Injection Molding Screw Design

 

Quality Control - Plastics processing requirements in injection molding are similar to the requirements in the extrusion industry. Many of the terms are different; for example, rate in extrusion is defined as pph/rpm, and in injection oz/sec. The primary difference is of course extrusion is continuous and injection is start - stop. 


Extrudate quality in extrusion is easier to monitor than in injection as it is continuous. Extrusion systems normally accurately monitor melt pressure, melt temperature, and amps. The end product is normally continuously measured in thousandths of an inch or better. With this type of continuous monitoring quality problems are quickly addressed. 
Extrudate quality in injection molding is normally only considered when obvious part discrepancies occur. Examples are: color streaking or mixing, recovery times that increase cycle times, melt temperatures that either are to low, when coupled with inadequate injection pressures, will not allow the mold to be filled (short shot), or conversely to high, causing part flash or mold drool. 


The reasons for this lack of proper extrudate quality monitoring is twofold: 

First - most of the parts that are molded are initially designed to use a specific resin with adequate physical properties. Parts are then tested and finally placed into production. The actual molding may be placed in a machine that has inadequate inject pressure. In this example to overcome the lack of adequate inject pressure, the setup man increases back-pressure and barrel temperatures to fill out the mold. Rarely will he check to see if the melt temperature is too high - his job is to fill the mold and make parts - he probably does not know that damage may be occurring to the resin due to increased shear or elevated temperatures. After the part is placed into production, physical testing usually only happens if failure occurs. 

Second - Injection Molding Machinery manufacturing companies have not been required by molders to improve process technology primarily because the molder is not aware of the need for an advanced level of process technology or the advantages derived from improved process technology. Very few process technologists have made the transition from extrusion to injection molding. The ones that have rarely have had the opportunity to work in a "hands on" production molding facility. 


Obvious hardware differences between Extrusion and Injection:

(1) L/D - Length divided by diameter (screw or barrel length /barrel inside dia. or screw outside dia.) in extrusion is normally 30:1 or greater; in injection molding 20:1 is still the normal l/d. The l/d in injection is further reduced as the screw reciprocates. The amount of effective screw length loss is in direct relation to shot size. Therefore the greater the shot size, the greater the starvation of resin due to the resin inlet being translated downstream in relation to the first feed flight. Injection screw designs normally have additional turns of feed section to compensate for this starvation.

Barrel and screw lengths in extrusion have constantly increased from 20:1(introduced in 1950) to 30:1 plus (introduced in 1960). The reason for this increased length is described in the formulas for rate and pressure flow. 
Rate in cubic inches per second = (Q total = Q drag + Q pressure - Q leakage) 
Pressure flow = Qp = p D h3 P sin2 f / 12 u L
Note the "L" variable is linear and the "h" variable is cubic in the pressure flow equation. This means any increase in depth must have a corresponding increase in length, or the pressure flow value will increase reducing total flow. This formula does not take heat transfer and melting into account, it is simplified to show only values for constant viscosity. 

The advantages that are utilized in extrusion from longer l/d are; 
(a) Increased rate (reduced recovery times) 
(b) Lower melt temperatures. 
(c) Less pressure and temperature variations. 
(d) Improved color mixing. 
(e) Improved energy efficiency. 

Items (a) and (b) offer cycle time reductions; (a) reduces cycle time if recovery is a limiting factor, (b) reduces mold close time requirements, thereby reducing cycle time. If the lower melt temperature causes shorts due to lack of adequate inject pressure or speed, or if the mold is opened during injection (inadequate clamp tonnage), then either the injection unit was not properly selected or the wrong machine size was selected. The intent is not to run the lowest melt temperatures possible, but to run melt temperatures that are within the manufacturers recommended specifications. In the majority of applications this author has observed melt temperatures are well above the recommended. 


Downsizing (reducing barrel and screw diameters) coupled with longer l/d can provide a solution to inadequate injection pressure. Shot size must be examined so that the proper diameters are selected. In many cases recovery can be maintained or increased. Vented applications in the injection molding industry that maintain the same barrel and screw l/d (20:1) are rapidly being replaced by non-vented systems with dryers - this is unnecessary. The utilization of a vent system to extract moisture and volatiles offers serious cost saving advantages - if properly designed. In extrusion 32:1 l/d is common for vented applications. Vent flow, in a properly designed system, is non-existent. The technology is available to utilize vented systems and enjoy their advantages without the disadvantages observed by misapplication and poor design. 


(2) Screw designs - Longer l/d allows for deeper section depths that in turn increase rate. The problem with deeper metering section depth is that unmelted particles are allowed to enter the metering section. This section is not capable of removing these particles, therefore they proceed downstream and at best produce viscosity variations in the molded part, with the worst case being unmelted particles in the molded part - if the part does not short, which is likely. Typical in the injection molding industry in the above case is to apply back-pressure. While this may allow the molder to mold parts it is not without sacrifice. When restriction (back-pressure) is applied, flow rate will decrease and melt temperature will increase. Also pressure stability may be decreased. Back-pressure is commonly used and is always a poor substitute for proper screw design. See graph 1 for rate reduction vs. back-pressure with a general-purpose screw design. It may be assumed that the flow passages downstream of the screw can provide further shear energy to complete the required melting to bring the melt temperature to unity. This is normally incorrect - a brief investigation of the viscoelastic nature of particularly the low viscosity resins used in injection molding will confirm this fallacy. 


In the extrusion industry, so called General-Purpose Screw designs are seldom used and 20:1 l/d is not used, this type of design was first used in the early 1950s. Extrusion calls this design the single stage square pitch design - the author would like to submit a new name for this type of design to the injection industry "The No Purpose Design." 
A common misunderstanding is that general-purpose designs are more forgiving and capable of running a wider range of resin viscosities. This is simply not correct! A properly designed mixing or barrier screw has a far greater "window of performance" due to the ability to disperse agglomerates that enter the metering section. Modern screw designs will provide proper mixing and color dispersion without the rate reduction associated with increased back-pressure. 


The introduction of mixing sections in 1968 by R.B.Gregory (patent # 3,411,179) led to improved rate, lower melt temperatures, and improved extrudate quality. A second patent by Gregory (patent # 3,788,614) patented in 1974, has been widely used in the Injection Molding Industry. With the advent of longer L/Ds barrier designs that provided melt separation allowed for even higher rates, lower melt temperatures, and with mixing sections downstream further improved extrudate quality.

 
The proliferation of mixing sections to the injection industry in recent years proves that virtually any type of device placed downstream in the metering section will improve a No Purpose Design - this does not intimate that all mixing devices are equal. Nor does it say that the upstream design does not have to be properly modified to enjoy the benefits of effectively designed mixing sections. 


Barrier designs that separate solids from melt in the transition section were first introduced in 1959 by Miallefer (Swiss patent 82,535/59). The most widely used barrier design in extrusion today was patented by R.F.Dray in 1972 (PATENT # 3,650,652). This design has also been successfully been used in high performance low recovery time injection molding applications - primarily with longer l/d's. 


The description for efficiency in extrusion is pounds per hour per rpm (pph/rpm) and pounds per hour per horsepower (pph/hp). You will note in graph 1, the longer metering section design yields far better rates with the same backpressure; as back pressure is reduced efficiencies further improve. No purpose designs, in many cases are not able to run at lower backpressure due to inadequate color mixing or poor extrudate quality. This example is only describing the metering section, this section's function is to develop pressure, if it is unable to develop the required pressure, the pressure development requirements are moved upstream. When this occurs melting may also be moved upstream reducing the upstream pressure developing capabilities and thereby reducing the rate of melting. 

See Figure 2 for examples of screw designs in extrusion and injection molding. 


(3) Torque readout - in the extrusion industry virtually all machines are equipped with an ammeter that is a direct reading of screw torque. If the operator is to find the optimum barrel heater settings, a torque reading is invaluable. What he is attempting to accomplish is to find the peak of the coefficient of friction curve. As you will note in graph 2 either side of the peak will reduce the coefficient of friction and reduce the screw's ability to convey and develop pressure. Increasing the coefficient of friction increases torque and screw efficiency in pph/rpm, which also results in lower melt temperatures. To locate the peak of this curve select a nominal manufacturers profile, allow the machine to settle at actual temperatures without cooling, set zones five degrees lower than actual temperatures. Lower zone one at ten-degree increments noting amp. or pressure changes. If amps or pressure increase, continue; if they decrease, stop and raise temperatures while continuing to monitor amps or pressure. If they increase, continue; if amps or pressure decrease, stop and select the setting that yielded the highest amp or pressure readout. In Injection molding torque can and should be read as hydraulic pressure on the screw drive. The availability of accurate torque readout will enable injection molders to establish the same efficiencies as the extrusion industry. It may be noted that the energy used by the screw drive is at least seventy percent of the total energy used in an IM machine. Proper screw efficiency can offer significant energy savings in the IM process. 


(4) Pressure readout - In extrusion head pressure is monitored accurately with a pressure transducer located downstream of the screw with a digital readout. In injection molding the readout is backpressure, that is a hydraulic pressure taken by a transducer in the injection cylinder. The ratio of injection cylinder or cylinders to barrel inside diameter is normally ten to one. Therefore the accuracy is ten times less than a transducer mounted downstream of the screw, as in extrusion. An example of End Cap Transducer mounting is shown in figure 3. Backpressure variation readout is normally not available in injection molding. In some injection systems accuracy is further sacrificed due to improperly sized relief valves that control poorly at low pressures. An accurate readout of pressure variation as normally observed in extrusion is one of the primary screw variables that determine screw performance as related to product tolerances and quality. In injection molding accurate pressure readout during recovery would enable the screw performance to be more accurately described. In most IM applications recovery time varies far more than other machine variables and is neglected. Recovery time and recovery time variations are normally the only indication of screw performance available in IM machines. In nearly all IM companies recovery variations are totally overlooked and accepted. In many IM companies recovery times that increase cycle times are neglected. It is unnecessary to accept these variations. With a proper screw design recovery time limitations can be eliminated and extrudate quality improved. IM machinery manufacturers in some cases have increased rpm to reduce recovery times - this may cause excessive shear heating and poor product quality if the screw is not properly designed. Conversely high rpm can also be an advantage in high temperature engineering resins with a properly designed screw. 
 

(5) Temperature readout - In extrusion melt temperature is taken downstream of the screw, the preferred location is in the discharge end of the adapter as shown in Figure 3. The most accurate thermocouple is an immersion thermocouple that may be adjusted to the centerline of the melt stream (Figure 4). The next best is a partially immersed (usually ¼ in.). The most durable is a surface type although it is the least accurate. Temperature variation can easily be monitored via the digital readout or recorded on microprocessor equipped machines. In IM extrudate temperature readout is normally not available. If melt temperature is taken a pyrometer inserted into the purge is the normal means. Accuracy in temperature readout is easier to achieve in extruders than in IM machines. If we were to monitor melt temperature in IM the same as we do in extrusion we would have to follow the discharge of the screw as it retracts. Needless to say this would be very difficult. The area that is more conducive to monitoring is in the end cap; while this monitoring does not accurately describe the temperature variation during recovery, it does give a good indication of the extrudate temperature during injection. This at least is a base point that the operator or process engineer can record and refer to, to improve consistency and determine if large variations are occurring, or if excessive temperatures are being used that may be damaging the resin. Presently when parts are being molded it is impossible to determine extrudate temperatures without interrupting the machine cycle.


Conclusion - Product quality monitoring in extrusion - for example cast film - can be measured in line and traverse within thousands of an inch or better. In IM the measuring is more difficult - but not impossible. New Injection Molding Machines are equipped with microprocessors that monitor and control machine functions. Many of these machines have Staticial Process Control (SPC), SPC is valuable if understood and used properly. As noted earlier in this paper the necessary inputs for extrudate quality and screw performance are missing in Injection Molding Machines. Accurate screw torque, melt pressure, and melt temperature readouts are standard and considered necessary in the Extrusion Industry, they are as necessary and should be standard on Injection Molding Machines. The author believes that the injection unit is the foundation of the molding process. The injection unit has been generally overlooked and process technology disregarded. The current technology used is from the 1950s. In the 1950s, 60s, and 70s the Extrusion Industry revolutionized process technology. The motivating force was the advent of measuring and monitoring equipment that could accurately describe product quality. This coupled with new resins required new process technology, this technology evolved and is available. The same type of revolution will occur in the Injection Molding Industry, it is overdue - we can observe changes now occurring - new mixing designs and even longer l/d in selected cases. The problem is that in many cases the Injection Molding Industry is trying to re-invent the wheel. Mixing designs that are capable of improving quality and recovery are coupled with poor screw design. Barrier designs are supplied with l/ds that are not capable of providing high performance and improved mixing. The Injection Molding Industry should not try to re-invent the wheel, it would be better served to examine the innovations of the Extrusion Industry and utilize this technology. The process requirements are very similar, the costs for longer l/ds, proper monitoring, and advanced screw designs is minimal compared to the benefits and can be justified by reduced scrap and lower resin consumption. If the injection unit provides the extrudate in the proper quality, viscosity, and rate with reasonable monitoring many part failures in the field would be avoided. Shot to shot repeatability also must be provided by the injection unit - when these two basic requirements are adequately performed by the injection unit, scrap reduction and part failure are greatly reduced. The Injection Molding Revolution in process technology will raise our industry to another level. If we are able to eliminate variables from the injection unit, provide proper extrudate quality, and eliminate the possibility of recovery time increasing cycle time, we can then begin to realistically look at mold design to improve flow and eliminate product quality problems associated with poorly designed molds. 

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