Development of a Retort-able Plastic Container
Developing a package capable of retaining its shape after the rigors of the retort process poses one of the most challenging problems for plastic package designers. The development process is further encumbered by the fact that performance feedback on a particular design does not occur until after production quality bottles have been molded and are available for testing.
Usually, this testing results in failure, which initiates a protracted round of trial and error development. All of this is happening long after significant development time and capital has been committed. By applying predictive analysis technologies that enable the performance of a design to be assessed before physical samples or prototypes are available, this entire trial-and-error development process directed at establishing a satisfactory structural design can be circumvented.
Retort packaging was initially developed using glass and metal. Consumer inconvenience, safety concerns and cost issues served as the genesis for the development of plastic resin formulations capable of sustaining the challenging retort temperatures. However, the transition to plastic was and is not without its own challenges. Plastic containers are much ?softer? or less stiff than their glass/metal counterparts. This poses a significant structural problem at retort conditions of relatively high temperature (220?F to 265?F) combined with an overpressure (30psig and more) in the retort vessel. If improperly designed. The bottle will emerge from the retort permanently deformed. The technical challenge for plastic retort package is to determine how to design the container to accommodate the abuses of the retort process, yet emerge from the retort in its intended shape.
The magnitude of the development challenge is proportional to the thermal and pressure history of the package in the retort. These attributes of the process are governed by the target Fo value required to achieve sterilization. As a separate topic, not discussed in this paper, retort process development can be significantly aided using computational fluid dynamics methods. This approach can save line-time and ultimately, be integrated into the package development program.
For purposes of this discussion it is assumed that the retort process (thermal and pressure history) and process equipment are already known. This paper focuses on illustrating how a computer-based design-by-analysis approach can be used to develop a retort-able plastic container, eliminating costly and time-consuming trial and error development.
In general, there are two levels of the design-by-analysis development methodology that can be deployed for the development of a retort-able plastic package; single package assessment and system level assessment. For a single package assessment only the container itself is analyzed. This analysis is focused on predicting the response of the package (for a given retort cycle) to sustained pressure differential (pressure or vacuum) at retort temperature. The goal is to predict both the recoverable and non-recoverable (permanent) deformation. Typically, a series of analyses are completed, leveraging the learning from previous analyses to achieve a satisfactory design.
At the system level, the container-to-container and container-basket interactions are taken into account and simulated. The issue of determination of adequate spacing to allow the container to ?breath? freely (relative to other immediate structures, either bottle or hardware) during retort cycle is addressed. The goal is to identify and avoid permanent deformations that are caused by ?over-constraint? of the package during processing.
As a general rule the process of applying predictive methods to retort container design is in two stages. Initially the work is focused on the single container where screening calculations can be efficiently completed and a foundation of performance assessed. After package performance has been assessed and optimized, system level calculations are made where the package design is fine-tuned.
Every retort-able plastic package has its own degree of uniqueness. This uniqueness is not limited to shape alone, but includes plastic material properties, manufacturing process, product contains (does the product consume or release gas?), head space volume, filling/capping process (stream or nitrogen purge) and warehousing situation. Although it is not necessary to know every one of these package and process attributes before the simulation process begins, the utility of the solutions are greatly enhanced if the information is available.
The Stress Engineering process for predicting the performance of a retort package has been developed and applied to dozens of package and process situations, and it occurs in four stages.
1. Determine Container Deformation When Subjected to Pressure and Vacuum
Initially, both pressure and vacuum analyses are performed. The objective is to establish the limiting and allowable container performance for pressure and vacuum conditions. This also establishes pressure and vacuum compliance for the appropriate temperature range.
Pressure allowable is the pressure that causes a container to have unacceptably high permanent deformation. Permanent deformation can be identified by examining the predicted strains/stresses developed in the package as a function of the retort process. Vacuum allowable is the vacuum level that causes the container to buckle or ?panel?. In many cases this is a driving design factor during the cooling period of the retort process, when the container is rapidly brought down to ambient temperature. Allowable loading levels and compliance are computed as part of the simulation process and are illustrated in Figure 1.
2. Determine Container Pressure/Vacuum Experienced During the Retort Cycle
The pressure and vacuum that a container experiences during a retort process is largely controlled by the following:
Filling process (filling temperature, headspace, stream or nitrogen purge)
Retort process (temperature and overpressure schedule)
Containing product expansion/contraction
Containing product oxygen consumption
Headspace air expansion/contraction (ideal gas law)
Container structural compliance
Container thermal or memory related expansion/contraction
For a given container design (structural compliance) and a given retort process, the container pressure and/or vacuum levels can be calculated based on well-established fluid/gas properties. At Stress Engineering, this calculation is accomplished by a proprietary program ? BIPATH (Bottle Internal Pressure Analysis & Test for Hotfill/retort). Figure 2 illustrates the predicted response of a retort container during a particular retort cycle. The container performance illustrated in Figure 1 is used to generate this curve.
3. Container Design Evaluation
Analysis of Figure 2 indicates that a container with the attributes illustrated in Figure 1 will have adequate performance in retort for the nominal container and retort process conditions, assuming the actual retort vessel overpressure operates from 34 to 40 psig (between the two vertical blue lines). However, to accommodate container and retort process variability, Stress Engineering has determined empirically that a safety factor (SF) of 1.3 or higher is needed. Given this added performance constraint, the package performance illustrated in Figure 2 is non-conservative and will likely fail because of paneling or permanent deformation as the controlling factors discussed in Section 2 vary. (See two vertical pink lines in Feature 2. The package can only tolerate retort vessel overpressure from 35 to 39.5 psig considering the safety factor)
4. Container Redesign and Optimization
Based on the results described in Section 3, it is necessary to redesign the package to widen the over-pressure operating window. The analysis procedures described in Sections 1, 2 and 3 in this paper can be repeated for each structural design iteration. Ideally, the higher the container pressure/vacuum allowable and compliance, the better the design. The increasing pressure/vacuum allowable will allow the container to tolerate higher pressure/vacuum without permanently damaging the container. The increasing container compliance will decrease the pressure/vacuum that the container will experience during the retort process. However, the container pressure/vacuum allowable and the container compliance often work against each other. Higher pressure/vacuum allowable frequently results in reduced compliance (a stiffer container) and vise visa. Multiple design iterations are needed to achieve a well-balanced container design. Based on Stress Engineering?s past experience, 3 to 5 design iterations are generally sufficient to achieve the optimized design. Every retort-able package is different, and the four-step procedure outlined here must be tailored to address the specific needs of the package to be developed. The intent of this paper is to shed light on the technical insight of the design-by-analysis development procedure that Stress Engineering uses for retort-able plastic containers, a process proven to be far more efficient than the traditional trial-and-error package development.
For more information contact
Stress Engineering in Cincinnati at 866-888-8333
Plastic Package Design | Product & Packaging Development | Plastic Materials Selection
Plastic Product Design | Package Performance | Product Testing