General Design Principles for DuPont Engineering Polymers - Module

More documents

Recommendations

Info

In all plastics, glass fibers or fillers reduce tensile elongation, so that stress concentrations are very harmful. Designers pay far too little attention to this fact. Occasionally one is also faced with the problem of joining plastics of different types, with different melting points. The greater the difference between the melting points, the more difficult welding will be, and one cannot call such a joint a true weld, as it is merely a mechanical adhesion of the surfaces. The strength of the joint will be low. It may even be necessary to have special joint profiles and work with very high weld pressures. In practice there are very few such applications, and in all these cases the parts are not subjected to stresses. Typical applications are oil-level gauges and transparent polycarbonate spy-holes welded into holders of Delrin ® . The following test results should give some idea of the possibilities of joining Delrin ® to other plastics. The float of Delrin ® shown in Figure 11.13 has a burst pressure of about 4 MPa. If a cap of some other material is welded onto a body of Delrin ® , the burst pressures are as follows: Zytel ® 101 (nylon resin) 0.15–0.7 MPa Polycarbonate 1.2–1.9 MPa Acrylic resin 2.2–2.4 MPa ABS 1.2–1.6 MPa It must be remembered that, in all these cases, the weld forms the weakest point. Spin Welding Soft Plastics and Elastomers The softer the plastic, with a few exceptions (e.g., fluoropolymers), the higher the coefficient of friction. Spin welding therefore becomes increasingly difficult with soft plastics, for the following three reasons: • The deceleration produced by a high coefficient of friction is so great that the flyweight is unable to produce heat by friction. Much of the energy is absorbed in the deformation of the component, without any relative motion occurring between the joint faces. If the amount of kinetic energy is increased, one is more likely to damage the parts than to improve welding conditions. It is sometimes possible to solve this problem by spraying a lubricant onto the joint faces (e.g. a silicone mold release). This reduces the coefficient of friction very considerably at first, so that the usual rotation takes place. The specific pressure is, however, so high that the lubricant is rapidly squeezed out, the friction increases, and the material melts. 94 • For soft plastics having a very low coefficient of friction a very much higher specific pressure is needed to produce sufficient heat by friction in a short time. Most components cannot stand such a high axial pressure without being permanently deformed, and there is to date no reliable way of making satisfactory joints between these materials by spin welding. • Soft plastic parts are difficult to retain and cannot easily be driven. Transmission of the high torque frequently poses an insoluble problem, particularly since it is scarcely possible to use tooth crowns. To sum up, it can be said that marginal cases of this sort should be approached only with extreme caution, and that preliminary experimental work is unavoidable. Figures 10.36–10.38 show only a few selected examples out of the great number of possibilities in this field. Examples of Commercial and Experimental Spin Welding Machines Figure 11.36 Commercial spinwelding machine. This machine is driven by a compressed air motor which allows the speed to be adjusted within broad limits. The specific model shown in the photograph is equipped with an 8-position turntable and jigs for welding a spherical container.
Figure 11.37 Commercial bench-type spinwelding machine. The basic model is equipped with a 3-phase squirrel cage motor. The rotating head with the jigs is fixed directly onto the double guided piston rod as shown in Figures 11.12 and 11.13. The machine an also be supplied with adjustable speed, turntable, automatic cycle control and feeding device. Figure 11.38 Universal inertia spinwelding machine, see also Figure 11.13 for welding parts from 15–80 mm. By the addition of a turntable and an automatic cycle control device the unit can be made semi-automatic without involving too much expense. Even with manual feeding of the turntable (two parts simultaneously), a remarkably short total cycle time of 2 sec can be easily acheived. 95 Ultrasonic Welding Introduction Ultrasonic welding is a rapid and economical technique for joining plastic parts. It is an excellent technique for assembly of mass produced, high quality products in plastic materials. Ultrasonic welding is a relatively new technique. It is used with ease with amorphous plastics like polystyrene which have a low softening temperature. Design and assembly, however, require more planning and control when welding amorphous plastics with higher softening temperatures, crystalline plastics and plastics of low stiffness. This report presents the basic theory and guidelines for ultrasonic welding of parts of DuPont engineering plastics. Ultrasonic Welding Process In ultrasonic welding, high frequency vibrations are applied to two parts or layers of material by a vibrating tool, commonly called a “welding horn.” Welding occurs as the result of heat generated at the interface between the parts or surfaces. Equipment required for ultrasonic welding includes a fixture for holding the parts, a welding horn, an electromechanical transducer to drive the horn, a high frequency power supply and a cycle timer. The equipment diagrammed in Figure 11.39 is described in detail later. Typical ultrasonic welding machines currently available are shown in Figure 11.40. Figure 11.39 Components of ultrasonic welding equipment Transducer or converter Welding horn Plastic parts Holding fixture Power supply Cycle timer
Page 1 and 2:
dDuPont Engineering Polymers Genera
Page 3 and 4:
8—Gears . . . . . . . . . . . . .
Page 5 and 6:
1—General Introduction This secti
Page 7 and 8:
Prototyping the Design In order to
Page 9 and 10:
2—Injection Molding The Process a
Page 11 and 12:
As a general rule, use the minimum
Page 13 and 14:
Figure 3.11 Cored holes Figure 3.12
Page 15 and 16:
Figure 3.19 Mold-ejection of rounde
Page 17 and 18:
• Zytel ® nylon resin—Parts of
Page 19 and 20:
4—Structural Design Short Term Lo
Page 21 and 22:
Form of section Area A d d R R 1 1
Page 23 and 24:
Table 4.03. Formulas for Stresses a
Page 25 and 26:
Form of bar; manner of loading and
Page 27 and 28:
Figure 4.01 Creep Stress (S), MPa (
Page 29 and 30:
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 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: Welding Double Joints The simultane
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
show all

General Design Principles for DuPont Engineering Polymers - Module

Create successful ePaper yourself

Delete template?

Save as template?