1. General Design Guidelines

2. 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

3. 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

4. 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

5. 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.

6. 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)

7. 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.

8. Wall Thickness

9. Wall Thickness
Core out thick sections of the part to create a uniform wall thickness

10. Wall thickness
When thickness changes are necessary use gradual transitions

11. 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

12. 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

13. 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 in of texture depth

14. Draft Guidelines

15. 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

16. Ribs Proper rib design involves five main issues Thickness Height
Location
Quantity
Moldability

17. 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

18. Ribs

19. 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

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

21. Gusset Design

22. 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

23. 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

24. Bosses Boss Sink Recess
A recess around the base of a thick boss reduces sink.

26. 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

27. 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

28. 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

29. Molded Threads
Common thread profiles used with plastics

30. 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