How Do Barrier Screws Work?
Barrier screw technology has been around for over half a century. These screw designs have been widely adopted in the extrusion industry for decades, while the injection molding industry has increasingly been leveraging the technology over the years. Barrier screws, as a category, consist of many basic designs and countless variations; this topic is worthy of its own discussion. However, the fundamental mechanism that makes barrier screws appealing is generally consistent. Exploring the melting mechanism of a basic barrier screw compared to that of a conventional screw is important for all resin processors. It is likely that your company has an application that could greatly benefit from a more optimized screw design.
The Transition Section: How Resin is Melted
Conventional screw designs, meaning a single flighted screw, most notably with a screw pitch, all share the same melting mechanism. The transition section of any given screw is the workhorse of the system. The transition section conveys, compresses, and melts the polymer along a root diameter that increases at a constant taper to the metering section. The transition section’s requirements are to continue the transport and pressure increase to the ideal maximum level, resulting in the desired rate of melting and melt temperature. The solid bed, provided by the upstream feed section, starts to melt instantly upon contacting the hot barrel surface and the hot screw surface. As the solid bed melts, a thin melt film is formed at the solid bed-barrel interface. This melt film is continually scrapped off the barrel wall by the advancing flight, collecting the molten resin in the melt pool. As the resin continues to melt down the length of the screw, the solid bed width reduces while the melt pool width increases.
Solid-Bed-Break-Up
As the compacted solid bed is conveyed and melted along the screw, the internal feedstock remains virtually at feed temperature. Again, as the solid bed melts, the solid bed width continually decreases while the melt pool width increases. The solid bed is strong under compression, but weak under tension. At some point near the end of the transition section the solid bed becomes so small that it can no longer withstand the internal pressure within the screw channel. The pressure penetrates the remaining solid bed causing internal un-melted pellets to disperse, or explode, into the melt pool. This phenomenon is known as “breaking up of the solid bed,” or simply “solid bed break up.” This phenomenon occurs with all general purpose and conventional single-flighted screws. Solid bed break up is a major source of quality limitations in both extrusion and injection molding applications and has been the source of inspiration for many of the advances in screw technology to date, most notably barrier screw technology.
Barrier Screws
Barrier Screws for Extrusion & Injection Molding
The SpiraMelt MVP Barrier screw family is a high-output barrier screw design that delivers 20%+ more output than conventional screw designs. The unique channel geometry of the MVP allows for approximately 1.5x the area available for melting compared to GP screws. Perfect for Olefins and other shear-tolerant resins, the SpiraMelt MVP barrier screw family is the ideal choice for improved throughput and melt uniformity!
What is a Barrier Screw?
A barrier screw is characterized as a feed screw that incorporates a secondary flight, or barrier flight, into the transition section. The barrier flight generates two separate channels: the primary channel and an auxiliary or barrier channel. As the solid bed is conveyed forward, it will remain in the original primary channel. The barrier flight is undercut at some clearance that is great enough for molten material to flow over the flight into the auxiliary channel, but not great enough for unmelted solids to pass over. As the solid bed melts and a melt pool is generated, the melt pool continually flows into the new auxiliary channel. Generally, the primary channel continually decreases in cross-sectional area, much the same as a conventional screw’s transition section. Meanwhile, the auxiliary channel’s cross-sectional area generally increases, allowing for more and more melt to accumulate within the channel.
Conventional Screws Have Limited Melting Capacity
With conventional screws, the channel width within the transition section is functionally narrowing as the solid bed melts. This gradually limits the area available for melting down the helical length of the screw because the channel becomes occluded by the melt pool itself. This provides less and less area available for adequate melt film generation for optimal dissipative melting. With barrier screws, the primary channel area remains unoccluded due to melt separation. In turn, the melting capacity of a barrier screw can be significantly higher than that of a conventional screw of the same diameter and L/D. Morerover, solid bed break up is no longer a primary concern with barrier screws. This allows for much higher throughput and melt quality vs conventional or “general purpose” screw designs.
When Should You Use a Barrier Screw?
Not every application warrants the use of a barrier screw, however, there are plenty of applications where significant gains in quality, output, and overall efficiency can be realized. By better understanding the core concepts of advanced screw technology, injection molding and extrusion firms can make better investments in equipment to maximize quality and profits for the company. This is great for industry, the business, and the end user.
SpiraMelt Technologies offers a range of barrier screw design options to suit your specific production requirements like the MVP Barrier Screw, the MVP LS, and the MVP HS. We specialize in custom, high-performance screw design. Contact SpiraMelt today to learn more about how we can help achieve optimal results in your extrusion or injection molding process!
Melt Separation
The addition of an auxiliary barrier flight in the MVP barrier section allows the melt pool to flow into an auxiliary channel separate from the solid bed. The additional surface area available in the primary channel allows for significant gains in melting capacity with superior melt quality!
Viscous Heat Dissipation
Melt separation in the SpiraMelt MVP barrier screw design leverages viscous heat dissipation as the primary melting mechanism, rather than relying largely on conductive heat transfer, as seen in conventional screw designs. The MVP is a far more efficient and superior screw design for melt quality and throughput when compared to conventional GP screws.
High Throughput
The SpiraMelt MVP barrier screw family is capable of exceptional pressure development, allowing for maximum output and melt quality. With the shearing action and melt separation seen in the MVP barrier section, solid-bed-break-up is eliminated, even at high rpm.
Why Choose Us?
We Specialize in Custom Solutions for Unique Customers Like You.
SpiraMelt is driven by innovation. We do not offer off-the-shelf solutions because we understand that each business and process is unique. Your company deserves the right solution – let us help you!
- Feed screws from Ø12mm to 450mm (0.5" to 17.5"). Single-piece construction up to 7,000mm length (23 feet).
- Bimetallic extrusion and injection barrels with a range of liner options. Single-piece construction to 6,000mm (19.5 feet ) length without seams.
- Non-return valve options from Ø12mm - 250mm (0.5" to 9.75"). Premium, through-hardened tool steels available.
