Printing on Horizontal Form-Fill-Seal Machinery
by John Merritt
Director, Medical Business Development, ALKAR-RapidPak
by John Merritt
Maintaining print registration is critical to smooth operation.
|At MD&M East 2005, RapidPak featured a rotary flexographic printer capable of printing 2-point type on Tyvek 1059B, heretofore a daunting task in the eyes of any medical device manufacturer.|
(click to enlarge)
Coordinating various functions on horizontal form-fill-seal (HFFS) machinery and making sure that they occur at consistent locations is a complex endeavor. In particular, the subtleties of print registration can create problems if not handled properly. However, if these complexities are understood and dealt with early in the process, such as while specifying machinery and materials, the challenges of print registration can be easily overcome.
The HFFS Process
There are several basic functions of the HFFS process. These include forming, product loading, sealing, labeling or printing, cutting, and discharging finished packages. For all of these operations to occur in the proper locations on the package relative to each other (which can be referred to as “in registration”), it is necessary that these discrete functions be spaced at a distance from each other in exact multiples of the index length of the package and that the machine advance this same distance each cycle. Otherwise, the location of the various operations (forming, printing, sealing, etc.) may be out of position, resulting in product in seals, narrow seals, cut-off printing, and other problems.
On HFFS machinery, the distance that the machine advances each index is controlled by one of two methods. It is either dictated by the machine or dictated by eye marks printed on the rollstock.
Machine-controlled index. A machine-dictated index is controlled electromechanically either by using cams, gears, and sprockets or by an encoder, which is either internal or external to the motor driving the system. This encoder provides a closed-loop system that measures the relative position of the drive shaft as it turns and feeds this information to the drive unit itself, which achieves the proper index by turning based on this feedback. A machine-prescribed index can be used for unprinted top webs, random preprinted rollstock, stretchable rollstock printed short of the nominal index, and nonextensible preprinted rollstock printed precisely to the nominal index.
Material-controlled index. This method uses an eyespot preprinted on the rollstock at the prescribed interval or repeat. This preprinted eye mark is read by a photocell on the HFFS machine, which then tells the machine to stop advancing. This method is used only with preprinted material containing an eye mark.
Common printing Practices
Primary packaging for sterile medical devices provides fundamental information relative to product description, product code, a statement of sterility, expiration dating, and corporate contact information. There are a number of options available for labeling in HFFS applications.
In-line printing. In many regards, printing on the HFFS machine, or in-line, is the most effective means of ensuring accurate print registration. It allows the machine to control the advance, and the printing location is dictated by this advance. It is ideally suited for situations in which there are a large number of product codes using the same package shape and size but requiring different labeling copy. It is also appropriate when the production lot size of the individual product codes are relatively small; although, in aggregate, they represent a sizeable volume. In-line printing employs a variety of technologies, of which the following are most commonly used:
• Programmable printing technologies. These systems are preferred when the amount of text to be printed is relatively small and must be changed frequently. For example, ink-jet, thermal-transfer, and laser printers are most appropriate for printing lot codes and expiration dates. These programmable systems are often used in conjunction with rollstock that is preprinted or rollstock that is printed in-line. The primary benefit of programmable printing methods is that text can be changed quickly with little cost implication in terms of setup.
• In-line flexographic printing. Small-scale flexographic printers mounted on the HFFS machine are commonly used where the amount of ink coverage is too extensive to make the use of programmable systems practical. The principles for these systems are the same as for commercial flexography. The ink is picked up by a rubber printing plate, which transfers the ink to the rollstock. In its infancy, in-line printing had limitations in terms of quality and cleanliness, but the industry has evolved to a point where today’s in-line printers are capable of printing high-resolution copy with finer adjustments, minimum ink spillage, and quick cleanup.
Preprinted rollstock. Preprinted material is often preferred for HFFS applications because the volume required to warrant the capital investment for HFFS is typically high enough to warrant having rollstock preprinted by the material converter. Preprinted material may be printed randomly or in registration.
Figure 1: Illustration of random-repeat print.
Random print. The easiest way to avoid registration issues when using preprinted rollstock is to use nonregistered or random print. When using random print, to make sure all of the necessary information is present on every package, the text is printed in repeats no more than half the index length of the package. This ensures that no matter where the text falls on the package, all of the information will be present on each package.
Registered print. Preprinted material requiring registered print can present a challenge, depending upon the material being used and the method of printing. Simply put, the challenge is in making sure that the index length printed on the rollstock and the distance of the machine advance match, and that both are consistent with the spacing between the various functions (forming, printing, etc.) on the HFFS machine. Converters use one of two methodologies for printing rollstock. Both methods have positives and negatives, as well as implications to print registration.
• Rotogravure printing. In this method of printing, the most precise means of printing, there is an etched cylinder used to pick up the ink. The ink is subsequently transferred to the packaging material via a transfer roller. A high degree of precision can be obtained with rotogravure printing, since the cylinder or image carrier is machined to a specific circumference, reflecting the needed repeat or index length. This distance can be matched precisely to the desired index of the machine. Unfortunately, while rotogravure printing offers the most precise means of printing, it is also the most capital-intensive method of printing. The cost of rotogravure cylinders can run into the thousands of dollars.
• Rotary flexographic printing. In flexography, the most common means of commercial printing for medical device packaging, the image carrier is a rubber plate mounted on a metal cylinder. While flexo plates cost just a fraction of rotogravure cylinders, they lack precision and the ability to print finer graphics that can be achieved with rotogravure printing. Certainly, the ability to precisely match the desired index of the machine is limited with flexographic printing.
Common Problems and Solutions
The challenge of print registration is in making sure that the repeat of preprinted rollstock and the machine index length match precisely. To effectively do this, the impact of material characteristics and printing methodology must be reconciled against the nominal index length of the HFFS machine. The method of registration for preprinted registered print is dictated by material and printing method as well as by machine constraints.
For instance, if the material is fairly extensible, this stretch can present problems for the converter. Tension must be tightly controlled to maintain a consistent print repeat.
On the other hand, the HFFS machine fabricator can use this stretch to ensure print registration. To do this, the machine index will be programmed at a distance slightly longer than the repeat length of the print. The machine index will be controlled through the drive, but a brake will also be used to stretch the rollstock into registration. The brake is activated when the eye mark is seen by a photocell mounted on the HFFS machine. When the eye mark is detected, the brake is activated, thereby stretching the rollstock into registration. This is the registration method of choice for Tyvek, since it has significant inherent stretch.
More often, problems occur when medical device manufacturers choose to use paper or foil with registered print. Since both materials have relatively little stretch in contrast to that of Tyvek, the system described above is often not practical. In these situations, it is crucial that the machine builder and the converter communicate early. If the converter will be using rotogravure printing, then the cylinders will be machined to the proper diameter and circumference. Such machining should eliminate any print registration problems. However, cylinder etching is time-consuming and expensive. Reworking rotogravure cylinders because of a lack of dialogue between machine fabricator and converter is particularly painful and costly.
Retooling the HFFS machine to match the index printed on the rollstock is also costly. A set of forming and sealing tools can easily run from $15,000 to $40,000.
Early dialogue between machine fabricator and converter is also essential when the converter is going to use flexographic printing, because cylinder choice and plate mounting are critical. In most cases, converters will maintain an inventory of cylinders of varying diameters. When asked to print to a specific index, the converter will canvass this inventory and find the cylinder that most closely matches the desired repeat. It may be that the circumference of the best cylinder, with the plate mounted, results in a repeat slightly more or slightly less than the target index. It is not likely that the circumference of the cylinder, with the plate mounted, will exactly match the index length arrived at in package design.
Mounting the printing plate is another source of variance. Flexo plates are more often mounted on the cylinder with a double-backed tape or an adhesive.
Because circumference is a product of roll diameter (circumference = 2pr), any change in the mounting of the plate on the cylinder in regards to height will have a six-fold impact on circumference. As a result, consistency in the mounting of plates can have a significant effect on print repeat consistency. This variance is inherent, and it must be anticipated when developing machine and packaging specifications.
Consider a real-world situation. A machine was built in which the package design used preprinted paper for a package length of 270 mm. Based on available cylinder diameters and the characteristics of the printing press itself, the converter had two choices that were closest to this target index. The converter could print to a nominal length of 269.88 mm or 273.00 mm, and in either case, the maximum tolerance would be ±0.5 mm. The shorter index was preferred, because even paper can be stretched enough to cover this variance of less than 0.025%. However, should the desired index length have been 272 mm, either of these choices could have been a problem and an adjustment in index may have been advised.
Successful medical device packaging is the product of a system that requires needs and capabilities to be reconciled from a number of perspectives. Therefore, the first step in the development of any effective system is to respect the implications of the different elements of the system and how they interact with each other. As Tony Delfosse, art and specification supervisor at Amcor Flexibles, so succinctly puts it, “Packaging typically involves three companies: machine manufacturer, material converter, and product packager. It is imperative that all three work in harmony to successfully achieve the final product.”
This is certainly the case when it comes to the application of graphics on HFFS machinery. Understanding the various elements of the system (i.e., material characteristics and limitations and capabilities of labeling systems and HFFS machinery) and reconciling them in advance will result in a robustly sustainable system. Failure to do so will result in unnecessary costs and project delays.