Injection screw design starts to 'get some respect.'

Injection Screw Design Starts to `Get Some Respect'

Screw design for injection molding has long been a neglected topic. But as increasing numbers of molders are required to provide ever faster cycles and tighter tolerances, this is beginning to change. As more and more molders encounter new families of high-performance resins for critical applications, requiring tighter temperature control and gentler mixing, screw design is growing in importance.

Most material suppliers tailor their resins to operate well with what are generally accepted as standard screws. But equipment suppliers are developing some specialized designs to improve critical areas of plasticating performance. Most of these newer designs are a result of the surge of new polymers, including reinforced compounds, polymers of very high molecular weight, and premium high-heat resins for especially demanding applications. The foregoing types of resins all require low-shear processing and tight temperature control if they are to retain their desired physical properties. General-purpose, single-flighted screws generally provided relatively high shear rates and fluctuating temperatures, causing overheating and degradation problems in many instances.

THE SHEAR PROBLEM

Conventional screws create a high level of shear between the barrel and the solid mass of resin in the screw flight, which results in uncontrolled, high melt temperatures and degradation of resin properties. Also, because of the eventual breakup of the solid mass when it becomes too small to stay cohesive, portions of unmelted resin can mix with the melt pool. This unmelted resin ends up in the center of the melt channel and can pass into the mold. The problem is thus twofold: too high a melt temperature coupled with incomplete and poor mixing.

Barrier screws have been employed for over 20 years to address these problems. Barrier screws reduce "unmelt" problems and lower melt temperature somewhat. But barrier screws rely on the same type of melting mechanism as a conventional screw - including high shear over the solid bed to melt the resin. Because of this, they still have difficulty running a low enough melt temperature to properly process many of the emerging materials, in the eyes of some screw manufacturers.

Screw and barrel suppliers, machinery builders, and even processors themselves have been working to come up with screw designs that circumvent these problems. In this article we'll take a closer look at some of these new designs. The examples given are meant to be representative of trends, and not an exhaustive list of every supplier conducting technical development in this area.

IMPROVED MELTING MECHANISMS

Robert Barr developed a few years ago the E.T. (Energy Transfer) screw, which, unlike conventional or barrier screws, uses a melting mechanism other than solid-bed melting. The melting mechanism of the E.T. screw is similar to that seen in many twin-screw extruders. Initial melting follows the conventional solid-bed mechanism, but at the end of the relatively short transition section where only about half of the resin is melted, it enters the E.T. section, which is designed to intentionally break up the solid bed and disperse unmelted resin into the pool of melt. Melting is then completed by conduction of heat from the molten mass into the unmelted solids. The melting rate is said to be greater than that of solid-bed melting because the mixture is well stirred by the flow profile and by being passed back and forth over the barriers, which have clearances that are at least resin pellet size. According to Barr, in some cases the melting rate is as much as 20-30% higher.

Barr says that, in addition to its higher melting capacity, the E.T.'s transfer of energy from the molten resin to the solids lowers the melt temperature, which is advantageous with many of the emerging resins.

The Barr E.T. screw is available from Great Lakes Feedscrews, whose president, Jeffrey Kuhman, reports that users of the Barr E.T. have reduced recovery times by as much as 50% and overall cycles by 25-30%, and lowered melt temperatures by 40-50[Degrees]F.

Spirex Corp. developed the Pulsar screw not long ago, which is aimed at solving a number of the problems posed by the emerging engineering resins (see PT, Feb. '86, p. 49). Marketing manager Mike Durina says that perhaps its biggest advantage is that it offers a median between high-compression and low-compression types of screws. High compression in shallow-flight screws does a reasonable job of mixing plastic melt, but does so by means of high shear, which often causes excessive melt temperatures. This can result in low production rates and poor part quality. Low-compression screws give more output, but provide poor mixing and part quality, and may even let pass unmelted pellets of resin. Durina says the Pulsar provides excellent mixing with little or no temperature rise and low shear.

The Pulsar has a metering section divided into constantly changing sections that are either deeper or shallower than the average metering depth. This requires all of the material to pass back and forth many times from the shallower depth and somewhat higher shear to deeper channels with low shear. A larger portion of resin is always contained in the deeper section, because there is a greater volume in these low-shear areas.

Each time the plastic changes from one section to another, it experiences a gentle mixing action. This interrupts the undesirable laminar flow and is said to cause excellent distribution and melt uniformity without high shear.

NEW BARRIER DESIGN

Van Dorn Plastics Machinery offers its own patented "high-performance" (HP) mixing screw, aimed primarily at providing better melt homogeneity, but also said to offer faster cycles and energy savings. It's a barrier type that's said to provide 25% higher output than Van Dorn's general-purpose screw. The barrier in the flow channel selectively adds more energy to lower-temperature particles (or "mini-masses") of plastic, due to their high viscosity or stiffness. At the same time, both macro and micro dispersion are taking place. All of the resin receives the benefit of this selectively increased energy as needed, because all of the plastic must cross the barrier to reach the front of the screw. Mixing-section channels are open alternately upstream and downstream, so material must cross the barrier to continue along the screw. The design simultaneously has forwarding characteristics to facilitate material and color changes, and a very long barrier to minimize pressure build-up behind the barrier.

Another reported advantage is energy savings. Since less heat is used to melt the plastic, a uniform melt can be produced at a lower temperature, requiring less cooling in the mold.

At NPE '88 in Chicago, Sumitomo Plastics Machinery (represented here by SPM Corp.) demonstrated its new subflight screw, which is said to offer accelerated plasticating rate, due to alternate flights that are kept free of resin. This is said to accelerate the separation of solid and melted particles for a more uniform melt. Higher capacity injection and kneading are also claimed.

MIXING TO THE MAX

Xaloy has developed a dispersive/distributive mixing device for injection molding that is said to be gentle to degradable materials. Marketing director Laura Turner says this "G1" device is self-purging and cleaning, facilitates color changes, and supplies a high degree of mixing.

The G1 mixer is added to the discharge end of the screw and allows deeper screw flights by augmenting the mixing function. This facilitates lower melt temperatures prior to the mixing section, consequently lowering the probability of material degradation.

Dispersive mixing (the deagglomeration of large particles) is accomplished by moderate shearing (shear rates of approx. 250 sec (1)) of the material flowing over the mixer, while distributive mixing (random spatial distribution of particles within the matrix) is accomplished by passage of material through the inlet and discharge channels.

Turner says that what this means for the processor is temperature, color and viscosity homogeneity. The helix angle of the G1 mixer, being greater than square pitch, results in a conveying rate greater than that of the rest of the screw, making the G1 mixer self-purging and cleaning.

New Castle Industries also has a new mixing head. Its Stratablend device is designed to maximize distributive and/or dispersive mixing. Its a free-flowing mixer, which selectively applies stratum-layer shear on an intermittent basis, while creating an infinite number of divisions of the melt stream, according to Jim Frankland, marketing manager.

The Stratablend is said to provide the mixing capability of static-mixing sections while creating little rise in temperature. New Castle recommends it for blending dissimilar resins or processes that require a high degree of color or additive blending.

SCREW SPREADS INHOMOGENEITY 

Dr. Robert Nunn of the University of Lowell, Mass., and Stephen Takashima, manager of research and development at HPM Corp., jointly developed the screw configuration shown below (left) to promote a mixing behavior in which elements of inhomogeneity can be distributed throughout the shot. Called the Two-Way Mixing Screw, its essential features include multiple parallel screw channels, provision of regions of compression and decompression (shallow and deep channel sections), and undercuts on the flight tips to allow forward and backward flow between the parallel channels.

In effect, the mixing section consists of a series of repeating unit geometries, each of which provides compression, decompression and forward and backflow mass transfers. The mixing action of the screw results from the positioning of the regions of compressions and decompression in the two channels relative to the flight undercuts. Areas of compression in one channel are in phase with areas of decompression in the other channel, and separated by undercut flight regions. As a result, pressure differences due to flow between material elements in each channel give rise to mass transfers of material between each channel. These crossflows give rise to forward and backward mixing.

As a local inhomogeneity enters a region of compression, a portion is transferred to the other channel by the crossflow, either in the forward or backward direction, depending on the local geometry. At subsequent compression regions, other portions of the inhomogeneity are similarly transferred from channel to channel. The extent of the resulting mix depends on the number of crossflow paths present in the overall mixing-section geometry. The quantity of material transferred depends on the crossflow rate, which is a function of geometry, flow rate and polymer viscosity, and can be prescribed by the screw designer.

In addition to the mixing action already described, Dr. Nunn says there is a potential for additional flow complexity due to effects that he and Takashima call "leakage" flows. The first of these is a drag flow at each undercut resulting from screw rotation, which is always directed backward, and the second is a pressure flow at each undercut resulting from the overall pressure gradient along the mixing section, which can be directed either backward or forward, depending upon whether the pressure gradient is positive or negative. The magnitude of this leakage flow can also be prescribed by the screw designer.

GROOVES IMPROVE FEEDING

At least one injection machine builder appears to be applying the grooved-feed-throat concept that has been shown to improve screw feeding in extrusion of polyolefins. Battenfeld of America is offering a new design that company officials say offers a wider processing scope, higher plasticating rates at lower screw speeds, good thermal and mechanical homogeneity, improved thermal efficiency, lower torque requirements, reduced wear, and other advantages. The so-called LT screw, first introduced at the K'86 show in Dusseldorf, W. Germany, differs from a conventional screw in that the LT is provided with a barrel which has a rectangular feed throat and small grooves in the feed zone. These grooves are said to improve the filling of the screw channel without a build-up of pressure. According to Battenfeld officials, this improved feeding brings about a higher throughput per revolution despite the shallower channel in the feed zone.

Material compression, and therefore the melting process, is said to be initiated sooner. At the same time, the shallower feed section intensifies the melting process. According to Battenfeld, this means that, given the same length of screw, mixing and homogenizing are improved still further.

The metering section of the LT screw is cut deeper. Uncontrollably high levels of shear are said to be avoided, even in the case of "difficult" materials. The LT screw reportedly affords a much wider processing range with controlled melt temperature.

Despite an increase in output that is said to be about 25%, the LT screw permits the installation of a mixing head; it is therefore possible not only to increase the plasticating capacity, but also to improve mixing performance.

On conventional plasticating units, the barrel or wall temperatures along the screw are generally either increasing or constant. But with plasticating units equipped with the LT screws, it's also said to be possible to work with a decreasing barrel wall-temperature profile. The barrel temperature at the hopper throat can be considerably higher than at the screw tip where the desired melt temperature is set. This method of operation reportedly isn't possible with conventional screws processing materials such as PP, PS and PE; the result would be a considerable drop in throughput rate. The higher barrel temperature at the feed sections of the LT screws means that the material starts melting much quicker. At the same time, Battenfeld officials report that experiments have shown that a decreasing barrel temperature profile helps to achieve better mixing and provides a more uniform melt temperature in the metering chamber.

Battenfeld is offering the LT screw as an option in diameters from 16 to 150 mm (0.62-5.85 in.). It's recommended especially for polyolefins, to increase throughput and/or reduce melt temperature. Likely applications include HDPE pails and PP dishwasher tubs.

SPECIAL FOR PET PREFORMS

In the specialized area of PET bottle preform molding, Husky Injection Molding Systems has developed screws said to significantly reduce acetaldehyde (AA) levels. AA is a harmless thermal degradation byproduct, but it can affect the taste of the bottled product. Leading French mineral-water companies have set standards allowing maximum AA levels in PET bottles of 2 micrograms per liter - less than half that allowed by major U.S. soft-drink producers.

PET screws must be designed to minimize processing temperatures in order to limit AA levels. AA is produced at a linear rate with time for a specific melt temperature, but the relationship becomes exponential when melt temperatures exceed 510 F. Therefore it is important to prevent temperature overrides during injection molding. Husky claims a 33% reduction in AA levels have been achieved by its screw design, providing a wider operating window than is possible with the standard screw design. Husky officials say a mixing head is one element of the special screw configuration, which is said to provide high throughput rates and very gentle mixing.

NEW USE FOR VENTING

Spirex has applied for a patent on a a vented-barrel system that will enable both injection molders and extruders to process ionomer resins (Du Pont's Surlyn) in a "wet" condition without predrying, using a special two-stage screw design. This approach is useful because polyethylene ionomers such as Surlyn reportedly can't be dried in a conventional manner. The system requires starve-feeding so as to allow the vaporized moisture to vent out the rear of the partially filled screw as well as through the barrel vent. The patent application names Du Pont Co. as the sole licensee for the process and further provides for Du Pont to grant sublicenses to its customers. Spirex supplies the two-stage screw, vented barrel and starve feeder.

BMC MACHINE HAS HYBRID SCREW

At the JP '88 trade show in Osaka, Japan, Ube Industries , Ltd. introduced a machine specifically for molding long-glass-fiber reinforced thermoset compounds (PT, Jan. '89, p. 64). The Ubemax LM Series machines have a screw that's said to combine the best features of reciprocating screws and plungers. It has a wider material passage area between the barrel and the screw, and the length of the screw has been kept to a minimum. Officials at Ube say that by adopting these concepts, mixing and kneading of the resin is decreased during the metering process and the loss of material impact strength is minimized.

A PROCESSOR'S NOVEL APPROACH

In an effort to get down to zero rejects, custom molder Wolf Engineering in Dearborn, Mich., has designed its own "thermally compensated" screw. This system "reuses" excess frictional heat by transferring it back from the front end of the screw to the transition zone, where it can aid in melting and plasticating. Heat conductors inside the screw are said to provide the precise compensation necessary to prevent the random overheating and underheating of the melt that occurs with conventional reciprocating screws.

Roger Holtslander, maintenance supervisor at Wolf Engineering, says the net effect is to eliminate all unmelted resin from the conventional melt. The internal heat conductor, he adds, stabilizes the melt temperature in the critical front zone, and allows dropping that zone temperature well below that used with an uncompensated screw, with the effect of increasing cycle speed.

Holtslander says the thermally compensated screw gives greater control of front-zone temperature; usually, it can only be increased, but never decreased. Both are said to be possible with the thermally compensated screw. He says other barrel zones can be run about 20[degree]F cooler because previously uncontrolled frictional heat is brought under control by the compensated screw.

Another reported benefit of the internal heat transfer provided by the screw is a noticeable reduction in screw and barrel wear. One Wolf engineer has described the antiwear effect of the heat-compensated screw on filled material as "eliminating the grindstone effect." Holtslander says that the material now assumes a "fluid flow pattern," with far less wear in the compression zone than is found on standard screws. He adds that tests have shown that fiber integrity is preserved better in filled materials, enabling the production of stronger parts.

Holtslander says that thermally compensated screw is limited by its heat conductor to a temperature ceiling of about 600 F.

IMS Co., Chagrin Falls, Ohio, is licensed by Wolf Engineering to supply thermally compensated screws or retrofit existing screws with internal heat transfer.

THE CONVERSION OPTION

Wayne Machine & Die Co. has been modifying screws for customers to reduce residence time in the barrel. Wayne calls this putting "fast leads on screws." For example, putting a 1.75-in. lead on a 1.5-in.-diameter screw - which usually has a 1.5 in. lead, also known as square pitch - reportedly avoids degradation by decreasing residence time, and promotes faster shot cycles by delivering melt to the nozzle faster. Wayne modifies screws in other ways, too. For example, a customer who wants to run PVC in an LDPE machine would require a screw with lower compression ratio. Other companies active in conversions include Xaloy and Spirex.

PHOTO : View inside oven where Van Dorn's HP mixing screws undergo ion-nitriding treatment (opposite page). The HP screw is primarily aimed at better melt homogeneity, but is also said to offer faster cycles and energy savings. At right, the ET screw from Barr, Inc. is said to save energy and allow a much lower melt temperature.

PHOTO : THERMALLY COMPENSATED RECIPROCATING SCREW Heat conductors inside the screw developed by by injection molder Wolf Engineering are said to provide precise compensation necessary to prevent random overheating and underheating.

PHOTO : SECTION OF UNWRAPPED SCREW CHANNEL Essential features of HPM's Two-Way Mixing Screw include multiple parallel screw channels, regions of compression and decompression, and underucts on flight tips to allow forwardd and backwardd flow between parallel chanels.

PHOTO : UNWRAPPED CHANNEL OF PULSAR MIXING SECTION Pulsar mixing screw from Spirex is aimed at offering a median between high-compression and low-compression screw types.

PHOTO : Table and graph depict the difference in (acetaldehyde) AA levels using a standard screw and a screw custom-designed by Husky Injection Molding Systems.

PHOTO : For hard-to-feed polymers such as PP, the E.T. section of the Barr E.T. screw is used at the end of a conventional tapered or transition section.