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 75-85% 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