General Design Guidelines

General Design GuidelinesPlastics are used in a variety of diverse and demanding applicationsThere are design elements that are common to most plastic partsWall thicknessRibsBossesGussetsDraft

General Design GuidelinesKeep walls as thin as possibleThick enough to meet strength requirementsIf too thick part will warp or crackThinner is betterUse a uniform wall thicknessAreas where the wall increases in thickness are subject to warping, cracking and showing sink marksChange must be gradual and not exceed 20% of thickness

General Design GuidelinesUse ribs and gussets to improve part stiffnessThey provide a good way to strengthen a part without making the wall thickerUse generous radii at all cornersEliminates stress concentration and will make it easier to remove the partDesign parts with draft to facilitate removal

Wall ThicknessWhat 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 ThicknessSolid 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 ThicknessTherefore 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 ThicknessCore out thick sections of the part to create a uniform wall thickness

Wall thicknessWhen thickness changes are necessary use gradual transitions

CornersCorners of the part should be rounded to reduce the stress concentration at the corner and make removal easierThey are the number one cause of part failure, stress concentration, poor flow patterns and increased tool wear

CornersCorners 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

DraftDraft is necessary for ejection of parts from the moldRecommended draft angle is 1 degree with degree on ribsDraft all surface parallel to the direction of mold separationUse standard one degree of draft plus an additional one degree of draft for every 0.001 in of texture depth

Draft Guidelines

RibsRibs are an economical means to improve stiffness and strength without increasing overall wall thicknessOther uses for ribsLocating components of an assemblyProviding alignment in mating partActing as stops or guides

RibsProper rib design involves five main issuesThicknessHeightLocationQuantityMoldability

RibsIn parts where sink marks are of no concern, rib base thickness, t, can be 75-85% of wall thicknessWhere sink marks are objectionable rib base thickness, t, should not exceed 50% of wall thickness if textured30% if not texturedMultiple ribs should be twice the wall thickness apart

Ribs

GussetsGussets are rib like features that add support to structures such asBossesRibsWallsLimit gusset thickness to one half to two thirds of wall thickness to prevent sink marks

GussetsContour lines show flow front position at incremental time intervals.Squared gussets can trap air in the corners.

Gusset Design

BossesBosses find use in many part designs as points for attachments and assemblyMost common variety consists of cylindrical projection with holes designed to receiveScrewsThreaded insertsOther types of fastening devices

BossesThe outside diameter of bosses should remain 2 to 2.4 times the outside diameter of the screw or insertTo prevent sink marks, keep the boss wall thickness to nominal wall thickness the same as for ribsBosses should have a blended radius at the base

BossesBoss Sink RecessA recess around the base of a thick boss reduces sink.

Holes and CoresCores are the protruding parts of the mold that forms the inside surface of features such as holes, pockets and recessesDesign parts so that cores can separate from the part in the mold opening directionOtherwise you will have to add slides or hydraulic cores

Holes and CoresThe depth to diameter ratio of blind holes should not exceed 3:1If the core is supported on both ends the depth to diameter ratio doubles to 6:1Holes will be no closer to each other than 2 times the part thickness or twice the hole diameter

Molded ThreadsThe molding process accommodates thread forming directly in a partExternal threads centered on the parting line add little to the molding costInternal threads require unthreading devices which add to molding costs

Molded ThreadsCommon thread profiles used with plastics

Molded ThreadsStop threads short of the end to avoid making thin, feathered threads that can easily cross threadLimit pitch to less than 32 threads per inch for ease of molding and to prevent cross threading