General Design Principles for DuPont Engineering Polymers - Module

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d) At the end of the welding period, the cross-slide moves back again to release the part a1, which immediately begins to rotate. e) The motor is switched off (or the spindle disengaged). f) Pressure must be kept up by means of the springloaded point for a short time, the duration of which will depend on the solidification properties of the particular plastic, before the parts can be taken out. Figure 11.01 Pivot welding on a lathe b a a1 c e d This sequence is often made simpler by not removing the stop e at the end of the welding cycle, but by merely disengaging or switching off. Since, however, the moving masses in the machine are generally fairly considerable, they will not decelerate fast enough, and the weld surfaces will be subjected to shear stresses during solidification, often resulting in either lowstrength or leaking joints. In general, the narrower the melting temperature range of the plastic, the more quickly does the relative velocity of the two parts have to be reduced to zero; in other words, either the fixed partner must be rapidly accelerated, or else the rotating partner must be quickly stopped. Using a lathe for spinwelding is not really a production method, but it can be used sometimes for prototypes or pre-production runs. It is, however, a very good way of welding caps and threaded nipples onto the end of long tubes. For this purpose the tailstock is replaced by a spring-loaded jig which grips the tube and at the same time exerts pressure on it; although the lathe needs to be fitted with a clutch and a quickacting brake, because a long tube cannot be released and allowed to spin. Pivot Welding on Drilling Machines Components up to 60 mm in diameter can very easily be welded on table-type drilling machines with special-purpose tools. This is the most suitable method for pre-production runs, hand-machined prototypes, or 78 repair jobs. The process can be made fully automatic, but this is not sufficiently economical to be worthwhile. Some practice is needed to obtain uniform welds, because the welding times and pressures are influenced by the human factor. The tool shown in Figure 11.02 has an interchangeable tooth crown a whose diameter must match that of the plastic part. With a set of three or four such crowns it is possible to weld parts ranging from about 12 to 60 mm in diameter. Figure 11.02 Pivot welding on drilling machines b The pressure of the point can be adjusted, by the knurled nut b, to suit the joint surface. The tightness and strength of the weld will depend on the pressure, and the correct pressure must be determined by experiment. To make a weld, the drill spindle is lowered slowly until the tooth crown is still a few millimeters above the plastic part (see Figure 11.03a). Contact should then be made sharply, to prevent the teeth from shaving off the material, and so that the part starts rotating immediately. In the form shown in Figure 11.03b, the pressure should be kept as constant as possible until a uniform flash appears. Then the tooth crown is pulled up as sharply as possible (see Figure 11.03c) until the teeth disengage, but with the point still pressed against the part until the plastic has hardened sufficiently. a
Figure 11.03 Drill spindle positions a b c The function of the point, therefore, is merely to apply the appropriate pressure. All the same, the plastic parts should be provided with a centering recess to guide the tool and to obtain uniform vibrationless rotation. For a good weld a certain amount of heat is needed, which will depend on the plastic in question; it is a product of the pressure, the speed and the cycle time. At the same time, the product of pressure times speed must not be below a certain minimum value, or else the joint faces will only wear without reaching the melting point. The coefficient of friction is important too. Clearly all these factors vary from one plastic to another, and must be determined for each case. As a first approximation, the peripheral welding speed for Delrin ® and Zytel ® should be chosen between 3 and 5 m/sec. Then the pressure must be adjusted until the desired result is obtained in a welding time of 2 to 3 seconds. For good results, a correct weld profile is of course essential. 79 Pivot Welding on Specially Designed Machines To make the method we have just described fully automatic involves a certain amount of machine investment, so that it is now very rarely used in largescale production. But special machines, based on an adaptation of this method, have been built which are much easier to operate (see Figure 11.04). Figure 11.04 Pivot Welding on Special Machines b a c d e f The machine has an electromagnetic clutch a, which makes it very easy to engage and disengage the working spindle b, which rotates in a tube c which also carries the air-piston d. The head e can take either a tooth crown or one of the other jigs described in a later section, depending on the particular plastic component to be welded. The welding procedure is as follows: • Both parts are inserted into the bottom holder f. • The piston (operated by compressed air) and its working spindle are lowered. • The clutch engages, causing the top plastic part to rotate.
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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
Page 21 and 22:
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
Page 27 and 28:
Figure 4.01 Creep Stress (S), MPa (
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Figure 4.05 Creep in flexure of Zyt
Page 31 and 32: Example 1 It there are no restricti
Page 33 and 34: 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: 11—Assembly Techniques, Category
Page 85 and 86: Inertia Welding By far the simplest
Page 87 and 88: Machines for Inertia Welding The pr
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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General Design Principles for DuPont Engineering Polymers - Module

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