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