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A system approach to screw design.

Accurate data regarding the screw, barrel, machinery, and material are among the essentials of successful screw design for both extrusion and injection molding.

The financial ramifications of poor extruder or injection press performance have plunged many machine operators into an ongoing battle with management and "the bottom line." Typically, performance issues result when output rates are increased without regard for machine maintenance. In other instances, an extrusion line or injection press may process some materials well but have difficulty with others. Problems also include high melt temperatures, color incorporation, surging output rates, and quality issues such as splay in injection molding or haze lines in blown film.

The problems have many possible explanations. One may reason, for instance, that polymer moisture content or predrying parameters have caused a splay problem, or that improper barrel zone settings have caused high melt temperatures or a loss in output. Similarly, die design, cooling capacity, and downstream equipment limitations may be implicated. However, screw design is often identified as the culprit while other potential causes receive little or no attention. Therefore, to evaluate a request for screw design requires that a series of questions be asked about the process, machinery, material, and the expectations of each.

This article discusses typical questions essential to a successful design. Often, such questions lead to further questions, permitting a better definition of the design request. Many resources, including resin suppliers and OEMs, may be required to satisfy all of the issues. Because answers to the questions are often used to measure success, it is imperative that they be as accurate as possible.

Project Definition

The first question to ask is "What is the problem?" The problem may, in fact, be low output, surging, poor color mixing, or high melt temperatures. On the other hand, a problem may not exist. If this is so, it is important to understand why you are investigating a new screw design. Most important, identifying the problem often leads to a solution.

A statement of goals is also necessary. The statement should define realistic expectations, carefully considered to ensure they are achievable. Unless goals are addressed early in the process, unrealistic expectations can lead to unnecessary problems.

Equipment Evaluation

Existing screw and barrel data are important to the consideration of a new screw design. Such information helps designers develop a sense of what had been used in the past that may have led to the current unsatisfactory condition. Are you using old, grossly worn equipment? If you have not used screw cooling in the past, would you be willing to try it? Do you typically run plated, hardfaced screws, or nitrided materials and tool steels? If hardfaced, what type? Do you use bimetallic barrels? If so, what is the inlay? Screw and barrel materials are subject to a compatibility index; incorrect alloy application can lead to galling and premature wear. Examples of bimetallic cylinder and screw overlay compatibility appear in the Table.
TABLE. Cylinder and Screw Compatibility Index.
Resin Filler/ Cylinder Screw
 Additive
ABS None X101 Stellite 12
FEP None X309 C#56/C-276
Nylon Glass X800 X830/C#56
PET None X101 C#56/S#12
PBT Glass/FR X800 X830/C#56
PE Carbon X101 C#56/S#12
 black
PVC Calcium X800 X830
 carbonate


Processors who run a vented operation should supply the linear location of the vent port to ensure proper two-stage screw geometry. Vent locations should be chosen to ensure all material is melted prior to the vent for effective extraction of volatiles. Also, the venting environment should maintain a vapor pressure less than that of the volatile vapor pressure at processing temperature. Depending on what volatiles are to be removed, a vacuum may be necessary.

A vented operation also requires a long L/D ratio, which is necessary to establish sufficient second-stage length to pump against discharge pressure. The equation for pressure flow is:

|Mathematical Expression Omitted~

As Equation 1 indicates, one of the biggest contributors to vent bleed is short second-stage length; small-diameter screws aggravate the situation. Five diameters of a 4-1/2-in screw are equal to 22-1/2 in; five diameters of a 2-1/2-in machine are equal to only 12-1/2 in.

Does the barrel have a grooved feed section? If so, it should be intensively cooled: High pressures generated by solid bed keying in the grooves can lead to early melting and cause problems of feeding if effective cooling is not achieved. Grooved-feed extruders operate according to physics of conveying and melting that are different from those under which smooth bore extruders operate, thus raising a number of additional questions. Figure 1 shows extruder barrels with grooved and intensively cooled feed throats.

Available screw torque is also necessary information; it can be viewed in light of resin enthalpy to determine maximum allowable output of the drive and resin system. The rate is then subjectively reviewed with respect to melt conveying rates achievable for the given screw geometry and process. It makes little sense for a screw to deliver 1000 lbs/hr to a die and cooling system that can handle only 600 lbs/hr. Those who have had the misfortune of breaking a screw realize the importance of screw strength, another criterion based on available screw torque.

With respect to machinery, it is important to know whether the DC drive field has been weakened. As Fig. 3 indicates, weakening of the drive field increases motor speed at the expense of torque. Belt and sheave ratios can be used to increase or decrease screw speed and, consequently, change torque. In some cases, it may be easier to determine armature current (amperage) and compare it to the full rated armature current. If the ratio is low and melt conveying rate is as predicted, it may be possible to deepen the screw to increase production rate or reduce melt temperatures.

In the case of injection molding, in which hydraulic drives are common, available screw torque is typically reported in ft-lbs/100 psi of hydraulic pump pressure. To evaluate whether a screw in this application can be deepened, pump pressure during screw recovery must be indexed against maximum hydraulic pump pressure. For example, a screw requiring only 1000 psi during recovery for a 2000-psi system is, for all intents and purposes, only pulling 50% torque and can be cut deeper.

Barrel zone settings should not be overlooked. Rear zones should be set to optimize solids conveying; discharge zones, to optimize melt temperatures. Barrel feed zone temperatures are intended to help locate areas where material sticks to the barrel I.D. surface, so that the advancing flight can scrape it off and advance it downstream. Figure 4 shows the differences between LDPE and PP. In the case of LDPE, segmental chain motion of the polymer begins at approximately 180|degrees~F, causing the material to become tacky. This condition promotes adhesion, and the curve continues to climb to its peak, representing the melting point of the material. At this point, adhesion begins to fade; eventually, a slip condition occurs as a melt film is created. In this case, allowing for a 50|degrees~F delta T through the barrel, optimization of zone one occurs between 250|degrees~F and 375|degrees~F. If the zone is too cold, the material will not stick. It will instead turn continuously around with the screw, starving the melt conveying zone. If the zone is too hot, the material may prematurely melt, causing an annular melt film that can act as a lubricant and starve the screw. In either case, the result is reduced output and increased occurrence of material degradation and surge.

In comparison, the curve for PP exhibits a very narrow operating window. Optimization of solids conveying zone settings for PP can be difficult, and a strong case can be made for screw cooling a portion of the feed zone. It is not surprising, therefore, that output rates for PP are much less than those for LDPE. Figures 5 through 7 show coefficient of friction vs. steel temperature for polypropylene, LDPE, and Du Pont Surlyn "A" resins.

Careful consideration should also be given to resin formulations because internal and external lubricants, slip agents, and other low molecular weight species can blossom to the surface, creating a starved condition.

If production or setup data are available, it should be used. Information on barrel zones can help resolve many processing problems and provide a much clearer understanding of the heat transfer capacity of the equipment. Typical production screw speed can be cross referenced with existing lbs/hr data to determine whether screw geometry is achieving its predicted melt conveying rate. If it is not achieving the predicted rate, what is preventing it from doing so? Is there reason to believe that it can? Information on current screw design and performance can prove to be extremely valuable.

Material Evaluation

Feedstocks vary greatly. It is important, therefore, that information be accurate. Screw design can only be as good as the information it is based upon. The type of material to be processed should be listed with trade name, grade, and supplier; when needed, the information may be used to track down more detailed information. Data regarding the percentage, form, and bulk density of virgin and regrind are required to evaluate such variables as polymer feed rates, melt conveying rates, compactibility, and melting rates. A completely amorphous virgin pellet behaves much differently than a blend of 80% crystalline pellet and 20% fluff regrind. The varying amounts of fillers and colors, their differing specific heats, densities, and rheological contributions, and their impact on solids conveyance, melting, and output rates, raise the question of whether a screw design can be anything more than a compromise.

Once the resin system is defined, rheological characteristics should be reviewed so that a fair interpretation of melt temperature and torque requirements can be made. A single point melt index test does not begin to explain the response of a material to temperature and shear; a better tool is a rheology flow curve depicting viscosity or shear stress response to varying shear rates and temperatures. Given the seemingly endless combinations of feedstocks, the need for accurate material information cannot be overstated.

General Data

With respect to general data, the extrusion and injection molding information begins to differ. The information sought is the same, but because of process differences, the methodology changes. Initially the questions (as to feed throat temperature, temperature of material when loading the hopper, and whether the material is predried) are the same. If the material is not predried, should it be? Is predrying used to remove moisture, or to decrease resin enthalpy, thereby yielding higher output rates? Is material fed warm, or allowed to cool? Is the system fed from an outside silo where temperatures reach extremes? It is necessary to identify the condition of the material as it is fed for extrusion or injection molding.

In extrusion, discharge pressure is extremely important; likewise in two-stage vented screw design. In the case of reduced output due to pressure flow, a cubic relationship exists with respect to metering depth "h," and a linear relationship exists with respect to output rate. Consequently, a screw design with excessive second stage metering depth may fail to perform. Incorporation of screw speed in the drag flow equation justifies attempts to overdrive pressure flow by running vented operations as fast as possible.

|Mathematical Expression Omitted~

Existing rate in lbs/hr and melt temperature become the benchmark against which screw performance is judged. It is important to ensure the accuracy of the information before beginning a new screw evaluation.

The injection molding equivalent of discharge pressure is back pressure, which is commonly used to enhance mixing. Existing lbs/hr, defined as the instantaneous rate, is determined from shot size and recovery time. Stroke length is extremely important. A machine with an L/D of 16:1 and a four-diameter stroke length has, in effect, an L/D of 12:1--hardly enough to perform feeding, compacting, melting, pumping, and mixing. If the overall cycle time is known and the cooling time of the part is long, reduced recovery time becomes less of an issue.

One of the most important questions relative to screw design is "What is the end product?" Without knowledge of the end product, screw design becomes a "best guess" game. For example, three 4-1/2-in, 24:1 machines with identical torque, running the same resin, and making blown film, cast film, and extrusion coating, respectively, will have very different screw designs. To keep melt temperatures low for increased outputs, blown film screws should be relatively deep. To yield a higher melt temperature, a cast film screw should be shallower; the film can then be quenched against a chill roll while in an amorphous state. An extrusion coating screw is shallower still. As such, it yields even higher melt temperatures to oxidize the melt and promote adhesion to a substrate.

Further questions need to be asked. In the case of blown film, what is the die diameter, and how is cooling achieved? Is it achieved with single- or dual-lip air rings? Is internal bubble cooling available? Are antiblocks or slip agents added to the resin? If crosslinked polyethylenes are being used for wire and cable coating, how is crosslinking achieved? If with peroxides, what is the decomposition temperature? The questions are seemingly endless, but should continue until the information is as accurate as possible. The evaluation may then begin.

Accurate information is the first step toward a successful screw design; with it comes greater confidence that a successful design can be achieved. Screw design is based in part on science. It is also based, in varying degrees, on art and politics. For any screw design to become a success, both purchaser and designer must consider all three aspects.
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Title Annotation:injection molding
Author:Black, Thomas
Publication:Plastics Engineering
Date:Jun 1, 1992
Words:2302
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