Packaging Critical Dimensions: Afterthoughts or Necessary?
As contributors to packaging design and development, we’re often tasked with development of packaging material specifications. It is possible that packaging engineers, often pressured by fast moving, ever-changing projects, design various components of a system in silos and don't even consider critical dimensions till the last moment, e.g. drawing or part release. It is possible that critical dimensions may end up as a clerical responsibility and may or may not even make it on the packaging material specifications. Packaging critical dimensions -- are they just afterthoughts or necessary evils? Hopefully the following discussion will stimulate a creative debate.
Hal Miller's Package Material Specifications & Your Suppliers provides great information on developing packaging material specifications. As part of the packaging material specifications, it is essential that we specify controlled or critical dimensions that will be used by suppliers and incoming or receiving inspection departments for quality control.
To exercise adequate and accurate quality control, it is quite critical how we determine, measure, and control the critical dimensions. For the purposes of this discussion, we will concentrate on how to determine critical dimensions. Web research can provide a plethora of definitions, but in general terms, specifically in case of packaging components, critical dimensions are the physical measurements that impact the functionality of the packaging system.
A common and conventional practice in the packaging industry is to measure length, width, and depth (LWD) as critical dimensions. Depending on the level of packaging (primary, secondary, or tertiary) and functionality of the component, the practice of measuring LWD may serve the purpose in certain cases, but not always. Consider the following system -- a medical device packaged in a heat-sealed double tray (vacuum formed) system, placed as single unit in a reverse tuck shelf carton, and shipped in a quantity of five per corrugated shipper (regular slotted style).
Stating LWD as critical dimensions on the shelf carton and corrugated shipper drawings is an accurate practice because these dimensions directly impact the functionality of the component, i.e. containment. However, in case of vacuum formed tray, measuring only LWD may not provide us with the most efficient controls. For example, if the inner tray is suspended in the outer tray, then measuring depth does not provide us with any additional advantage other than documenting a reference measurement. Additionally there are other dimensions that impact the functionality of the tray and must be considered, e.g. in case of inner trays, I would focus on:
- L and W -- directly impact the fit of inner tray in the outer tray.
- Seal flange width -- directly impacts seal integrity.
- Seal flange thickness -- directly impacts seal integrity.
- Minimum wall thickness -- impacts overall sterile barrier integrity.
For the outer tray, however, we should also consider depth, because the dimension impacts outer-tray fit in the shelf carton.
A few other considerations to make while setting up critical dimensions for formed trays:
- In case of formed trays, consider controlling design features that impact device retention or snap, e.g. depending on intricacy of design we may need to measure the thicknesses of the snap as well.
- Critical dimensions should be validated during the forming process unless a 100 percent inspection is performed. Certain critical dimensions such as minimum wall thickness may not be agreed upon until completion of forming process validation.
- Is it a male or female mold? All critical dimensions that are validated during the forming process may not be required for measurement during incoming inspection. For example, in the case of female molds, mold-side dimensions are fixed, and therefore measuring depth and flange-width may not always be necessary.
- Tolerance for forming process are not universally applicable to the entire tray design, at least not in most cases, e.g. form to form, form to cut, and cut to cut are different values.
A few general considerations to make are:
- Tolerance stated on the drawing should accurately reflect supplier capabilities.
- A tolerance stack-up should be performed to ensure that all components of the packaging system will work with each other.
- Measurement test methods should be validated.
Each process (die-cutting, cold forming, hot forming, injection molding, extruding, etc.) has its limitations and each level of packaging (primary, secondary, and tertiary) is usually manufactured by a separate process. Therefore it is important that we consider critical dimensions, not as an afterthought, but as integrated and very important aspects of packaging system design and development activities.
Abhishek Gautam, Packaging R&D Manager, ConMed