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Spirex Screw Technology
 
 
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Spirex Screw Technology
 
 
Screws
The functions of a plasticating screw are: feeding, melting, mixing, and pumping.

The feeding process is the conveying or axial forwarding portion. Solid pellets or powders are pushed forward when the screw turns if there is drag on the barrel wall and slipping and sliding on the screw root. This forwarding action is called plug flow. There are many variables that affect this process. Some examples are coefficient of friction of the resin, pellet geometry, screw feed depth, root finish, feed length, barrel finish and screw, resin and barrel temperature in the feeding zone.

The melting process is explained graphically by viewing the melt model. As resin is conveyed forward it picks up heat from the barrel wall and forms a melt film. The melt film is collected when the screw turns and forms a melt pool in the channel. As more melting occurs, the melt pool gets larger and the solids bed gets smaller. Eventually, the solids bed breaks up and is dispersed into the melt pool. Heat transfer is very difficult here and the remaining solids are hard to melt. Unless complete melting occurs, efficient mixing will never happen.

Mixing, then, can be in the distributive or dispersive form.

Distributive Mixing:
  • Requires a substantial shear strain with a continuous reorientation of the material
  • Typically a low shear melt homogenizing process characterized by a series of disruptions to the flow of the polymer melt stream. These directional changes separate and rejoin varying bits of polymer melt tending to cause a uniform blending of the material.
  • The objection in a distributive mixing section is to mix the fluid by reorienting the flow many times while minimizing the shear stress. (Good for both shear and temperature sensitive materials.)
  • Analogy: An ocean wave that crashes onto a rocky shoreline. As the wave hits the rocks, energy is absorbed by the rocks and the wave action is reversed, causing a tremendous stirring action of the water, i.e., the more directional changes, the better the distributive mixing.
Disperse Mixing:
  • Mixing of a melted, semi-solid polymer, i.e. the materials yield point has not yet been completely exceeded.
  • Dispersive mixing is typically associated with a shearing of the polymer to eliminate gels and unmelt and to give added homogenization or blending to the polymer melt.
  • The objective with this type of mixer is to break down the particle size of the filler and to evenly distribute it throughout the fluid.
  • Analogy: Buttering a piece of toast with cold butter simulates the type of shear that may be found in a typical dispersive mixer. The more force exerted by the knife on the butter, the faster the butter melts, i.e., the higher the shear rate, the better the dispersive mixing.
There are certain rules that must be followed when designing mixers. The general rules of mixing are:
  • The mixing section should have a minimal pressure drop, preferably by having a forward pumping capability.
  • There should be no hang-up or dead spots in the mixing section.
  • The mixing device should completely wipe the barrel surface each revolution.
  • The mixing device should be easy to clean, preferably by using normal purging techniques.
  • The device should be reasonably priced.
Pumping efficiency can be calculated by using the drag flow equation.
 
SPI Guidelines for Single Screws


The following recommendations for single screws of injection molding machines and extruders have been prepared as a guide to manufacturers and processors. These guidelines have been developed to provide working tolerances that produce effective performance with economy of manufacture. Manufacturers are encouraged to meet or exceed these guidelines and processors are entitled to expect screws that they purchase to be in conformance with the guidelines.


Lengths
The following tolerances apply to most linear dimensions of a screw including, but not limited to the overall length of the screw, the flighted surface and the drive. The tolerances increase with the linear dimension involved.

English Measurement Metric Measurement
Linear Dimension Tolerance Linear Dimension Tolerance
To 12” ± .010” To 300 mm ± .25 mm
12-60” ± .030” 300-1500 mm ± .75 mm
60-120” ± .045” 1500-3000 mm ± 1.00 mm
120-200” ± .060” 3000-5000 mm ± 1.50 mm
over 200” ± .090” over 5000 mm ± 2.25 mm


Flight Width
The tolerances set forth below relate to the screw flight width at any point in the length of the flighted surface. The tolerances increase with the size of the screw and, therefore, the width of the flight.

English Measurement Metric Measurement
Specified Flight Width Tolerance Specified Flight Width Tolerance
To .500” ± .015” To 12 mm ± .38 mm
.500-1.000” ± .020” 12-25 mm ± .50 mm
over 1.000” ± .030” over 25 mm ± .75 mm


Channel Depths
The tolerance guidelines set forth below are for the channel depths of the feed and meter sections of a screw. As the channel depth increases, the tolerance also increases.

English Measurement Metric Measurement
Channel Depth Tolerance Channel Depth Tolerance
To .100” ± .003” To 2.5 mm ± .08 mm
.100-.500” ± .007” 2.5-13.0 mm ± .18 mm
over .500” ± .012” over 13.0 mm ± .30 mm


Barrier Flight Undercut
The barrier flight (or secondary flight) in a barrier screw is undercut (or a reduced diameter) from that of the primary flight, permitting the flow of melted polymer over it. The undercut is expressed as the difference in radius of the barrier flight from the primary flight. This barrier flight undercut has a greater tolerance in screws with larger diameters, as follows.

English Measurement Metric Measurement
Screw Diameter Tolerance Screw Diameter Tolerance
To 6.0” ± .002” To 152 mm ± .05 mm
Over 6.0” ± .003” Over 152 mm ± .075 mm


Screw Section Lengths
Screw section lengths (also referred to as zones) such as the feed, transition or meter sections, are defined by their length and also toleranced by a fraction of a turn (or diameter).

The tolerance for screw sections for all sizes of screws, expressed in turns (or diameters) is ± 1/8 of a turn.


Keyways & Splines
Keyways and splines are two methods used to drive or rotate the screw with the quill of the machine. The keyway is a rectangular groove extending forward from the rear end of the screw drive. A spline is a series of grooves (usually straight-sided) located similar to the keyway and performing the same function.

Keyway tolerances for all sizes of screws are recommended as follows:

English Measurement Metric Measurement
Specified Depth + .005” Specified Depth + .13 mm
Specified Width + .002” - .000” Specified Width + .05 mm-.00 mm

Spline tolerances are recommended as set forth in the following table, using metric measurements.

D (mm) Designation Teeth D (mm) B (mm)
11 6x11x14 6 14 3
13 6x13x16 6 16 3,5
16 6x16x20 6 20 4
18 6x18x22 6 22 5
21 6x21x25 6 25 5
23 6x23x28 6 28 6
26 6x26x32 6 32 6
28 6x28x34 6 34 7
32 8x32x38 8 38 6
36 8x36x42 8 42 7
42 8x42x48 8 48 8
46 8x46x54 8 54 9
52 8x52x60 8 60 10
56 8x56x65 8 65 10
62 8x62x72 8 72 12
72 10x72x82 10 82 12
82 10x82x92 10 92 12
92 10x92x102 10 102 14
102 10x102x112 10 112 16

Tolerance Zones for External (Shaft) Dimensions
Basic Size  
From - To B(d10) D(a11) d(f7)
0 - 3 -.020 to -.060 -.270 to -.330 -.006 to -.016
3 - 6 -.030 to -.078. -.270 to -.345 -.010 to -.022
6 - 10 -.040 to -.098 -.280 to -.370 -.013 to -.028
10 - 14 -.050 to -.120 -.290 to -.400 -.016 to -.034
14 - 18 -.050 to -.120 -.290 to -.400 -.016 to -.034
18 - 24 -.065 to -.149 -.300 to -.430 -.020 to -.041
24 - 30 -.065 to -.149 -.300 to -.430 -.020 to -.041
30 - 40 -.080 to -.180 -.310 to -.470 -.025 to -.050
40 - 50 -.080 to -.180 -.320 to -.480 -.025 to -.050
50 - 65 -.100 to -.220 -.340 to -.530 -.030 to -.060
65 - 80 -.100 to -.220 -.360 to -.550 -.030 to -.060
80 - 100 -.120 to -.260 -.380 to -.600 -.036 to -.071
100 - 120 -.120 to -.260 -.410 to -.630 -.036 to -.071
120 - 140 -.145 to -.305 -.460 to -.710 -.043 to -.083
140 - 160 -.145 to -.305 -.520 to -.770 -.043 to -.083
160 - 180 -.145 to -.305 -.580 to -.830 -.043 to -.083
180 - 200 -.170 to -.355 -.700 to -.950 -.050 to -.096
200 - 225 -.170 to -.355 -.740 to –1.030 -.050 to -.096
225 - 250 -.170 to -.355 -.820 to –1.030 -.050 to -.096
250 - 280 -.190 to -.400 -.920 to –1.240 -.056 to -.108
280 - 315 -.190 to -.400 -1.050 to –1.370 -.156 to -.108
315 - 355 -.210 to -.440 -1.200 to –1.560 -.062 to -.119
355 - 400 -.210 to -.440 -1.350 to –1.710 -.062 to -.119
400 - 450 -.230 to -.480 -1.500 to –1.900 -.068 to -.131


Hollowbores
In some cases, a screw is cored for cooling by boring a hole from the drive end of the screw well into the flighted section of the screw. The tolerance for the cored length (or hollowbore) is the same for all sizes of screws.

Screw Diameter English Measurement Metric Measurement
  Specified length ± .030” Specified length ± .76 mm


Nose Thread Pilots
A nose thread pilot is an internal cylindrical surface at the meter end of a screw used to accurately locate a non-return valve or other attachment connected to the end of the screw. The tolerance for the length of the pilot is especially important on an injection screw which will be fitted with a valve. The tolerance is the same for all sizes of screws.

  English Measurement Metric Measurement
Injection Specified length ± .005” Specified length ± .13 mm
Extrusion Specified length ± .015” Specified length ± .40 mm


Flight & Bearing Diameters
The diameters of the flighted section and the bearing surface of the screw are vital to the performance of the screw. The flight diameter is the outside diameter of the screw flights. The bearing diameter (or Hub) is the diameter of the screw immediately behind the flighted length which prevents the escape of material and provides a seal between the screw and the barrel. The tolerances for these two diameters is stated below:

Screw Diameter English Measurement Metric Measurement
To 6.0” +.000 - .002” +.00 - .05 mm
Over 6.0” +.000 - .004” +.00 - .10 mm


Shank Diameter
The shank is the non-flighted section of the screw, also referred to as the drive end. The tolerance for the diameter of the shank is the same for all sizes of screws.

  English Measurement Metric Measurement
  +.000 - .002” +.000 - .50 mm


Hollowbore Diameter
The tolerance for the length of the hollowbore is stated in a previous paragraph. The tolerances for the diameter of a hollowbore are dependent upon the size of the screw, as shown below.

Screw Diameter English Measurement Metric Measurement
To 3” ± .020” ± .5 mm
3” to 6” ± .040” ± 1.0 mm
Over 6” ± .060” ± 1.5 mm


Nose Thread Pilot Diameter
The depth of the nose thread pilot is stated in a previous paragraph. The tolerances for the diameter are the same for all sizes but differ between injection and extrusion screws.

  English Measurement Metric Measurement
Injection +.001” - .000” +.025 - .000 mm
Extrusion +.002” - .000” +.050 - .000 mm

The tolerance for injection screws is particularly important because they will be fitted with valves.


Concentricity Of Outside Diameters
Concentricity of cylindrical surfaces of a screw exists when all of the cylindrical shapes share the same axis (and the axis is the true center of the screw.) The deviation in the concentricity of one surface to another is measured as the maximum reading on a dial indicator, also referred to as Total Indicator Reading (TIR). The tolerance in concentricity deviation (also known as runout) varies with the length of the screw, as follows:

  English Measurement Metric Measurement
  Screw Length TIR Screw Length TIR
  To 100” .004” To 2500mm .100mm
  100-200” .006” 2500-5000mm .150mm
  200-300” .010” 5000-7600mm .250mm
  Over 300” .015” Over 7600mm .400mm

These tolerances apply to the concentricity of the outside diameter of the screw, the bearing surface and all portions of the screw drive. The concentricity of diameters in flighted sections cannot be accurately measured due to the interrupting effect of the flight. Flight depth variations taken from a true OD are used as a measure of concentricity in this area.


Concentricity Of Inside Diameters
The tolerances for inside diameters are the same for all sizes of screws. However, there is a different tolerance for the register of the screw as compared with the other inside diameters.


Nose Thread Pilots, Nose Threads and Hollowbores
  English Measurement Metric Measurement
  .001” .025 mm


Screw register
  English Measurement Metric Measurement
  .0005” .013 mm


Flight Radii
Unless otherwise specified, the flight radii connecting the flight with the root of the screw should not be less than ½ of the flight depth, up to a 1” or 25mm radius. The tolerances should be as follows:

  English Measurement Metric Measurement
  Specified ± .030” Specified ± .75 mm


Perpendicularity & Parallelism
All flights, unless otherwise specified, should be perpendicular to the screw axis from the root radius to the OD on both sides. Other surfaces perpendicular to the screw axis can be tested by use of a surface plate and an adjustable height table indicator or a precision square. Other perpendicular surfaces would include the register face and the rear drive face. The tolerance for these surfaces are the same for all sizes of screws and may be measured in distance or degrees, as follows:

  English Measurement Metric Measurement
  ± .001” in./in. ± .0025 mm/mm
  .006 degrees .006 degrees

Parallel surfaces can be determined by TIR or by using a surface plate and an adjustable height gauge. All dimensions meeting the concentricity and/or flight depth guidelines are considered acceptable.


Screw Threads
The variation in threads used in the manufacture of screws is too broad to be addressed by these recommendations. It is suggested that whenever thread selection is made, either ANSI or ISO standards are observed for ease of measurement and compatibility.


Surface Finish
Surface finish tolerances are different for plated vs. unplated surfaces, as indicated below:

  English Measurement Metric Measurement
Chrome-plated surfaces:    
Channel 16 microinches .40 micrometers
OD 32 microinches .80 micrometers
Bearing/shank 32 microinches .80 micrometers
Unplated surfaces 32 microinches .80 micrometers


Hard Surfacing
All hard surfacing materials should be specified as to alloy, width of weld, and depth of weld.


Measurement Temperature
All measurements should be taken at room temperature of 72 degrees F (± 20 degrees F) or 22 degrees C (± 11 degrees).