General Design Principles for DuPont Engineering Polymers - Module

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the spindle is not fully transmitted until the coupling reaches the end of its stroke, there will be a delay in gripping the part, with the result that the teeth tend to shave off the plastic, especially when the spindle does not descend fast enough. A lined flat clutch (as shown in Figure 11.13) can of course be used instead of a hardened ground cone clutch. The following rules must be observed when using inertia tools in drilling machines: • The spindle must be lowered sharply. The usual commercial pneumatic-hydraulic units fitted to drilling machines are too slow. Figure 11.10 Inertia welding, mechanical coupling b d c a 82 • The pressure must be high enough to bring the tool to rest after 1–2 revolutions. This is particularly important with crystalline plastics with a very sharply defined melting point. (See general welding conditions.) • Inertia tools must be perfectly round and rotate completely without vibration. If they have a Morse cone, this must be secured against loosening. It is best to use a Morse cone having an internal screw thread within anchoring bolt (hollow spindle). Fatal accidents can result from the flywheel coming loose or the shaft breaking. • The downwards movement of the spindle must be limited by a mechanical stop, so that the two jigs can never come into contact when they are not carrying plastic parts. Although uniformly strong welds can be made when operating these drilling machines by hand, the use of compressed air is firmly recommended even for short production runs. Such a conversion is most easily done by adding a rack and pinion as shown in Figure 11.11. Moreover, it is advisable to have a machine with variable-speed control, so as to be able to get good results with no need to modify the mass. It is only worthwhile converting a drilling machine if this is already available; if starting from scratch, it is better to buy a machine specially designed for spinwelding. Figure 11.11 Inertia welding, rack and pinion conversion
Machines for Inertia Welding The principle of the inertia welding machine is so simple that it is possible to build one with very little investment. If the machine is mainly used for joining one particular pair of components, it will not generally require to have facilities for varying the speed. If this should prove necessary, it can be done by changing the belt pulley. Except for the welding head, the machine shown in Figure 11.12 is entirely built from commercially available parts. It consists basically of the compressed air cylinder a, which supports the piston rod at both ends and also the control valve b. The bottom end of the piston rod carries the welding head c (see Figure 11.13), driven by the motor d via the flat belt e. The machine also incorporates a compressed air unit f with reducing valve, filter and lubricating equipment. The welding head shown in Figure 11.13 (designed by DuPont) consists of a continuously rotating belt pulley a, which carries the coupling lining b. In the drawing, the piston rod is at the top of its stroke and the movement of rotation is transmitted via the coupling to the flywheel c. As the spindle descends, the coupling disengages and the tooth crown grips the top of the float, shown as an example. Figure 11.12 Inertia welding machine d e f a c b 83 Figure 11.13 Inertia welding machine head a b c d If it is impossible to grip the part with a tooth crown, and it has to be fitted into the top jig by hand (as in Figure 11.06, for example), an extra control will be necessary. The piston will have to pause on the upstroke just before the coupling engages, to enable the parts to be inserted. This can be managed in various ways. For example, one can buy compressed air cylinders fitted with such a device. A pulse passes from the travelling piston directly to a Reed switch on the outside. So that the parts may be taken out conveniently, the piston stroke must generally be about 1.2 times the length of the entire finished welded part. Long parts require considerable piston strokes, which is impractical and expensive. Figure 11.14 shows a typical example—a fire-extinguisher—for which a piston stroke 1.2 times the length of the part would normally have been needed. However, there are various ways of circumventing this: • The bottom holder a, can be fitted with a device for clamping and centering, so that it can easily be released by hand and taken out sideways. • Two holders are fitted, a and b, which can swivel through 180° about the axis X-X by means of a
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dDuPont Engineering Polymers Genera
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8—Gears . . . . . . . . . . . . .
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1—General Introduction This secti
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Prototyping the Design In order to
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2—Injection Molding The Process a
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As a general rule, use the minimum
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Figure 3.11 Cored holes Figure 3.12
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Figure 3.19 Mold-ejection of rounde
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• Zytel ® nylon resin—Parts of
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4—Structural Design Short Term Lo
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Form of section Area A d d R R 1 1
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Table 4.03. Formulas for Stresses a
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Form of bar; manner of loading and
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Figure 4.01 Creep Stress (S), MPa (
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Figure 4.05 Creep in flexure of Zyt
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Example 1 It there are no restricti
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Determine the moment of inertia and
Page 35 and 36: Figure 4.11 Stress curves The compu
Page 37 and 38: Rim Design Rim design requirements
Page 39 and 40: Figure 5.08 Chair Seat Zytel ® 71
Page 41 and 42: Cantilever Springs Tests were condu
Page 43 and 44: 7—Bearings Bearings and bushings
Page 45 and 46: debris is moved around continuously
Page 47 and 48: Table 7.01 Coefficient of Friction*
Page 49 and 50: Figure 7.15: Connecting rod spheric
Page 51 and 52: 8—Gears Introduction Delrin ® ac
Page 53 and 54: 9—Gear Design Allowable Tooth Ben
Page 55 and 56: Delrin ® 500. See the section on D
Page 57 and 58: Figure 9.06 Sizes of gear teeth of
Page 59 and 60: The following additional examples i
Page 61 and 62: In practice, the most commonly used
Page 63 and 64: Driving Gear Meshing Gear Table 9.0
Page 65 and 66: A full-throated worm gear is shown
Page 67 and 68: Worm Material Gear Material Possibl
Page 69 and 70: 10—Assembly Techniques Introducti
Page 71 and 72: For these materials, the finer thre
Page 73 and 74: Table 10.01 Self-Threading Screw Pe
Page 75 and 76: Figures 10.07 and 10.08 show calcul
Page 77 and 78: Undercut Snap-Fits In order to obta
Page 79 and 80: Cantilever Lug Snap-Fits The second
Page 81 and 82: 11—Assembly Techniques, Category
Page 83 and 84: Figure 11.03 Drill spindle position
Page 85: Inertia Welding By far the simplest
Page 89 and 90: This is not always easy, because ap
Page 91 and 92: The holder a, will have equal and o
Page 93 and 94: Calculations for Inertia Welding To
Page 95 and 96: If, for example, the angles of the
Page 97 and 98: Welding Double Joints The simultane
Page 99 and 100: Figure 11.37 Commercial bench-type
Page 101 and 102: Heat is generated throughout the pa
Page 103 and 104: Figure 11.45 Tapered or stepped hor
Page 105 and 106: Weld strength is therefore determin
Page 107 and 108: ) General Part Design The influence
Page 109 and 110: eticule in the eye piece is suitabl
Page 111 and 112: For this reason, if the strength of
Page 113 and 114: Figure 11.64A shows a variation whi
Page 115 and 116: 3. Vibrations, generated either by
Page 117 and 118: If one part is only vibrated at twi
Page 119 and 120: Another joint design, with flash tr
Page 121 and 122: Figure 11.82. A linear welded motor
Page 123 and 124: B) Commercial linear and angular we
Page 125 and 126: Hot Plate Welding of Zytel ® One o
Page 127 and 128: Figure 11.94 Applications of riveti
Page 129 and 130: Tolerances may be difficult to hold
Page 131 and 132: are used in either hand- or power-o
Page 133 and 134: ather than scrape, these drills wil
Page 135 and 136: The polishing operation is performe
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