Sam Iuliano, chief technologist, Nordson EDI, explains how new sheet die technology can enhance quality and consistency of medical packaging
Mind the gap: A Nordson EDI sheet die equipped with SmartGap mechanism.
A breakthrough technology for extrusion dies enables manufacturers of thermoformable sheet for medical and pharmaceutical packaging to make thickness changes in minutes rather than hours, extend the range of thicknesses that they can produce and achieve these improvements while enhancing sheet quality and consistency.
The quality improvements resulting from the new technology are the result of more uniform polymer orientation in the sheet and lower residual stress. They are particularly important for preventing warpage when the plastic is heated during sterilisation. In general, enhanced quality and consistency make the sheet more readily thermoformed into blisters, trays, and procedure kits, more resistant to defects that can compromise the sterile barrier and less prone to streaks and other appearance problems in applications that demand excellent clarity.
Called SmartGap, the new, patent-pending system uses a single-point adjustment mechanism that changes the lip gap while simultaneously modifying the length of the lip land to provide the most appropriate conditions for the newly adjusted thickness as the sheet exits the die. The lip land is the final gap in the die that forms the lip exit. By mechanically linking the adjustment of these two key process variables for the first time, the SmartGap system ensures a proper set-up of the die and takes substantial time and guesswork out of the process for achieving desired sheet properties.
The system, available only on new dies, eliminates the extended shutdowns for changes in lip components that have often been necessary when transitioning to new job runs, reducing to a matter of a few minutes changeovers that used to take two hours or more. Also overcome are previous limitations on thickness range caused by the complexities of die modification; the system readily encompasses die gap adjustments over a range of 0.400 in. (10.2 mm), enabling processors to efficiently run with multiple product changes per day.
Simultaneous adjustment of lip land and lip gap
Operators set the thickness-change process in motion by using an adjustment mechanism at the end of the die to make a new setting for the lip gap. Simultaneous change in the length of the lip land automatically takes place on an as-needed basis, depending on the target sheet thickness. Thus, for thin sheet, the lip land is short, which helps to control back-pressure at smaller die gaps, reduces deflection, and lessens the need for draw-down. For mid-range sheet, the lip land changes from short to medium length, providing sufficient pressure for effective adjustments to the flexible lip of the die while helping to reduce swelling of the extrudate. For thick sheet, the lip land is extended to full length, increasing back pressure at large lip gaps and helping to reduce extrudate swell.
Previously there have been two methods of varying the lip gap. The simplest was to adjust the flexible upper lip of the die. This has a limited range of gauge variation, or ‘travel’, requires several adjustments across the width of the die and necessitates an extended shutdown period to exchange a removable lower or fixed lip to effect a step-wise change in both lip land and lip gap.
A more recent method involves use of a single-point adjustment system to bend the fixed lip, extending the range of travel but without providing for change in the lip land—so that the lip land used is a compromised design of medium length.
Bending die lips over significant distances results in lip lands that are not parallel, causing undesirable diverging or converging flowpath geometry. This can exacerbate certain problems such as lip contamination that leads to streaks. When there is either no provision for a lip land change or lip land change procedures are too time-consuming, processors will often struggle with sheet quality. This is especially true at the thinner and thicker extremes of their product range, where a medium-sized lip land is less appropriate.
Improvements in sheet quality and consistency
In addition to avoiding extended shutdowns and broadening the available thickness range, SmartGap technology aims to provide these advantages:
Avoiding excessive sheet shrinkage. Since it is now convenient to set smaller lips gaps, processors do not need to draw down as much to achieve target gauge. As a result, there is less orientation of the sheet and, subsequently, fewer problems with shrinkage.
Tighter tolerances and better lay-flat sheet. Now that more appropriate lip land lengths are available for small and large lip gaps, use of the flexible lip to fine tune the gauge profile is now more effective. In addition, a proper lip land to lip gap ratio prevents excessive extrudate swell, which causes visible corrugation defects. Improved lip geometry also yields a more fully developed flow, which results in more uniform orientation and less warpage.
Better appearance. Reduction in extrudate swell helps to avoid streaks and reduces the tendency of lip faces to become contaminated. Defects from running with a die gap that is too small, such as ‘shark-skin’, can be avoided by using systems that provide good thickness control at larger die gaps.
Improved external deckle performance. Since the SmartGap system slides the lip components, instead of flexing them through an arc, the lip lands will remain parallel and the lip faces remain flush and normal. This keeps the lip geometry ideal for use with external deckles since the deckle seals do not need to conform to irregular lip faces. The result is that external deckles seal better and require less maintenance.
SmartGap technology can be employed with most of the polymers typically used in medical and pharmaceutical packaging, such as polystyrene and high density polyethylene. Ultimately the quality benefits that it provides translate into packaging that is safer, more reliable, and better looking.