General Purpose Screw Designs:
Dysfunctional and Expensive
A screw of “general purpose” design is purported to have the ability to process a wide range of polymers equally well. To the contrary, a screw of general-purpose design cannot effectively process any polymer. To fully understand the use of general-purpose designs, there are three areas we need to examine:
- When they were initiated
- Why are they still being used
- What are the detrimental effects of their use.
Reifenhäuser (Troisdorf, Germany) first developed the general-purpose screw design for an injection molding application. This design was then used as part of the original reciprocating-screw injection unit design manufactured by Egan Mfg. in the United States. Paul Squires first developed this design in 1950 at DuPont for use in the extrusion industry. General-purpose screws cost less to manufacture than specialty screws. However, the molder is generally not aware of the hidden costs associated with a general-purpose screw design—costs that arise from its inability to properly process virtually any polymer. Also, machinery manufacturers normally do not have the technical capability to provide modern screw designs, and their screw manufacturing is neither set up for, nor (in many cases) capable of machining specialty designs. Therefore, injection molding machinery manufacturers have little desire, or ability, to promote special screw designs.
As compared to specialty designs, general-purpose designs create longer cycle times, more rejects, higher resin cost, and the potential of part failure in use. These are all important, but part failure can be the most expensive. Furthermore, the cause of failure can be easily discovered if melt flow analysis is used to check the physical properties of the failed part.
Processing Requirements and Screw Design
Polymer melting requirements are the same for extrusion and injection molding. We must take a solid and make it into a viscous liquid. This is accomplished mostly by friction, shear, and normally less by conductive heat transfer.
The percentage of the energy input in the form of heat varies with the size of screw, as the watt density is greater in a smaller diameter machine. Therefore, it is far easier to design a 1-inch-diameter screw than a 6-inch-diameter, as the percentage of conductive heat transfer is far greater in the smaller screws. For this reason, there are many designers and manufacturers of small injection screws and few competent designers of generally larger diameter extrusion screws.
In the extrusion industry, the OEMs traditionally designed and manufactured the screws for their own equipment. The first non-extruder OEM company to provide screw design and manufacturing was Feed Screws Inc. (Hamilton, TX).
The injection molding industry initially had mostly small-screw diameters and general-purpose designs. Many of the injection molding machinery manufacturers did not make their own screws. New, independent screw manufacturers provided this service.
As the only competition in injection molding was among general-purpose designs, these new companies competed by adding some form of mixing section to the injection screw. These companies were and still are unable to compete in the extrusion industry and have proliferated in the injection molding industry where the design competition is less formidable.
As compared to specialty designs, general-purpose designs create longer cycle times, more rejects, higher resin cost, and the potential of part failure in use.
The problem with supplying a general-purpose design and simply adding a mixing section is that the mixing section’s job is to melt and disperse the remaining solids and has little other influence on screw performance. The upstream sections of the screw provide the necessary rate of melting and stability and are far more complicated to design.
These independent companies have provided modern designs, derived in many cases from existing designs. This is far less expensive than developing entirely new designs. Therefore, in most cases their screws are less expensive than a screw designed from scratch. Purchasing a general purpose screw on price only can be a costly practice, as a custom-designed screw with enhanced performance can reduce rejects, decrease cycle times, and decrease resin cost.
Breaking up the Solids Bed
General-purpose screw designs are not used in the extrusion industry because of the phenomenon called “breaking up of the solids bed.” This occurs in all non-barrier screw designs. In the transition section of a screw, the melt pool on the forwarding side of the flight constantly increases as the solids bed on the trailing side of the flight decreases, and melting progresses downstream. When the solids bed width decreases to a point near the end of the transition section, the increasing internal pressure penetrates (explodes) the remaining solids bed causing it to disperse into the metering section (Figure 1).
The metering section does not have the ability to effectively melt these remaining solids. The metering section has little flow agitation; therefore, the melting of the remaining solids is by heat transfer only. This is not very effective as the time is short to exit and the temperature gradient in a given pellet can potentially be from almost ambient to melt. At best this creates viscosity variations; at worst the molder can have unmelted pellets in the molded part. Poor dispersion can readily be seen when colors emerge as streaks in the part.
Dispersive melting downstream of the screw can have little influence on melting, as time is an important factor in melting and injection times are relatively short. High velocities and shear can deform the unmelted particles but as injection time is short the molecular structure is not permanently deformed and as polymers are viscoelastic they will reform into their prior configuration. These variations in viscosity can cause non-uniform stresses in the part and stress concentration areas that may cause failure.
Barrier Screw Designs
Barrier screw designs separate the melt from the solids, therefore keeping the solids from entering the metering section (Figure 2). The first barrier screw design was by Maillefer (pat. 3,358,327, 12/19/1967). The most widely used barrier design in extrusion is by the author, R. F. Dray (pat. 3,650,652, 3/21/1972). These designs eliminate the “breaking up of the solids bed” and thereby improve melt quality, decrease melt temperature, and increase rate. For further information on barrier screw designs see "How to Compare Barrier Screws.”
Mixing sections, if designed correctly, melt the solids that remain after the breaking-up of the solids bed. There are two types:
- Dispersive - These mixers should be used for low-viscosity polymers or polymers that are not shear sensitive. The first dispersive mixer was designed by R. B. Gregory (pat. 3,411,179, 11/19/1968). This and other dispersive mixers have inlet and outlet channels that have a barrier flight separating them (Figure 3). This flight typically has a clearance that provides a restriction to any remaining solids. As these low-viscosity polymers are viscoelastic, they are elongated (Figure 3), creating greater surface area for the adjacent melt through heat transfer. This promotes melting of the remaining solids, thereby bringing them to temperature unity with the adjacent melt.
- Distributive - These mixers should be used for high-viscosity polymers, filled polymers, and shear-sensitive polymers. They promote melting and mixing by agitation of the flow utilizing the increased heat transfer characteristics of the polymer. It should be noted that dispersive-type mixers also provide some distributive mixing, and conversely distributive mixers may provide some dispersive mixing. The amounts depend on the particular configuration of the mixer.
True General Purpose Screw Design
Barrier and mixing screw designs provide a wide window of performance and should be classified as “general purpose” because they can process a wide range of polymers with differing viscosities. Adapting these designs from the extrusion industry can help reduce rejects and improve part quality, but injection molders still face the following challenges:
- High melt temperature costs - With either barrier or mixing designs, melt temperatures in the extrusion industry are normally 50 to 100 deg F lower than in injection molding. These lower melt temperatures could also be used in the injection molding industry if the molds and injection units were capable of handling the higher viscosities. Many mold designs and injection units are not capable of running lower melt temperatures. Molders are therefore forced to use more energy for heating and cooling the polymer than would otherwise be necessary, and consequently live with longer cycle times.
- Checking melt temperature - Injection units normally do not provide the molder with any way to determine how damage was done to the molded part during molding. Melt temperature can only be measured by inserting a pyrometer in purged material. The temperature indicated is normally far lower than actual run temperatures. In extrusion, temperature measurement with an adjustable depth pyrometer can show an increase of 60 deg F from the barrel ID to center line of the melt stream.
- Physical properties - In many molding operations the molder is not concerned about a part’s physical properties and is unable to monitor or properly control melt temperature due to limitations in injection unit capabilities and screw design. The molder’s only recourse to a dispersion problem is to add backpressure. This increases melt temperature and recovery time. To make matters worse, in many cases the melt temperature has to be excessive to meet mold fill requirements. Resin suppliers provide recommended processing temperatures for their polymers, but they do not show the damage to the polymers at elevated temperatures.
Modern designs are available from designers who have experience in both injection and extrusion. This enables the molder to see the advantages that extrusion designs offer and how they can be advantageously applied to the molding operation. The best test of the capabilities of the screw designer or manufacturer is to require a guarantee that the screw will provide the performance required. Time spent to investigate the screw supplier’s design capabilities can eliminate the cost of down time if the screw does not perform properly, and can assure the molder that he has the best possible design for his application. The injection screw is a major factor in determining the product quality, ease of molding, and profitability of the molding operation and should be investigated and purchased as such.
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