MACHINERY | FLAT DIES


Fabrication methods In the early 1960s, co-extrusion concepts were developed to obtain a high number of layers, such as the ‘comb die’ proposed by Schrenk et al. in 1965 [2]


. However, these tooling approaches were too


complex and difficult to machine, resulting in high cost. An alternative method was then developed to combine a traditional feedblock co-extrusion step with several steps of layer multiplication, using a series of multiplication elements[3]


. The method –


named the ‘interfacial surface generator’ and developed by Tollar – was similar to a static mixer. With this method, the theoretical number of


layers is:


Total number of layers


=


Initial number of layers × 2N


Where N is the number of multiplication elements


Several variations of this approach have been developed over the years, such as the layer multiplier developed by Schrenk, using side-by- side channels [4]


(seen in Figure 1). Research


groups – such as that led by Professors Anne Hiltner and Eric Baer at the Case Western Reserve University – have successfully used similar layer multiplication systems to study the benefits of nano-layered polymer structures. Today, there are two commercially available nano/micro-layer co-extrusion tooling solutions. One, from Nordson EDI, is essentially based on a variation of the layer-multiplier system, combined with a flat die[5]


. The second system was developed by Cloeren


in the early 2000s and takes a different approach. Its Nanolayer Feedblock was designed to create layers sequentially, each formed by its own dedicated flow channel[6]


.


Figure 1: Schrenk’s layer multiplier design uses side-by-side channels


cially true with large viscosity ratios or in some cases, for polymers with matching viscosity but very different melt elasticity. Designers have worked on design iterations


. These flow channels are


machined as flow inserts and assembled in stacks. The flow inserts are therefore designed for a specif- ic structure and material flow properties[1]


Right: This arrangement of Cloeren’s NanoLayer Feedblock with a flat die is used to make stretch film


22


Tooling considerations Co-extrusion can be technically difficult on its own and designing the hardware for these applications with thin layers in high numbers can be a major challenge. A typical issue is layer thickness non-uniformity. This is espe-


from the original layer multiplier to mitigate the problems, with more or less success. In some cases, layers can become discontinuous. Cloeren’s feedblock design aims to reduce the manipulation of the flow streams by forming each layer individually with a dedicated flow insert. The challenge with this approach consists of balancing the flow rate through each flow insert. The design is obtained through numerical optimisation and uses polymer rheology. Design iterations can be calculation intensive, but can help to obtain acceptably uniform layers – even with rheologically mismatched polymers, such as EVOH and TPU. While the feedblock technology is critical for successful nano-layer co-extrusion, the die is equally important as it needs to shape the co-extrusion struc- ture into a thin and wide web, without creating any layer disturbances. When designed correctly, the die and feedblock system work together to deliver a high-quality co-extrusion. The best example is probably stretch film: today’s most advanced structures use 55 layers for a 12-20 micron film total thickness and die slot width close to 5.5m. The die flow channel design is critical, as it must


preserve the layer integrity. Best co-extrusion practices are therefore a requirement for the design.


Industrial applications Optical films were probably the first area of commercial development for nano-layered films, emerging in the early 1960s. Some


FILM & SHEET EXTRUSION | April 2018


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