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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.
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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.
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If the flow orientations are strong, the isotropic shrinkages will require adding 0.001 mm/ mm
length to the overall tolerances of the parts.
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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.

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
Design for assemblyand 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.

where:
I = diametrel interference (d – d1), mm
Design for assembly
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:
l   Annular snap-fit joints
l   Cantilever snap joints
l   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.

FIGURE 4. Typical annular snap-fit joint. The assembly force, w, strongly depends on the lead angle, ,
and the 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.
Design for assembly
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 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 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:
where:
$= angle of twist
y1 and y2 = deflections
l1 and l2 = lengths of lever arms (see Figure
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:

r pm = permissible shear strain
e pm = permissible strain
v = Poisson’s ratio (approx. 0.35 for plastics)

FIGURE 8. Torsional snap-fitting arm with torsional bar. Symbols defined in text above.
Fasteners
Screws and rivets, the traditional methods of fastening metal parts, can also be used with plastics.
Several important concerns are:
l   Over-tightening the screw or rivet could result in induced stress.
l   Threads might form or be cut as the screw is inserted.
Design for assembly
Burrs on the screw head or nut or on the head of the rivet could act as stress risers and cause
early failure.
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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.

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
Design for assembly
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 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
l   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.
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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.

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