Ask the Expert: How Does a Die Flow Channel Work?
With 33 years of experience in the applications and technology areas of extrusion dies and coextrusion feedblocks, our Chief Technologist, Sam Iuliano, has been a pioneer in flow distribution channel development. In this edition of our Ask the Expert series, we asked him a fundamental question: “How does a die flow channel work?”
From first principal theories to application-specific developments, Sam shares valuable insight on how flow distribution is achieved and what features can be beneficial for cast film applications.
Flow Channel Basics
Extrusion dies are tasked with uniformly distributing molten plastics or polymers. These are fluids with flow characteristics that vary with not only temperature but also with shear rate (how quickly adjacent layers of fluid move relative to one another). Forcing molten polymer through a smaller channel gap or at a higher velocity makes it flow more readily (lowering its viscosity due to a phenomenon called shear-thinning). Put more simply, the harder you work the plastic, the more easily it flows. As the material is spread through the flow channel of a die its viscosity varies as velocity changes and as the size of each flow chamber changes. Channel designers use flow modeling software along with the polymer rheology data to optimize the design.
Coat Hanger Flow Distribution Channel
A coat hanger flow distribution channel is a common design well suited for mono-layer and sheet applications. It consists of a primary manifold, preland, secondary manifold, and lip land.
The primary manifold is the largest chamber in the die. It has a relatively low pressure-drop and ports the flow from center to each end.
The preland is a distributor chamber with a relatively small
gap. Notice that the preland is somewhat
triangular in shape, with the longest land at center and the shortest land at
the ends. This difference in land length
is what tunes the flow distribution, otherwise most of the flow would come out
of the central region of the die (the shortest path). With the right design, each flow path along
the die has the same pressure drop, resulting in balanced flow.
The secondary manifold is a lower pressure relaxation
chamber that bridges the distance from preland to lip land. It is required because the location of the
flexible lip hinge places the primary manifold relatively far upstream – so a
relieved region is needed to prevent excessive pressure. An additional benefit is that the extrudate
will have more uniform orientation as a result of passing through this
relaxation chamber.
The lip land is the final gap before the material exits the die. Its length impacts the effectiveness of die lip profiling system and also the amount of extrudate swell, which can be important in preventing or slowing lip face build-up and contamination.
More Advanced Channels for Cast Film Applications
The classic coat hanger flow channel is still in use today, mostly for monolayer and sheet applications. However, distinct challenges arise when running thin, multi-layer films – requiring new, more advanced flow distribution channels.
Elongated Teardrop Shaped Primary Manifold
The classic coat hanger manifold has a teardrop shaped primary manifold. This provides good flow streamlining and works well for single-layer and lower pressure applications. However, when coextruding multiple layers, the layer interfaces can become distorted due to what is known as viscous encapsulation.
This is the tendency for lower viscosity layers to migrate to channel wall areas of higher shear stress to lubricate the path for the stiffer, higher viscosity layers. Shear stress is the force that acts on the channel walls (shear stress = viscosity x shear rate). Often, the result is that some layers overtake others to the ends of the die, resulting in excessive trim and extra material consumption for critical layers to achieve the minimum acceptable thickness. What drives this interface distortion is viscosity difference between layers and die wall channel areas of high shear stress.
By changing the primary channel shape from a teardrop to an
elongated teardrop, we can reduce the shear stress, therefore, improving the layer uniformity.
Notice in the flow simulations below, that the reduced shear stress along the back wall of the elongated teardrop manifold compared to the classic tear drop manifold. The shear stress is cut in half (from 0.8 psi down to 0.4 psi) by using this primary manifold shape, resulting in more uniform coextrusion.
Teardrop Manifold
Elongated Teardrop Manifold
Straight Manifold Backline
Cast film production often involves high line speeds, cooler processing temperatures, and smaller operating die lip gaps, resulting in higher back pressure. Consequently, the significant hydraulic forces acting on the die body can lead to lip gap distortion. If the die body fasteners follow an angled manifold backline (as in the classic coat-hanger manifold), then there will be more lip deflection in the center of the die than at the ends, due to difference in the moment arm.
A straight backline channel is more desirable for cast film,
since the lip gap will deflect uniformly.
This results in less need to retune the flow distribution when changing output
rates and resin melt index. Since the
lip deflects in a parallel way, any basic lip profiling remains in place even
when the die pressure drop changes.
The Multiflow™ 10 distribution channel, with its straight manifold backline, is dimensionally stable for a broad range of flowrates and materials, making it ideal for high pressure cast film applications. It also includes an elongated teardrop primary manifold for excellent coextrusion performance. Placed in between this primary manifold and the preland is a new bow-tie shaped chamber. This geometrically creates the straight manifold backline. It also allows the primary manifold to diminish in volume, which promotes fast purging and low inventory time.
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