general design guidelines. plastics are used in a variety of diverse and demanding applications...

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


• 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


• 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

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