Working with Flexible Foam in Medical Device Manufacturing

Flexible foam is readily used in medical device manufacturing ranging from simple shapes of single layer foam to more complicated adhesive bordered foam or island placed advanced wound care dressings. Foam wound care dressings are often used by clinicians due to their high absorptivity and flexibility, which greatly improves wound debridement through exudate management and promotes tissue formation. The high porosity and low density of flexible foam gives wound care dressings high absorptivity and flexibility. While these features make for a functional wound care dressing, they often create difficulties during the manufacturing process.

Whether you are developing a simple flexible foam wound care dressing or a trilaminate island placed wound care dressing, chances are one of the most difficult aspects in the manufacturing process will be the cutting and placement of the foam piece. Some common problems we see at Boyd Biomedical that occur during the medical device manufacturing process are foam stretching, tearing, and tracking. These problems can be associated to a number of aspects of the manufacturing process, which are important to understand.

• Controlling Tension
• Managing Vacuum Power of Island Placements
• Setting Gear Ratio
• Working with Guidance Systems

Controlling Tension

Tension control is one of the most important process capabilities to understand when dealing with flexible foam. A high tension on an unwind spindle or rubber nip roller can quickly cause a stretching of the foam. This means that the foam will tighten and create a smaller width and thickness in the center of the web. This issue can cause numerous problems, which includes variances in the shape of the foam after it relaxes leaving the product out of specification, as well as variances in the cut angle of the foam also resulting in the product being out of specification. 

One of the best ways to mitigate these issues is to create little to no tension on the unwind spindle of the flexible foam. Ideally, zero tension on the flexible foam roll will create no stretch as it is pulled off the roll and into the machine. However, zero tension can create little to no web control for the foam which can also be problematic.

Without web control tracking can occur - which is the tendency of a web of flexible material to stray left or right as it runs through a medical device manufacturing line. In order to reduce tracking a passive or active web guidance system can be used. A passive web guidance system, will guide the web using plates or panels along spindles in order to keep the web moving along the correct path. An active web guidance system, will monitor and record the location of the web, then move the spindle accordingly in order to keep the web moving along a straight path.

Managing Island Placements & Vacuum Power

Managing vacuum power is an important capability when working with flexible foam and frequently arises when trying to island place foam. When island placing flexible foam, a foam web is run through a rotary die to cut out a specified shape. Once the shape is die cut it is then pulled against the vacuum anvil using suction at which point it can be aligned and laid onto another web of material, like an adhesive film or nonwoven.

Often, when a flexible foam roll goes through this part of the process it will stretch as it is being applied to an adhesive layer. This can occur even if the foam is being fed into the machine with zero tension. This problem occurs when the vacuum power of the island placement module is set too high. The high suction power causes the foam to remain attached to the vacuum anvil as one end of the foam is being pulled away by the adhesive web. The result is a stretched piece of foam attaching to the adhesive layer. Once the final product is produced it can also cause curl in the dressing itself as the foam relaxes into its original shape and pulls the other layers of the composite with it. This curl issue is often misdiagnosed due to the foam relaxing back to the correct shape and appearing like it was not the culprit!

 Setting Gear Ratio

Understanding gear ratios and setting them appropriately is a key capability to understand when working with flexible foam in medical device disposables applications like wound care dressings. Thicker foam has a higher tendency to stretch. When die cutting or sheeting a thicker flexible foam you will most likely use a rubber nip roller to feed the foam web into the die cutter or rotary cutoff. This rubber nip roller will create stability as it helps pull the foam from the zero tension unwind spindle and provides a steady web of foam into the machined die. However, if the gear ratios of both the rubber nip roller and rotary die do not match, the foam will either stretch as it is being cut decrease the cut quality of the foam piece. Two guidelines for setting and understanding your gear ratios are below.

• If the gear ration of the nip roller is lower than that of the die, the nip roller will be moving slower than the rotary die itself, which means the foam will be under high tension before entering the die and effectively stretch out the foam web.
• If the gear ratio of the nip roller is higher than that of the rotary die, it will be moving faster than the die itself, which means it will feed too much foam into the rotary die station. This will cause the foam to track to one side or the other as well as decrease the cut quality of the foam cutout.

Working with Guidance Systems

As previously mentioned, tracking is a common issue that arises when working with advanced flexible materials, in particular flexible foam. Tracking is simply when a flexible material, like flexible foam, strays in one direction or another (left or right) while being processed. This can be caused by two separate reasons - zero tension of the foam unwind spindle, or misalignment of unwind and rewind spindles. If the rolls on the unwind and rewind spindles are not aligned correctly the material web will always pull towards the direction of the rewind spindle roll.

Even though zero tension of the foam unwind spindle helps to prevent foam stretching it can still cause issues in the medical device manufacturing process like tracking. When issues like this start to occur - where solving one problem creates another - it is often the case that some type of guidance system is needed. These web tracking issues can easily be reduced with the addition of an active or passive web guidance system. There are a number types of guidance systems that can be used to help mitigate processing challenges when working with advanced flexible materials like flexible foam.

Conclusion

Working with flexible foam materials can be challenging in medical device manufacturing. However, the functionality of this material really makes it an essential part of medical disposable products and therefore working with it in a manufacturing environment is a critical capability. A few key items to consider as you establish your process are controlling tension, managing vacuum power of island placements, understanding and setting your gear ratio, and working with guidance systems.