General Design Guidelines
General Design Guidelines
• Plastics are used in a variety of diverse and
demanding applications
• There are design elements that are common to
most plastic parts
–
–
–
–
–
Wall thickness
Ribs
Bosses
Gussets
Draft
General Design Guidelines
• Keep walls as thin as possible
– Thick enough to meet strength requirements
– If too thick part will warp or crack
– Thinner is better
• Use a uniform wall thickness
– Areas where the wall increases in thickness are subject
to warping, cracking and showing sink marks
– Change must be gradual and not exceed 20% of
thickness
General Design Guidelines
• Use ribs and gussets to improve part
stiffness
– They provide a good way to strengthen a part
without making the wall thicker
• Use generous radii at all corners
– Eliminates stress concentration and will make it
easier to remove the part
• Design parts with draft to facilitate removal
Wall Thickness
• What are the considerations for deciding wall
thickness?
– It must be thick and stiff enough for the job. Wall
thickness could be 0.5 to 5mm.
– It must also be thin enough to cool faster, resulting
lower part weight and higher productivity.
– Any variation in wall thickness should be kept as
minimum as possible.
– A plastic part with varying wall thickness will
experience differing cooling rates and different
shrinkage. Where wall thickness variation is essential,
the transition between the two should be gradual.
Wall Thickness
• Solid shape molding is not desired in injection
molding due to following reasons.
– Cooling time is proportional to square of wall
thickness. Large cooling time for solid will defeat the
economy of mass production. (poor conductor of heat)
– Thicker section shrink more than thinner section,
thereby introduce differential shrinkage resulting in
warpage or sink mark etc. (shrinkage characteristics of
plastics and pvT characteristics)
Wall Thickness
• Therefore we have basic rule for plastic part
design; as far as possible wall thickness should
be uniform or constant through out the part.
This wall thickness is called nominal wall
thickness.
• If there is any solid section in the part, it should be
made hollow by introducing core. This should
ensure uniform wall thickness around the core.
Wall Thickness
Wall Thickness
• Core out thick sections
of the part to create a
uniform wall thickness
Wall thickness
• When thickness
changes are necessary
use gradual transitions
Corners
• Corners of the part should be rounded to
reduce the stress concentration at the corner
and make removal easier
• They are the number one cause of part
failure, stress concentration, poor flow
patterns and increased tool wear
Corners
• Corners should always
be designed with a
minimum fillet radius
of 50% of wall
thickness and outer
radius of 150% of
thickness to maintain a
constant wall
thickness
Draft
• Draft is necessary for ejection of parts from the
mold
• Recommended draft angle is 1 degree with ½
degree on ribs
• Draft all surface parallel to the direction of mold
separation
• Use standard one degree of draft plus an additional
one degree of draft for every 0.001 in of texture
depth
Draft Guidelines
Ribs
• Ribs are an economical means to improve
stiffness and strength without increasing
overall wall thickness
• Other uses for ribs
– Locating components of an assembly
– Providing alignment in mating part
– Acting as stops or guides
Ribs
• Proper rib design involves five main issues
–
–
–
–
–
Thickness
Height
Location
Quantity
Moldability
Ribs
• In parts where sink marks
are of no concern, rib base
thickness, t, can be 7585% of wall thickness
• Where sink marks are
objectionable rib base
thickness, t, should not
exceed 50% of wall
thickness if textured
– 30% if not textured
• Multiple ribs should be
twice the wall thickness
apart
Ribs
Gussets
• Gussets are rib like features that add support
to structures such as
– Bosses
– Ribs
– Walls
• Limit gusset thickness to one half to two
thirds of wall thickness to prevent sink
marks
Gussets
• Contour lines show
flow front position at
incremental time
intervals.
• Squared gussets can
trap air in the corners.
Gusset Design
Bosses
• Bosses find use in many part designs as
points for attachments and assembly
• Most common variety consists of
cylindrical projection with holes designed to
receive
– Screws
– Threaded inserts
– Other types of fastening devices
Bosses
• The outside diameter of
bosses should remain 2 to
2.4 times the outside
diameter of the screw or
insert
• To prevent sink marks,
keep the boss wall
thickness to nominal wall
thickness the same as for
ribs
• Bosses should have a
blended radius at the base
Bosses
• Boss Sink Recess
– A recess around the
base of a thick boss
reduces sink.
Holes and Cores
• Cores are the protruding parts of the mold
that forms the inside surface of features
such as holes, pockets and recesses
• Design parts so that cores can separate from
the part in the mold opening direction
– Otherwise you will have to add slides or
hydraulic cores
Holes and Cores
• The depth to diameter ratio of blind holes
should not exceed 3:1
• If the core is supported on both ends the
depth to diameter ratio doubles to 6:1
• Holes will be no closer to each other than 2
times the part thickness or twice the hole
diameter
Molded Threads
• The molding process accommodates thread
forming directly in a part
• External threads centered on the parting line
add little to the molding cost
• Internal threads require unthreading devices
which add to molding costs
Molded Threads
• Common thread
profiles used with
plastics
Molded Threads
• Stop threads short of
the end to avoid
making thin, feathered
threads that can easily
cross thread
• Limit pitch to less than
32 threads per inch for
ease of molding and to
prevent cross
threading