The Pioneers of Screw Design

 

At R. Dray Manufacturing we have almost 50 years of feed screw design and manufacturing experience in virtually any molding application. Our resume includes the development of many of the standard screw manufacturing and rebuilding techniques used in the industry today as well as many industry advancing patents. Our experience in handling a wide range of materials in both extrusion and injection molding applications allows us to offer proven technology that can improve your productivity - guaranteed.

We can design and manufacture screws ranging from 3/4" to 20" in diameter and sections up to 40' in length with 10 ton overhead lifting capacity. Whether we design a screw to fit your application or manufacture screws according to your own design specifications, we offer the same strict quality. 

Our full range of mixing devices and innovative screw technology is guaranteed to improve your productivity, widen your processing window, and give you the competitive advantage in the global marketplace.

 
 

Feed Screw Design

Over the past several decades manufacturing firms have begun to adopt new manufacturing philosophies, techniques, and technology in order to maintain any competitive advantage in the global economy. This is especially true for injection molding and extrusion firms as they are faced with growing competition abroad. Whether a custom molding operation, prototyping operation, or large scale operation, in any sector, plastics manufacturing firms are seeking new and better ways to reduce scrap, increase quality, and increase production. These new practices, techniques, machinery, sensing technology, and automation can be very costly in the quest for efficiency, especially when one of the basic, key components to a successful production is neglected – the feed screw.

Investing thousands or even millions of dollars in advanced technology and training while relying on “general purpose” screws, or poor screw designs, is counterproductive at best. Choosing the right screw significantly impacts the throughput and quality of your operation, which ultimately impacts the success or profitability of your project. The idea of using a general purpose screw, or one-size-fits-all approach is counterproductive given the many number of design variables in a feed screw. By making just a few design changes, the overall performance of a screw can be changed drastically. At R. Dray Manufacturing we are dedicated to supplying the best performing screw designs in the industry. We are so confident that we can improve your productivity that we back our custom designed screws with our Performance Guarantee. We will outperform any competitors design, guaranteed - this has been our company policy since 1973!

 

What Makes up a Screw?

Melting requirements are the same for injection molding and extrusion. One must take a solid and make it a viscous liquid through friction, shear, and conductive heat transfer. While the idea is simple, the execution can be very complex. General purpose screws or “GP” screw designs were developed in the 1950s. These designs are simple, inexpensive to produce, and most importantly – not effective. On one hand we have billions of dollars invested over the past several decades in advances in materials, techniques, and parts produced, while on the other hand we are relying on screw technology that is over half a century old. It is commonly accepted that a general purpose screw can adequately process a range of materials equally well. Not only is this incorrect, it is a costly philosophy to adopt.

The basic design for a typical feed screw consists of three zones:

  • Feed Section
  • Transition Section
  • Metering Section

Unmelted plastic first enters the feed section where it is compacted and conveyed along a constant root diameter to the transition section. The transition section conveys, compresses, and melts the plastic along a root diameter that increases at a constant taper to the metering section. The metering section conveys the material along a constant root diameter while ideally reaching a desired temperature and viscosity.  

The screw profile is typically described as the length in diameters of each of the three sections described above. A typical 20:1 L/D GP screw has a 10-5-5 profile or 10 diameters in the feed section with 5 diameters in the transition and metering sections. The screw profile is a crucial variable in the screw’s performance. The lengths of each section greatly impacts how a resin is processed and should be taken into account when designing a screw.

 

Solid Bed Break Up

It is important to discuss the phenomenon associated with GP screws known as “solid bed break up,” as all GP screws allow this phenomenon to occur. While the compacted solid bed is conveyed along the transition section, the melt pool on the forwarding side of the flight constantly increases as the solid bed on the trailing side of the flight decreases. Melting occurs on the surface of the solid bed while the interior of the solid bed remains virtually at feed temperature. The solid bed is strong under compression but can easily split or break up under tension. At some point near the end of the transition section the increasing pressure will penetrate the solid bed, dispersing unmelted interior pellets into the metering section.

As discussed, the metering section is designed to establish a desired temperature and viscosity which has little ability to melt these remaining solids. This leads to obvious viscosity variation and poor melt uniformity, and can even lead to unmelted solids included in molded parts. To combat this phenomenon, molders are forced to make adjustments in backpressure and heat. This not only increases cycle time and energy consumption but also increases wear on components and greatly reduces molded part physical properties.

Click here to learn more about the downfalls of general purpose screw designs

 

Solid Bed

Solid Bed Break Up

 
 

Mixing Screws

To be competitive, many screw manufacturers that provide GP screws simply include a mixing section to improve processing performance. A mixing section is responsible for melting and dispersing the remaining solids and has little other influence on overall screw performance. As discussed, the upstream sections of the screw must provide the desired rate of melting and stability and are far more complicated to design properly. These modern GP mixing screws in many cases are derived from existing antiquated mixing designs. This is easier and cheaper than the R&D and engineering costs associated with developing new screw technology. At R. Dray Manufacturing we have developed a full range of dispersive and distributive mixing technology to accommodate virtually any molding challenge.

If designed properly, mixing sections melt the remaining solids after solid bed break up. Different polymers have different processing requirements, therefore not all mixing sections are alike. There are two types of mixing: distributive and dispersive. Distributive mixing refers to particles being spread throughout a medium through agitation of flow without a high shear rate, achieving good spatial distribution and uniformity. Dispersive mixing occurs when high shear forces are acted upon particles or agglomerates; the particles are elongated, exposing more surface area to the adjacent polymer for heat transfer. In reality, distributive mixing involves some degree of dispersive mixing while dispersive mixing involves some degree of distributive mixing. When designing a screw, it is important to take the materials being processed into account. Dispersive mixers should be used with low viscosity polymers or polymers that are not shear sensitive while distributive mixers should be used with high viscosity polymers, filled polymers, and shear sensitive polymers.

 

Barrier Screws

Unlike the injection molding industry, the extrusion industry does not use GP screws, or even refer to them as general purpose. Instead, they are commonly referred to as “square pitch” screws, as the pitch of the single flight is equal to the diameter. The extrusion industry has leveraged the benefits of mixing screws and barrier screws for years. As seen in Fig. 1, barrier screws introduce an auxiliary flight, or barrier flight, within the transition section; this effectively separates a “solid channel” and “melt channel.” The solid channel is open to the feed section while the melt channel is open to the metering section. While the solid channel depth decreases along the length of the screw, the melt channel depth increases. As the solid bed melts along the length of the screw, the melted polymer flows over the barrier flight into the melt channel through a tight clearance. The barrier clearance prevents any unmelted pellets from flowing into the melt channel. By separating the melt pool and solid bed, barrier screws increase melting efficiency and eliminate solid bed break up, allowing for more control and stability. Much like mixing sections, not all barrier designs are alike, a screw designer cannot simply add a barrier section and expect superior performance. The metering and feed sections of a given barrier screw must be properly designed in conjunction with the barrier section. When designed properly, barrier screws can accommodate a wide range of materials and provide a greater potential throughput. The most widely used barrier screw design in the plastics industry was patented by R. F. Dray (1972,Patent # 3,650,652).

Click here to learn more about how to compare barrier screws.

 

Fig. 1 - R. Dray Barrier Screw (left hand Engel injection screw)

 

With the current state of the global economy, molders and resin processors are rightfully seeking ways to increase profitability. In the name of continual improvement companies are willing to invest time and money on advanced technology and training while ignoring the key component that determines melt quality – the screw. In the quest for efficiency, molders should take careful consideration in choosing a screw. All the training, automation, and sensors in the world can only exemplify the need for proper screw designs. The screw has a major impact on the output, quality, and overall profitability of any molding operation. When a low quality melt is produced, low quality parts are produced. Molders should not simply adjust to the downfalls of outdated technology but rather seek the most cost effective ways to eliminate waste and increase profitability. Innovation is the driving force to any industry. It is clear that treating the screw as a simple replacement part and relying on the performance and price of “general purpose” screws is a costly endeavor. Properly designed mixing screws and barrier screws offer a wider window of increased performance. Molders and resin processors should seek experienced screw designers that offer innovative technology that best suits their production requirements. This is vital, not only for the growth of a given company, but for the advancement of the industry as a whole.