Words of Wisdom: The need for a general purpose standard screw

Over the last 50 years there has been a considerable amount written about the plasticating screw designed for injection molding machines.  From introductory plastics textbooks like Processing of Thermoplastic Materials, originally printed in 1959 when the plunger was still being compared to the reciprocating screw, to more recent publications such as Melt Rheology and its Role in Plastics Processing, printed in 1995, the subject matter focuses on a single stage compression screw known as the general purpose (GP) design.

Research into currently used designs shows how the GP design has evolved. The GP design (Figure 1) is a single flighted compression screw with a feed zone, a compression (transition) zone, and a metering zone, representing 50 percent, 25 percent, and 25 percent of the flighted length, respectively. The pitch or lead of the screw is equal to the diameter (square pitch) with the thread of the screw having a flight width equal to 1/10 of the diameter. The taper of the compression zone is cut on an involute pattern, creating a compression ratio that varies from 2 to 3:1, calculated by dividing the flight depth in the feed section by the flight depth in the metering section. The L/D ratio, or the flighted length of the screw divided by the diameter of the screw, is between 16:1 and 23:1.

The feed and metering depths are missing in this description. Without these depths, the compression ratio is meaningless. It is not uncommon to see a 60-mm, 20:1 L/D GP screw from different suppliers with a feed depth range between 5 and 9 mm. The depths affect the feeding or solids conveying of the feed-stock, the volume of plastic present in the screw channel, the throughput rate, shear rate, and overall melt quality.

Regardless of the exact depths, lengths, and flight widths used, the GP screw brings value to the industry. The screw supplier and plastics processor both recognize two resilient characteristics of the GP design that are testament to it?s continued usage: The design can process a variety of resins in a variety of shot sizes and cycle times without destroying the properties of the polymer, and the GP design is the least expensive screw design to manufacture.

Functions of a Plasticating Screw
Whether the molding application requires that we process an LCP material with a cycle time of 30 seconds or a rigid PVC with a cycle time of 95 seconds, every plasticating screw has to perform certain functions in order to process the polymeric material effectively and produce a high quality molded part. The functions are:

The GP screw, if designed properly, can perform these functions satisfactorily, but within a narrow processing window at best (see Figure 2).

Cycle time, shot size, pellet geometry, back pressure, and screw geometry determine residence time. Looking at the graph in Figure 2, when the cycle time/shot size relationship is outside of the curve, the GP design has a difficult time performing the melting and mixing functions. When the application is on the low side of residence time it is most likely that the design will not have enough melting capacity and/or mixing ability to properly prepare the melt pool. Conversely, when there is too much residence time, the screw is most likely to be too deep in the metering section to ensure good melting and mixing, and a good melting strategy plays a major role in achieving an adequate melt pool.

Difficult Task
In defense of the GP screw, the injection process requires that the plasticating screw be very versatile.  Not only does it have to perform the above-mentioned functions, but it has to reciprocate the equivalent of five diameters in length. Effective melting length is changed and the solids bed is disrupted. Cycle times generally vary from 4 to 120 seconds, which affects the residence time of the polymer. The RPM reaches 1000 mm/sec in circumferential speed to facilitate an extremely short cycle time, and the hydraulic back pressure control can reach 5000 psi of plastic pressure. This affects the pumping ability and the pressure flow of the drag flow equation.

If the application were similar to an extrusion process where most of the variables are known and the screw is in a fixed position, it would be easier to study the process and design a screw to meet the objectives. This explains why all of the math models and melt simulation packages are supplied for the extrusion process. In an injection application there are more variables involved and more importance put on adopting a melting strategy that requires good decisions by the technician.

The Need for a GP Standard
The GP design has been around for more than 50 years. There is no secret as to the geometry, there are no patent infringements to concern us, and there is no great difference in perceived value or performance, so why can?t we adopt a standard? 

A standard would do wonders for the industry, providing a benchmark against which all other iterations are measured. Let?s discuss what a standard would consist of, how to achieve a standard throughout the industry, and the ultimate benefits to the molder. There is a mechanism in place to develop such a standard or guideline; the work has to be done by the group that stands to benefit the most from the document?the injection molding community.

The GP Design
The development of a standard for a GP screw requires considerable research and testing. Collaboration is needed among machinery builders, molders, screw designers, resin manufacturers, and independent testing laboratories.

A specific design would be available for all to see. It would have a known melt temperature, throughput, and melt quality rating, established by using a scale of between 0 and 10 (as shown in Figure 2) for a specific residence time used on the molding machine over the full range of screw diameters and L/D?s.

A graph similar to Figure 2 would be generated. With this graph, a molder could determine if the GP design is adequate to mold parts to requirements. Similar graphs would also help the molder decide what rpm, back pressure, and barrel temperature settings should be used. While melt preparation can be more of an art than a science, this work to develop a guideline would be a scientific step forward. Injection molding machinery builders as well as independent screw designers and manufacturers could also use the testing mechanism developed to evaluate proprietary designs.

A Standard Benefits the Industry
The current situation in injection molding is not conducive for molding perfect parts for these reasons:

Without these tools to work with, the molder can?t formulate a precise scientific molding strategy.

It is my opinion that the molding community should insist that screw design guidelines be developed. The weak arguments that have been given in the past for not adopting a standard because of proprietary concerns or for the protection of intellectual property rights have to be overcome for the benefit of the injection molding industry.