability challenges can be addressed via currently available analytical and computational simulation methods. The scope of available virtual prototyping methods for the polyester packaging industry is described in Figure 1. The goal of using these technologies is to dramatically reduce technical risk and costly, time-consuming iterative trial-and-error development. The result of their application will typically be 50%+ reduction in development schedule and cost, when compared to traditional approaches. Successful case histories are summarized below, including development and structural performance optimization of the Pepsi Cola 32-oz All Sport bottle; blow molding simulation of a 20-oz PET water bottle; and predictive closure sealing and removal torque simulation for a wide-mouth, hot-fill PET package.
Structural Development of the 32-oz All Sport Bottle
A common economic constraint in developing a new PET bottle is the desire to use an existing preform. This was one of the primary economic requirements Pepsi established during the development of the new 32-oz All Sport Package in late 1997 (Figure 2). Faced with the challenge of quickly producing a new bottle for this fast-growing market, Pepsi elected to develop the new package using predictive structural analysis methods, and limit the production of fully functional prototypes. The development relied on virtual prototyping and performance evaluation via computer simulation.
Provided with the aesthetic requirements for the design, the development effort focused on identifying the primary structural details required to develop a bottle that met the prescribed gram weight using an existing preform, while satisfying the top load and low pressurization distortion criterion that had been established. Over a several month period, 22 production-quality virtual prototypes were developed and evaluated. During the development, several limited-function prototypes were blow molded for intermediate marketing reviews and simulation model validation. Figure 3 illustrates the validation of the computer-based structural simulation model via a comparison of the predicted bottle deformation with test results. Figure 4 presents a quantitative comparison of the predicted top load with the measured response for the same test.
This modeling method was used to develop all structural details that presently exist on the new All Sport bottle. These structural features were designed into the geometry to improve the top-load strength of the bottle during filling and distribution. Based on the simulation results, the production tooling was commissioned prior to the production of trade-quality prototype samples. Once production samples of the All Sport bottle became available, a comparison was made between the predicted top-load response of the final bottle design that was developed and optimized using virtual prototyping methods, and the production samples that were being manufactured (Figure 5).
In addition to top load simulation, the same structural methods can be used to predict the drop impact capabilities of a container, squeeze/handling characteristics, structural and material response due to pressurization as well as package shape due to internal vacuum.
Blow Molding Simulation
A companion technology to structural analysis, blow molding simulation offers the opportunity to dramatically accelerate development and optimization of preform geometry and the molding process. In addition, when used in conjunction with other virtual prototyping technologies, blow molding simulation facilitates a fully integrated approach to bottle development.
Sufficiently accurate stretch blow molding simulation technology is a more recent evolution of extrusion blow molding simulation technology. Much of the key technical work related to the inflation of the preform was completed a number of years ago. However, until recently, predictive capability of this technology for stretch blow molding was limited. Recent advances have come about primarily as a result of new insights and enhancements to algorithms used to model the complex behavior of polyester materials in stretch blow molding processes. Figure 6 illustrates the 20-oz water bottle and preform geometry that were used to demonstrate the predictive capability of stretch blow molding simulation technology.
Figure 7 shows the intermediate shapes or snap shots of the preform as it is being stretched and blown into its final shape. Actual bottle wall thickness and predicted thickness distribution are compared in Figure 8.
Thickness variation between the actual as-manufactured 20-oz bottle and the predicted wall thickness is somewhat greater that typically experienced, but not uncommon for this tool in its current state of development. For this particular example, the maximum deviation in any particular region from the as-molded wall thickness is about 0.1 mm (0.004 in). Although predictions are very reasonable in their current implementation, over the next 12 months it is expected that further improvements will be made in the material models for stretch blow molding that will reduce this variation and further improve robustness of the predictions.
Simulation of Closure Sealing & Removal Torque for Hot-Filled Container
Managing the stability of the seal over the life of the product while offering a removal torque that is sufficiently low for consumers, are common challenges for hot-filled PET bottles. This is of particular concern as the neck finish diameters for PET bottles continue to increase, such as in the case of wide-mouth hot-filled containers for ready-to-serve soups, sauces, juices and other products. Larger diameters impact both the geometric stability of the neck finish after filling as well as removal torque that must be overcome by the consumer.
Both package sealing and removal torque for threaded closures are related to the region of intimate contact between the bottle and closure that is so small and remote that it is not possible to anticipate the design performance when contact is made, via empirical means. Further complicating the problem of sealing and removal torque performance is the combined effects of dimensional variability, transient thermal load associated with sterilization and hot-filling, friction, and material relaxation characteristics. Using traditional design processes, the designer must rely on experience and judgment to develop the sealing system. Because of the complexity of the problem, this often leads to a significant investment in expensive and time-consuming trial and error prototype testing and development.
Computer-based simulation methods can be used to predict the quality of the seal and removal torque characteristics for conventional and wide-mouth, hot-filled packages. Simulation models can be developed that account for most of the critical factors that influence the effectiveness of the seal and the removal torque, including:
Thermal heating of the cap as a result of sterilization before application.
Thermal heating of the bottle and neck finish as a result of hot filling
Variability of application torque by capping equipment
Molding related dimensional variability of the bottle finish and closure
Variability in friction (wet or dry)
Material creep (for pressurized containers) and relaxation (for non-pressurized)
Figure 9 illustrates a computer simulation model of a wide-mouth PET container that was hot-filled with product at 180F. The liner is a thermoplastic elastomer and the cap is molded of polypropylene. Prior to capping, the closure is sterilized at 90F. While at this temperature, the closure is assembled with 24 lb-in of torque. The filled and capped package was then transported to a 48F chilling tunnel for 45 minutes. The product is then packed in corrugated cases and warehoused. Simulation methods can be used to predict evolution of the contact seal pressure and removal torque for a combination of thermal or structural events throughout its entire history. The example used for this demonstration is focused on the period through the chilling tunnel.
Figure 10 illustrates the temperature gradient in the bottle finish, liner and cap at select times after filling. Figure 11 is a typical representation of the contact area and magnitude of the contact sealing pressure between the bottle finish and the liner. This particular graph was made after the package had completed 45 minutes in the chilling tunnel. Characteristics of this sealing profile will change as the package ages and experiences different storage conditions.
When using simulation methods to support development of a new package, it is very useful to understand the details of the sealing profile and how it changes with materials selected or after the package experiences critical events. As an illustration of this capability, Figure 12 shows local contact seal pressure along the top of the bottle finish for the wide-mouth bottle geometry in Figure 9.
Figure 13 shows seal pressure and removal torque predictions as a function of the time after filling for the particular materials used in this sealing system design. About 3 hours was simulated with the most significant changes occurring within the first hour and a half. However, this method can also be used to simulate many months after filling.
The Opportunity for Integrated Development
Predictive simulation methods and software tools have advanced to the point where it is possible to predict many of the critical performance attributes of polyester packaging. Integration of these technologies -- blow molding simulation, predictive structural and material performance analysis, closure seal and removal torque optimization, as well as two emerging predictive methods for ESC and shelf life predictions -- offer the opportunity to change the economic and technical landscape of package development. Some of the key benefits of this approach are:
Dramatically reducing tooling iterations
Documented understanding of economic and technical performance tradeoffs
Significant reduction in development schedules
Opportunity to investigate innovative concepts and ideas to an extent never before possible, in a time frame and for a cost that is dramatically less than alternative methods
Physics-based understanding that enables seemingly impossible packaging ideas to be engineered for successful application
The development process described above and the associated benefits are available today.
Call Stress Engineering in Cincinnati at 866-888-8333