General Design Principles for DuPont Engineering Polymers - Module

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Figure 11.66 Ultrasonic inserting Ultrasonic horn Metal insert Plastic Before After Safety Ultrasonic welding can be a safe process. However, certain precautions should be taken to insure such safety. • Ultrasonic welding machines should be equipped with dual actuating switches, thereby insuring that the operators’ hands remain clear of the welding horn. Stop or safety override switches should also be installed so that welding machines can be stopped at any time during its cycle or downward travel. • Vibration welding horns should not be squeezed or grabbed, nor should the unit be brought down by hand under pneumatic cylinder load. Light skin burns may result from the former, and severe burns as well as mechanical pinching will result from the latter. • Welding machines operate at 20,000 cycles per second, above normal audibility for most people. However, some people may be affected by this frequency of vibrations and by lower frequency vibrations generated in the stand and parts being welded. An enclosure with absorption lining similar to that shown in Figure 11.67, may be used to reduce the noise and other possible effects of the vibrations. The enclosure should be complete and not just a barrier. If this is not possible, ear protectors should be worn by all production line operators and by others working near the welding equipment. Laboratory technicians working occasionally with ultrasonic welding machines should wear ear protection if sounds from the welding machine produce discomfort. Some welding horns, shaped very much 110 like bells, may produce intense sound vibrations when improperly operated. These vibrations may cause nausea, dizziness, or potentially permanent ear damage. Figure 11.67 Noise shield Vibration Welding Introduction Vibration welding as such has been known for many years and applied in some special fields. The DuPont Company, however, has further developed and improved the technique to the extent at which it can be used in the broad field of engineering plastic materials. In addition, DuPont was the first to produce adequate prototypes of equipment to demonstrate the feasibility and usefulness of this method for joining industrial plastic parts. Vibration welding is a simple technique and does not require sophisticated mechanical or electrical equipment. The welding cycle can be divided into the following steps: 1. The two parts are put into suitably shaped jigs on the machine. 2. The jigs move towards each other to bring the joint surfaces into contact under continuous pressure.
3. Vibrations, generated either by a gear box or by an electric magnet, are transmitted to the jigs and through them to the joint surfaces. The motions of the two parts take place in opposite directions, thus creating a relative velocity at the weld surfaces. As a result of friction, temperature rises immediately, reaching the melting point of the plastic, usually in less than a second. 4. After a pre-set time, an electrical control device stops the vibrations whilst pressure on the joint is maintained. Simultaneously the parts are brought into the correct position relative to each other. 5. Pressure is maintained for a few seconds to allow the melt to freeze. Then the jigs open and the welded parts are ejected. Basic Principles The various weld methods for joining parts in thermoplastic materials differ essentially in the way heat is built up on the joint surface. The presently known procedures can be split into two basically different groups: 1. The heat required to reach the melting temperature is supplied by an outside source. This is the case with hot plate welding, induction welding and hot air welding. 2. The necessary heat is generated directly on the joint surfaces by means of friction. The best known methods using this procedure are spinwelding and ultrasonic welding. They have the obvious advantage that the melted resin is never exposed to open air, in this way preventing decomposition or oxidation which, for some plastics, must be avoided. Spinwelding, however, is limited to circular shaped parts which, in addition, do not require positioning. If the two items are to be joined in an exact position relative to each other spinwelding equipment becomes quite costly because there are no simple mechanical means to fulfill this requirement. Vibration welding belongs to the second group since it produces heat by means of friction on the two joint surfaces. Unlike the spinwelding procedure, vibration welding is not limited to circular parts. It can be applied to almost any shape provided that the parts are designed to permit free vibrations within a given amplitude. Definition of Motion Center The center around which two parts vibrate can be located: a) inside the joint area b) outside the joint area 111 c) at an infinite distance, in which case the motion becomes linear Based on this, two distinct variations can be defined: Angular and linear welding. a) Motion center inside the joint area All parts having a perfectly circular weld joint would logically vibrate around their own center as shown in Figure 11.68A. Such parts can be provided with a V-shaped weld joint as described in chapter “Circular Parts.” All parts having a non-circular shape must of course be provided with flat joint surfaces. If the weld area has an irregular shape, as for instance shown in Figure 11.68B, it can still vibrate around an internal center. The latter would, however, be chosen in a place which produces the least possible difference in circumferential velocity. From experimentation it has been found that if the ratio of X/Y exceeds ~1.5, the motion center must be placed outside the joint. Figure 11.68 Weld joint shapes A B X Y X = max. distance to center Y = min. distance to center Parts having a rectangular weld area similar to that shown in Figure 11.69A can also vibrate around their own center provided the above mentioned ratio is not over ~1.5 to 1.0. With a shape like that shown in Figure 11.69B, the motion center would have to be located externally in order to obtain similar weld velocities all over the joint. b) Motion center outside the joint area In cases where the above described conditions are not fulfilled, parts must be placed far enough from the motion center to obtain again a ratio of X/Y
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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
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Figure 4.11 Stress curves The compu
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Rim Design Rim design requirements
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Figure 5.08 Chair Seat Zytel ® 71
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Cantilever Springs Tests were condu
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7—Bearings Bearings and bushings
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debris is moved around continuously
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Table 7.01 Coefficient of Friction*
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Figure 7.15: Connecting rod spheric
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8—Gears Introduction Delrin ® ac
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9—Gear Design Allowable Tooth Ben
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Delrin ® 500. See the section on D
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Figure 9.06 Sizes of gear teeth of
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The following additional examples i
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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 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: Figure 11.64A shows a variation whi
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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