General Design Principles for DuPont Engineering Polymers - Module

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Figure 3.08 Use of exterior or interior radii Radii on Exterior of Corner Bosses Bosses are used for mounting purposes or to serve as reinforcement around holes. Good and poor design is shown in Figure 3.09. As a rule, the outside diameter of a boss should be 2 to 21 ⁄ 2 times the hole diameter to ensure adequate strength. The same principles used in designing ribs pertain to designing bosses, that is, heavy sections should be avoided to prevent the formation of voids or sink marks and cycle time penalty. Boss design recommendations are shown in Figure 3.10. Figure 3.09 Boss design GOOD Radii on Interior of Corner 8 Figure 3.10 Boss design Ribbing Reinforcing ribs are an effective way to improve the rigidity and strength of molded parts. Proper use can save material and weight, shorten molding cycles and eliminate heavy cross section areas which could cause molding problems. Where sink marks opposite ribs are objectionable, they can be hidden by use of a textured surface or some other suitable interruption in the area of the sink. Ribs should be used only when the designer believes the added structure is essential to the structural performance of the part. The word “essential” must be emphasized, as too often ribs are added as an extra factor of safety, only to find that they produce warpage and stress concentration. It is better to leave any questionable ribs off the drawing. They can easily be added if prototype tests so indicate. Holes and Coring Holes are easily produced in molded parts by core pins which protrude into the mold cavity. Through holes are easier to mold than blind holes, because the core pin can be supported at both ends. Blind holes formed by pins supported at only one end can be offcenter due to deflection of the pin by the flow of molten plastic into the cavity. Therefore, the depth of a blind hole is generally limited to twice the diameter of the core pin. Design recommendations for cored holes are shown in Figure 3.11. To obtain greater hole depth, a stepped core pin may be used or a side wall may be counterbored to reduce the length of an unsupported core pin (see Figure 3.12).
Figure 3.11 Cored holes Figure 3.12 Holes with an axis which runs perpendicular to the mold-opening direction require retractable core pins or split tools. In some designs this can be avoided by placing holes in walls perpendicular to the parting line, using steps or extreme taper in the wall (see Figure 3.13). Core pins should be polished and draft added to improve ejection. Where weld lines caused by flow of melt around core pins is objectionable from strength or appearance standpoint, holes may be spotted or partially cored to facilitate subsequent drilling as shown in Figure 3.14. 9 Figure 3.13 Figure 3.14 Drilled holes
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: As a general rule, use the minimum
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
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Figures 10.07 and 10.08 show calcul
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Undercut Snap-Fits In order to obta
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Cantilever Lug Snap-Fits The second
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11—Assembly Techniques, Category
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Figure 11.03 Drill spindle position
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Inertia Welding By far the simplest
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Machines for Inertia Welding The pr
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This is not always easy, because ap
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The holder a, will have equal and o
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Calculations for Inertia Welding To
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If, for example, the angles of the
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Welding Double Joints The simultane
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Figure 11.37 Commercial bench-type
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Heat is generated throughout the pa
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Figure 11.45 Tapered or stepped hor
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Weld strength is therefore determin
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) General Part Design The influence
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eticule in the eye piece is suitabl
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For this reason, if the strength of
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Figure 11.64A shows a variation whi
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3. Vibrations, generated either by
Page 117 and 118:
If one part is only vibrated at twi
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Another joint design, with flash tr
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Figure 11.82. A linear welded motor
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B) Commercial linear and angular we
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Hot Plate Welding of Zytel ® One o
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Figure 11.94 Applications of riveti
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Tolerances may be difficult to hold
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are used in either hand- or power-o
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ather than scrape, these drills wil
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The polishing operation is performe
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General Design Principles for DuPont Engineering Polymers - Module

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