Introduction
To mold plastic products, the plastic is plas-ticized, that is, it is melted. The plasticator is the device that does so. Different methods can be used. The common types are those found in the single-stage (or reciprocating) and the two-stage IMMs (Chap. 2). In the re-ciprocating type, plastic is fed through a screw and into a shot chamber (front of screw).
In the two-stage plastic is fed into the first-stage screw, where it is plasticized prior to entering the second stage. In the first-stage plasticator the screw motion generates con-trollable low pressure [usually 50 to 300 psi (0.34 to 2.07 MPa)], which causes the screw to retract slightly, preparing the melt to en-ter the second stage. Depending on the plas-tic's melt flow characteristics and pressure required in the mold cavity or cavities, the injection pressure at the nozzle is between 2,000 and 30,000 psi (14 and 200 MPa). Ade-quate clamping pressure must be used to keep the mold from opening (flashing) during and after the filling or packing of the cavities with the plasticized melt.
Figures 3-1 and 3-2 as well as Tables 3-1 and 3-2 provide general descriptions of the screw.
The term L/ D (length-to-diameter) ratio of a screw is important in determining the plasti-cizing action, its best value depends on the in-terplay among many variables, including the
screw geometry as well as the screw drive, which must be carefully selected based on re-quirements such as the melt volume, charac-teristic behavior, and rate of travel.
Many different screw designs are available to meet the desired performance for the dif-ferent types of plastics being processed. Many thousands of plastics are processed, but a few hundred make up the majority of those in commercial use (Chap. 6). There is great variation in equipment and plastic materials, requiring control of the plasticator perfor-mance (see the subsection on Plastic Material and Equipment Variables in Chap. 11).
Plastic homogenization largely depends on the melt temperature. By varying the ro-tational screw speed, screw back pressure, and barrel temperature profile, a controlled change of the temperature profile along the feeding stroke is achieved. The effect of the speed is small compared to back-pressure variations.
Plasticators
A very important component in the melt-ing process for injection moldmelt-ing is the pi as-ticator with its usual screw inside a barrel (Figs. 3-2 and 3-3). The screw rotates to con-vey and melt plastic from the hopper en-trance to the front of the barrel (Fig. 3-4). If 151
D. V. Rosato et al. (eds.), Injection Molding Handbook
© Kluwer Academic Publishers 2000
TRAILING EDGE OUTSIDE DIAMETER
U
FRONT RADIUSKEYWAY FEED POCKET FEED SECTION
OVERALllENGTH---~
FEED SECTION --+---- IMELTING) - - - -... - METERING
125" II TRANSITION SECTION
150" II 125!. II
Fig.3-2 Typical metering-type screw with barrel: Ds
=
screw diameter (nominal); ¢=
helix angle=
17.8°; s
=
land width=
0.250 in.; hF=
flight depth (feed); hM=
minimum flight depth for metering=
0.22 in.; L
=
overall length; 8=
radial clearance=
0.005 in.; L/ D=
ratio of length to diameter=
16 to 24; hF/ hM = compression ratio = 2.0 to 2.2.Fig. 3-3 Schematic of a reciprocating screw plasticator.
3 Plasticizing 153
Table 3-1 Examples of dimensions in typical screw designs for different plastics
Rigid Impact Low-density High-density Cellulose Dimension (in.) PVC Polystyrene Polyethylene Polyethylene Nylon AcetlButyrate
Diameter 41 2 41 2
Total length 90 90
Feed zone (F) 131 27
Compression zone 76
1
2 18 Metering zone (M) 0 45DepthinM 0.200 0.140
Depth inF 0.600 0.600
the proper screw design is not used, products may not meet or maximize their performance and meet their cost requirements. Hard steel shaft screws usually have helical flights, which rotate within a barrel to mechanically pro-cess and advance (pump) the plastic. There are general-purpose and dedicated screw de-signs. The type of screw used is dependent on the plastic material to be processed.
·Cv
CENTER OF OlANtR t{) TRA SVERSE
FLO~, NO MIXING
Fig. 3-4 Example of the plastic melt flow in the screw and barrel.
41 Z 41 2 4~ Z 41 z
90 90 90 90
22~ 36 67! 0
45 18 41 Z 90
22! 36 18 0
0.125 0.155 0.125 0.125
0.600 0.650 0.650 0.600
The plasticizing capacity is the amount of plastic that can be melted and homogenized with heat in the barrel per unit of time (lb/h or kg/h). If the plasticizing capacity is too low in relation to the shot size required, the chances are that the injected plastic will not be com-pletely melted. With too high a capacity, ther-mal degradation of the plastic due to exces-sively long barrel dwell times can occur. The continuous plasticizing capacity is the maxi-mum quantity of a specific plastic that can be raised to a uniform and moldable tempera-ture in a unit of time. It is usually expressed in lb/h or kg/h.
The temperature of the melt has a direct ef-fect on the cycle time. The heat that is used to melt the plastic material must be removed in the mold in order to cool and solidify the part before it can be ejected. The lower the tem-perature of the melt as it enters the mold, the less time it will take to remove the heat from that mold, and the shorter the total molding cycle (Chaps. 4 and 9).
Table 3-2 Examples of gradual-transition screws Screw Diameter D
[in. (mru)]
1.5 (38) 2.0 (51) 2.5 (63) 3.5 (89)
Feed-zone Depth hI [in. (mru)]
0.250 (6.35) 0.320 (8.13) 0.380 (9.65) 00400 (10.16)
Metering-zone Depth hz [in. (mm)]
0.080 (2.03) 0.100 (2.54) 0.120 (2.79) 0.125 (3.17) For IMMs the following general configurations are suggested:
Upper Lower Recommended
Section Range (%) Range (%) Fraction (%)
Feed 60 33~ 50
Transition 331 20 25
Metering 33
1
3 20 25The injection end performs two basic func-tions. First, it melts the plastic pellets and de-posits the melt in front of the screw in the barrel, ready for injection. The controls used to perform this task include:
• Heat profile on the barrel (the temperature settings of the various heat zones)
• Screw rpm (the speed of screw rotation)
• Screw torque (the torque used to rotate the screw)
• Screw stroke (the distance the screw pumps back for the desired shot size)
• Back pressure (the amount of pressure required by the screw to pump the melt through to the front of the screw)
The second function of the injection end is to inject the melted resin into the closed mold. The controls for this function include:
• Injection pressure (the hydraulic pressure applied to the melt during mold filling)
• Holding pressure (hydraulic pressure ap-plied after the mold is full to control pack-ing of the cavities and shrinkpack-ing of the molded pieces)
• Injection speed (the rate at which material is forced into the mold)
• Programmed injection (a way to vary the injection speed in stages during filling) A nonreturn valve is also needed to ensure accurate and efficient injection. Although this device is not considered to be a control, the absence of such a valve would result in inefficient operation.
Other controls required for the injection function include:
• Shutoff nozzle (sometimes used to prevent melt from drooling out of the nozzle)
• Decompress (suckback) control (a way to hydraulically pull the screw back into posi-tion after the next shot is prepared, which helps eliminate drool)
• Sprue break (a method of pulling back the nozzle from the sprue bushing after injec-tion to prevent nozzle freeze-off)
The controls to be mastered for efficient injection-end function are numerous, but the rewards of proper adjustment are great in terms of both part quality and the efficient
cycle times that can be achieved. Knowledge of these various controls and how they inter-act to produce high-quality parts and efficient speeds is the heart of injection molding exper-tise (Chap. 7).
In many cases, controls can be retuned to shorten injection molding cycle times by 15 to 35%. A lack of knowledge and ex-perience regarding control of the injection end is costing molders a lot of money, stem-ming from inefficient control setup, improp-erly conditioned and heated melt, and actual abuse of the clamp and mold equipment as machine operators experiment in an attempt to obtain better cycle times.
It is not an unusual practice to install a mold from a lO-year-old machine in a new piece of equipment. It is also not uncommon to use the mold-run information from the old machine to set up the new machine because it is quick and easy-or so it seems at first.
Generally speaking, the screw in a 10-year-old machine is not as efficient as the screw commonly found in today's state-of-the-art equipment. The heat profile required to run the mold in the old machine is usu-ally much higher than is needed for the new, more efficient screw; hence, relying on the old mold-run data sets the melt temperature in the new machine hotter than it needs to be (Chap. 2).
As a result, the quality of the melt suffers.
A high-quality melt has a uniform tempera-ture throughout its mass. Because most plas-tics change in viscosity as the temperature changes, a melt without a uniform temper-ature profile is not going to flow readily into the mold and produce good parts.
Use of old mold-run data not only results in a higher than needed temperature; it also pro-duces an uneven melt, as the more efficient screw processes the plastic through the barrel at a generally faster rate than was achieved on the older machine.
Plastics Melt Flow
To meet part quality and performance requirements, it is best to understand the molding process and, in particular, the heart of the process: plastic melt flow (57). The
3 Plasticizing 155 general science of flow is called rheology
(Chap. 6). Rheology started many centuries ago, but a major landmark was the discov-ery of Poiseuille's law in the mid-nineteenth century. Poiseuille, who was interested in the flow of blood in the human body, found that the quantity of water flowing through a tube increased directly with the fourth power of its diameter and directly with the pressure. Also, the quantity decreased with increased viscos-ity and length of the tube. Years later, at the turn of the century, a man named Bingham developed the science and coined the name from the Greek "rheos," flow. It relates to the factors that influence flow in the injection molding process.
Flow of the plastic melt into the cavity of the mold affects the characteristics of the molded part as much as do the mold, the de-sign geometry of the part, and the selection of the plastic itself. Flow affects orientation, warp, surface finish, strength, etc. It is neces-sary to control the flow of the melt into the cavity to control the process and make re-peatable characteristics of the finished part.
Factors that influence flow are:
• Flow distance
• Wall thickness-cubed!
• Characteristics of the material
• Melt temperature
• Mold temperature and cooling rate (skin formation)
• Pressure
The mathematics of equating these factors has been worked out for some time, but un-til the arrival of computer programs, it was not extensively used because of its complex-ity. Now that it is practical to determine these factors and provide the conditions that can make the molding process optimum and re-peatable, improvements can be accomplished in quality, cost, product design, and future planning.
Flow distance The geometry of the shot needs to be divided up into flows. When the path of the melt divides (as when the sprue intersects with the main runner or the main runner branches into sub runners, or when using more than one gate), a number of
flows are distinguished. Each flow then is di-vided again into sections, or elements. These sections each have a channel shape-round, rectangular, tapered, for instance. Each sec-tion also has a specific wall thickness, width ( or diameter), and length (distance). If the wall thickness changes, or the type of chan-nel, another section is created. The width may change without a change in section, however.
The volume of the section is determined and an average, or equivalent, width is used.
The gates are located intuitively prior to laying out the mold plan. Then, after the pro-gram is run, if the flows are found not to bal-ance, the gates can be relocated again and again and new layouts made until a balance is obtained. It is so much less work and ex-pense to do this on a computer that doing it by trial and error in steel should be a thing of the past.
In a like manner, sprues and runners can be sized to an optimum diameter and distance.
Also, the economics of having a hot run-ner can be evaluated with more confidence (Chap. 4).
Wall thickness One of the early discover-ies in the science of rheology was the impor-tance of the thickness or diameter of the flow channel. In injection molded parts, the wall needs to be uniform and thick enough to flow, but thin enough to cool and stay fluid. Know-ing what this thickness should be from the processing standpoint, therefore, is a major consideration when designing a plastic part.
The designer usually considers thickness for strength and economy, but with knowledge from the processing standpoint, he or she can further optimize the wall thickness.
Characteristics of the material Every ma-terial has its own ability to be heated, moved, and cooled. This is caused by the physical characteristics of the polymer, which in turn depend on the molecular size, type, and con-figuration. The facility with which heat moves from one point to another in a body is called thermal diffusivity. It is measured by the thermal conductivity divided by the prod-uct of the density and specific heat at con-stant pressure. The thermal conductivity and
specific heat vary with temperature, so the measurements needed for calculating flow are the values at melt temperature. The val-ues published in the data files are at room temperature, so special values need to be ob-tained. Flow analysis software programs have a library of these rheology numbers for some materials, and some can be obtained from the manufacturers.
Viscosity is a concept that needs effort to understand. Molders know plastics are "hard to push." Viscosity, the resistance to flow is the opposite of fluidity. We know there is a temperature or a transition temperature range where the material softens enough to flow. There are a freezing temperature and a no-flow temperature. But plastics have an ad-ditional behavior that makes their viscosity change more than that of normal materials.
This is the variation with shear rate. Shear rate is essentially fill speed. Each material, having its own molecular characteristics, has a specific viscosity vs. shear rate curve.
So each material responds in its own way to changes in temperature, pressure, and fill speed. The rheology numbers in a typical computer flow analysis program are:
1. Thermal conductivity (J/m-sec-°C) 2. Specific heat (J/kg_°C)
3. Density (kg/m3)
4. Freezing temperature (0C) 5. No-flow temperature (0C) 6. Viscosity factor
7. Shear factor 8. Temperature factor
Shear rate (filling speed) The velocity of injection is one of the most critical controls in the molding process. This is because the viscosity of the polymer reduces dramatically with increasing injection rate. A maximum is reached whereby further increases in speed only use excess energy, and the optimum is at the lower fill rates. When the fill is too slow, small variations in speed will cause large vari-ations in viscosity, which cause irregularities in the process and resultant shot.
It is very important to fill the cavity using volume as the cutoff and making sure the
ma-chine is using enough of its pressure capabil-ity to assure a uniform fill rate from shot to shot. The fill rate used should be an optimum rate for the material and the job. This rate can be found experimentally with successive tryouts, but can also be estimated from a com-puter program.
Melt temperature Flow needs a melt with a consistent and homogeneous temperature.
It is affected more by shear-rate changes than by small temperature changes, but neverthe-less the desired temperature needs to be con-trolled and held constant. At least half the heat is provided to the material by the me-chanical work of the screw, so the tempera-ture needs to be monitored on a regular basis by using a preheated needle pyrometer in an air shot.
Mold temperature and cooling rate The cooling of the shot, if not planned carefully, can cause many problems. Skin formation af-fects the flow. The cooling rate afaf-fects the cycle time. The appropriate temperature for the mold depends on the polymer, geometry of the shot, fill rate, and characteristics re-quired in the finished part. The mathematics involved for the skin formation are propri-etary for each flow analysis program and are well-kept secrets.
The water lines in the mold are difficult if not impossible to change once the mold is built, so here is a place where heat-transfer technology can be used to great advantage during the tool design. The computer ana-lysts who provide these cooling layouts can provide both reduced cycle times and quality improvements.
Pressure This is the molding foreman's fa-vorite! When something changes in the oper-ation, raise (or lower) the injection pressure;
the results are immediate. These changes of-ten overcompensate and have a whipsawing effect on the process, making it difficult to get back to normal operation.
The injection pressure is leveraged at least 10 times, and lately machine cylinders and screws have been built to produce 20 and 30 times the injection pressure. Then there is a pressure drop as the melt passes through
3 Plasticizing 157 the system. The cavity sees half or less of the
pressure developed at the nozzle.
In flow analysis programs, pressure is one of the outputs. Each flow requires the same pressure to balance. If the system does not balance, a change needs to be made to the runners, gates, wall thickness, flow distance, fill speed, molding conditions, etc., until a bal-ance is obtained.
Barrel Temperature Override
The screw-barrel combination tends to be a complex heat-transfer system. To un-derstand something as simple as a zone override can require a complete analysis of the system. Just a few of the factors that can cause a zone override are screw de-sign, barrel mass, thermocouple placement, heating- and/or cooling-jacket fit, barrel and screw wear, head pressure, overall melt tem-perature profile, defective temtem-perature con-troller, and inadequate cooling. Before as-suming that zone override is strictly a screw design problem, analyze the complete system as a heat-transfer mechanism. Although the screw is responsible for most of the heat in-put, it cannot control the heat distribution in the equipment.
Screw Sections
The screw is usually a simple appearing device, but it accomplishes many different operations at the same time. These include (1) conveying or feeding solids; (2) compress-ing, meltcompress-ing, and pressurizing melt; and (3) mixing, melt refinement, and pressure and temperature stabilization. A simplified ver-sion of the screw plasticating process follows and is divided into the three sections or zones as shown in Figs. 1-20,3-1, and 3-2.
Feed Section
Unmelted plastic in pellets or another form enters the beginning of the feed section. The plastic is carried forward in the same man-ner as grain in a farm auger. Gravity holds the plastic down to the bottom of the
bar-reI, and it is pushed forward much like snow in front an advancing snowplow. In this case, the screw flight is angled in the direction of travel through the solid resin particles. As the resin proceeds further down the feed section, a densifying (compaction) occurs as the pel-lets or particles are pressed more closely to-gether.
The channels of the feed section become filled as resistance to motion is transmitted back toward the feed section from the re-striction caused by the tapered transition and shallow metering sections. This further com-pacts the bed of solid particles, which are pressed against the heated barrel. From this point, the compacted solids bed acts as a sin-gle semielastic mass and moves more or less as a unit. Movement of this solids bed is af-fected by many factors, including the flight helix angle, the depth of the feed channel, and the friction between the plastic and metal surfaces of the screw and barrel. A large por-tion of extrusion problems are related to poor or inconsistent transport of the solid feed material.
The movement of solid is always enhanced by anything that increases the friction be-tween the plastic and the internal surface of the barrel or decreases the friction between the plastic and the surface of the screw. In other words, it feeds well if it adheres to the barrel and slips on the screw. Reduction of friction on the screw surface can be achieved by improved surface conditions or chrome plating.
If a screw has a pitted or rough surface in the feed section, a polishing will usually help.
The brightest mirrorlike finish, however, is not always the best for a low coefficient of friction. Sometimes, a fine matte finish ob-tained with a fine grit blast provides better release and improved sliding.
Chrome or chrome-based platings can help to maintain the screw finish so that feeding conditions do not change rapidly. For ma-terials that are very difficult to feed, it may be necessary to provide a barrel that has ax-ial grooves in the internal surface from the beginning of the feed pocket (throat) to a position three to four flights forward. See Tables 3-3 and 3-4 for the materials of con-struction and protection of screws.