4/18/2017
`
`Design for assembly
`
`Design for assembly
`Molding one part vs. separate components
`
`A major advantage of molding plastics parts is that you can now mold what were previously
`several parts into one part. These include many of the functional components and many of the
`fasteners needed to assemble the molded part to other parts. However, due to the limitations of
`the mold and the process, functional requirements, and/or economic considerations, it is still
`sometimes necessary to mold various components separately and then assemble them together.
`Tolerances: fit between parts
`Punched and machined parts can be made to tighter tolerances than molded parts because the large
`shrinkage from the melt to the solid state make sizing less predictable. In many cases, the solidification is
`not isotropic, so that a single value of mold shrinkage does not adequately describe the final dimensions of
`the parts.
`
`Fit between plastics parts
`
`If the two plastics parts are made of the same material, refer to the tolerance capability chart supplied
`by the material supplier.
`If the two parts are of different material families or from different suppliers, add 0.001 mm/ mm of
`length to the tolerances from the supplier's tolerance capability charts.
`If the flow orientations are strong, the isotropic shrinkages will require adding 0.001 mm/ mm length
`to the overall tolerances of the parts.
`Add steps, off-sets, or ribs at the joint line of the two parts to act as interrupted tongue-and-groove
`elements to provide alignment of the two parts and ease the tolerance problem on long dimensions
`(see Figure 1).
`
`FIGURE 1. Matching half-tongue and groove align the two parts edges, within normal tolerances.
`
`Fit between plastics parts and metal parts
`Make sure that the joint between the plastic and metal allows the plastic part to expand without regard to the
`expansion of the metal part.
`
`http://www.dc.engr.scu.edu/cmdoc/dg_doc/develop/design/part/33000004.htm
`
`1/9
`
`AMT Exhibit 2003
`CORPAK v. AMT IPR2017-01990
`Page 1 of 9
`
`
`
`Design for assembly
`
`Design for assembly
`Molding one part vs. separate components
`
`A major advantage of molding plastics parts is that you can now mold what were previously
`several parts into one part. These include many of the functional components and many of the
`fasteners needed to assemble the molded part to other parts. However, due to the limitations of
`the mold and the process, functional requirements, and/or economic considerations, it is still
`sometimes necessary to mold various components separately and then assemble them together.
`Tolerances: fit between parts
`Punched and machined parts can be made to tighter tolerances than molded parts because the large
`shrinkage from the melt to the solid state make sizing less predictable. In many cases, the solidification is
`not isotropic, so that a single value of mold shrinkage does not adequately describe the final dimensions of
`the parts.
`
`Fit between plastics parts
`
`If the two plastics parts are made of the same material, refer to the tolerance capability chart supplied
`by the material supplier.
`If the two parts are of different material families or from different suppliers, add 0.001 mm/ mm of
`length to the tolerances from the supplier's tolerance capability charts.
`If the flow orientations are strong, the isotropic shrinkages will require adding 0.001 mm/ mm length
`to the overall tolerances of the parts.
`Add steps, off-sets, or ribs at the joint line of the two parts to act as interrupted tongue-and-groove
`elements to provide alignment of the two parts and ease the tolerance problem on long dimensions
`(see Figure 1).
`
`FIGURE 1. Matching half-tongue and groove align the two parts edges, within normal tolerances.
`
`Fit between plastics parts and metal parts
`Make sure that the joint between the plastic and metal allows the plastic part to expand without regard to the
`expansion of the metal part.
`
`http://www.dc.engr.scu.edu/cmdoc/dg_doc/develop/design/part/33000004.htm
`
`1/9
`
`AMT Exhibit 2003
`CORPAK v. AMT IPR2017-01990
`Page 1 of 9
`
`
`
`4/18/2017
`
`Design for assembly
`
`
`FIGURE 2. Design the joint between plastic and metal to allow for greater thermal expansion and contraction of the
`plastic. This includes use of shouldered fasteners and clearance between the fastener and the plastic.
`Press-fit joints
`Simple interference fits can be used to hold parts together. The most common press-fit joint is a metal shaft
`pressed into a plastics hub. A design chart recommended by the resin suppliers or interference formula can
`be used to design a press-fit joint at a desirable stress, so the parts will not crack because of excessive stress
`or loosen because of stress relaxation.
`
`Interference chart
`Figure 3 plots the maximum interference limits as a percentage of the insert shaft diameter. Note that this
`chart is material specific and the maximum interference limit depends on the shaft material and the diameter
`ratio of the hub and insert. The recommended minimum length of interference is twice the insert diameter.
`
`
`
`FIGURE 3. Maximum interference limits, pressing a metal shaft into a plastics hub. These curves are specific to the
`material. The max. interference limit (d - d1) as a percentage of the insert diameter, d, depends on the shaft material
`and the diameter ratio of the hub and insert (D/d). The recommended minimum length of interference is twice the insert
`diameter, 2d.
`
`Interference formula
`If the relevant design chart is not available, the allowable interference (difference between the diameter of
`the insert shaft, d, and the inner diameter of the hub, d1, see Figure 3) can be calculated with the following
`formula.
`
`http://www.dc.engr.scu.edu/cmdoc/dg_doc/develop/design/part/33000004.htm
`
`2/9
`
`AMT Exhibit 2003
`CORPAK v. AMT IPR2017-01990
`Page 2 of 9
`
`
`
`4/18/2017
`
`Design for assembly
`
`
`where:
`
`
`I = diametrel interference (d - d1), mm
`Sd = design stress, MPA
`D = outside diameter of hub, mm
`d = diameter of insert shaft, mm
`Eh = tensile modulus of elasticity of hub, MPa
`Es = modulus of elasticity of shaft, MPa
`h = Poisson's ratio of hub material
`s = Poisson's ratio of shaft material
`W = geometry factor
`
`Tolerance
`Check that tolerance build-up does not cause over-stress during and after assembly and that the fit is still
`adequate after assembly.
`
`Mating metal and plastic parts
`Do not design taper fits between metal and plastics parts, because stress cracking will occur from over-
`tightening.
`Snap-fit joints
`Snap-fit joints rely on the ability of a plastics part to be deformed, within the proportional limit, and
`returned to its original shape when assembly is complete. As the engagement of the parts continues, an
`undercut relieves the interference. At full engagement, there is no stress on either half of the joint. The
`maximum interference during assembly should not exceed the proportional limit. After assembly, the load
`on the components should only be sufficient to maintain the engagement of the parts.
`
`Snap-fit joint designs include:
`
`Annular snap-fit joints
`Cantilever snap joints
`Torsion snap-fit joints
`Annular snap-fit joints
`This is a convenient form of joint for axis-symmetrical parts. You can design the joint to be either
`detachable, difficult to disassemble, or inseparable, depending on the dimension of the insert and the return
`angle.
`
`http://www.dc.engr.scu.edu/cmdoc/dg_doc/develop/design/part/33000004.htm
`
`3/9
`
`AMT Exhibit 2003
`CORPAK v. AMT IPR2017-01990
`Page 3 of 9
`
`
`
`4/18/2017
`
`Design for assembly
`
`
`, and the
`FIGURE 4. Typical annular snap-fit joint. The assembly force, w, strongly depends on the lead angle,
`undercut, y, half of which is on each side of the shaft. The diameter and thickness of the hub are d and t, respectively.
`
`Hoop stress
`Figure 5 demonstrates that the outer member (assumed to be plastic) must expand to allow the rigid (usually
`metal) shaft to be inserted. The design should not cause the hoop stress,
`, to exceed the proportional limit
`of the material.
`
`
`
`FIGURE 5. Stress distribution during the joining process.
`
`Permissible deformation (undercut)
`The permissible deformation (or permissible undercut, y, shown in Figure 4) should not be exceeded during
`the ejection of the part from the mold or during the joining operation.
`
` Maximum permissible strain
`The maximum permissible deformation is limited by the maximum permissible strain, pm and the hub
`diameter, d. The formula below is based on the assumption that one of the mating parts is rigid. If both
`components are equally flexible, the strain is half, i.e., the undercut can be twice as large.
`
`y = cpm x d
`
` Interference ring
`If the interference rings are formed on the mold core, the undercuts must have smooth radii and shallow
`lead angles to allow ejection without destroying the interference rings. The stress on the interference rings
`(see the equation above) during ejection must be within the proportional limit of the material at the ejection
`temperature. The strength at the elevated temperature expected at ejection should be used.
`Cantilever snap joints
`This is the most widely used type of snap-fit joint. Typically, a hook is deflected as it is inserted into a hole
`or past a latch plate. As the hook passes the edge of the hole, the cantilever beam returns to its original
`http://www.dc.engr.scu.edu/cmdoc/dg_doc/develop/design/part/33000004.htm
`
`4/9
`
`AMT Exhibit 2003
`CORPAK v. AMT IPR2017-01990
`Page 4 of 9
`
`
`
`Design for assembly
`4/18/2017
`shape. The beam should be tapered from the tip to the base, to more evenly distribute the stress along the
`length of the beam.
`
`
`
`FIGURE 6. Typical cantilever snap-fit joint. The interference between the hole and the hook, y, represents the deflection
`of the beam as the hook is inserted into the hole.
`
`Proportional limit
`Assembly stress should not exceed the proportional limit of the material.
`
`Designing the hook
`Either the width or thickness can be tapered (see Figure 6). Try reducing the thickness linearly from the base
`to the tip; the thickness at the hook end can be half the thickness at its base. Core pins through the base can
`be used to form the inside face of the hook. This will leave a hole in the base, but tooling will be simpler
`and engagement of the hook will be more positive
`
`Designing the base
`Include a generous radius on all sides of the base to prevent stress concentration.
`
`
`
`FIGURE 7. Design the snap-fit features for ejection.
`Torsion snap-fit joints
`In these joints, the deflection is not the result of a flexural load as with cantilever snaps, but is due to a
`torsional deformation of the fulcrum. The torsion bar (see Figure 8) is subject to shear loads. This type of
`fastener is good for frequent assembly and disassembly.
`
`Design formula
`The following relationship exists between the total angle of twist
`
`and the deflections y1 or y2:
`
`http://www.dc.engr.scu.edu/cmdoc/dg_doc/develop/design/part/33000004.htm
`
`5/9
`
`AMT Exhibit 2003
`CORPAK v. AMT IPR2017-01990
`Page 5 of 9
`
`
`
`4/18/2017
`
`Design for assembly
`
`
`where:
`
`
`= angle of twist
`y1 and y2 = deflections
`l1 and l2 = lengths of lever arms (see Figure 8)
`
`The maximum permissible angle
`
`pm is limited by the permissible shear strain pm :
`
`
`
`where:
`
`
`pm= permissible total angle of twist in degrees
`pm= permissible shear strain
`l = length of torsion bar
`r = radius of torsion bar
`
`The maximum permissible shear strain pm for plastics is approximately equal to:
`
`where:
`
`
`pm = permissible shear strain
`pm = permissible strain
`= Poisson's ratio (approx. 0.35 for plastics)
`
`
`
`
`
`FIGURE 8. Torsional snap-fitting arm with torsional bar. Symbols defined in text above.
`
`http://www.dc.engr.scu.edu/cmdoc/dg_doc/develop/design/part/33000004.htm
`
`6/9
`
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`CORPAK v. AMT IPR2017-01990
`Page 6 of 9
`
`
`
`4/18/2017
`
`Design for assembly
`
`Fasteners
`Screws and rivets, the traditional methods of fastening metal parts, can also be used with plastics. Several
`important concerns are:
`
`Over-tightening the screw or rivet could result in induced stress.
`Threads might form or be cut as the screw is inserted.
`Burrs on the screw head or nut or on the head of the rivet could act as stress risers and cause early
`failure.
`
`Screws and rivets
`Use smooth pan-head screws with generous pads for the head. Washers under the screw or rivet head should
`be burr-free or the punch-face should be against the plastic (die-face will have burrs from the stamping
`process). Figure 9 provides recommendations for the diameter of clearance holes for various screw sizes.
`
`
`
`FIGURE 9. Recommendations for clearance between the machine screw and hole in the plastic. The pan-head style of
`the screw is recommended.
`
`Use
`Thread-forming screws: ASA
`Type BF
`Thread-cutting screws: ASA
`Type T, (Type 23) or Type BT
`(Type 25).
`A metal, threaded cap with one
`screw thread on the boss.
`
`Counter-bore hole with pan-head
`screw
`Rivets to join plastic parts for a
`permanent assembly
`
`
`
`If
`The modulus of the plastics is less than 200,000 psi
`
`The modulus is greater than 200,000 psi, since thread-forming screws
`can cause stress cracking in this case
`
`The screw is to be removed and replaced many times. This will assure
`that later insertions do not cut or form a new thread, and destroy the old
`one.
`The screw head must be below the surface of the part.
`
`The design prevents over-tightening of the joint or washers are used to
`prevent the head from cutting into the plastic.
`
`
`Do not use
`Countersunk
`screw heads
`Pipe threads
`
`Since
`They are easily over-tightened and cause stress-cracking.
`
`
`
`The tapered nature of this thread style can allow the joint to be easily over-tightened
`
`http://www.dc.engr.scu.edu/cmdoc/dg_doc/develop/design/part/33000004.htm
`
`7/9
`
`AMT Exhibit 2003
`CORPAK v. AMT IPR2017-01990
`Page 7 of 9
`
`
`
`4/18/2017
`
`Design for assembly
`and over-stressed. Stress-cracking will result
`
`
`Molded threads
`Molding threads into the plastic component avoids having to use separate fasteners such as screws and
`rivets. If the threads are molded, tool-making will be easier if you provide a lead-in diameter slightly larger
`than the main diameter and about one screw flight long. Figure 10 shows how to design an unthreaded lead-
`in.
`
`
`
`FIGURE 10. Recommended design for molded threads.
`
`Below are some guidelines to designing molded threads:
`
` Thread size
`Threads should be strong enough to meet the expected loads. Threads that are too small, especially if they're
`mated with metal threads, tend to become deformed and lose their holding power.
`
` Inside radius of the thread
`The thread design should avoid sharp inside radii. The corollary is that the peak of the thread should also be
`rounded to ease tool making.
`
` Orienting threads to the parting line
`If the axis of the thread is parallel to the mold parting line, half of the diameter can be molded in each mold
`half. You can reduce the effects of the parting line mismatch by partially flattening the threads at that point.
`Retractable mold components must be used if the axis of the threads is not parallel to the parting line.
`
` Demolding the threads
`Internal threads usually require un-screwing the mold component from the part, either manually or by action
`of the mold. Large internal threads can be formed on collapsing mold components.
`Inserts
`An insert is a part that is inserted into the cavity and molded into the plastic. The insert can be any material
`that will not melt when the plastic is introduced into the cavity. Metal inserts are used for electrical
`
`http://www.dc.engr.scu.edu/cmdoc/dg_doc/develop/design/part/33000004.htm
`
`8/9
`
`AMT Exhibit 2003
`CORPAK v. AMT IPR2017-01990
`Page 8 of 9
`
`
`
`Design for assembly
`4/18/2017
`conductivity, to reinforce the plastic, and to provide metal threads for assembly. Plastics inserts can provide
`a different color or different properties to the combinations.
`
`Balancing melt flow
`Place the gate so that equal melt flow forces are placed on opposing sides of the insert. This will keep the
`insert from moving or deforming during mold filling. Design adequate flow paths so that the melt front
`proceeds at the same rate on either side of the insert.
`
`Support posts
`Design support posts into the mold (these will be holes in the part) to support the insert.
`
`Shrinkage and weld lines
`Allow for shrinkage stress and for the weld line that will typically form on one side of the boss around the
`insert.
`Welding processes
`Ultrasonic welding uses high-frequency sound vibrations to cause two plastics parts to slide against each
`other. The high-speed, short-stroke sliding between the two surfaces causes melting at the interface. When
`the vibrations are stopped, the melted interface cools, bonding the two surfaces. Other welding processes are
`generally not reliable or involve considerable hand work.
`
`Design rules for welding
`
`The two materials must be melt compatible.
`The design of the ultrasonic horn that transfers energy to one of the plastics parts is important to
`success.
`Design axis-symmetrical parts with an interference at the joint. This is melted and the parts are forced
`together.
`The design of the contact surfaces is critical to success. You'll need to design an energy director, a
`small triangular raised bead, on one of the faces to be welded.
`
`
`
`FIGURE 11. Recommendations for the design of ultrasonic welded joints.
`
`http://www.dc.engr.scu.edu/cmdoc/dg_doc/develop/design/part/33000004.htm
`
`9/9
`
`AMT Exhibit 2003
`CORPAK v. AMT IPR2017-01990
`Page 9 of 9
`
`
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