Design Guidelines

Design Guidelines


1. Use of uniform wall thickness throughout the part.

This will minimize sinking, warping, residual stresses, and improve mold fill and cycle times.

2. Use generous radius at all corners.
        
The inside corner radius should be a minimum of one material thickness.
        
3. Use the least thickness compliant with the process, material, or product design requirements.

Using the least wall thickness for the process ensures rapid cooling, short cycle times and minimum shot weight. All these result in the least possible part cost.

4. Use ribs or gussets to improve part stiffness in bending.

This avoids the use of thick section to achieve the same, thereby saving on part weight, material costs, and cycle time costs.

5. Design parts to facilitate easy withdrawal from the mold by providing draft in the direction of mold opening or closing.


Wall Thickness


The typical plastic part may be considered to have a shell type configuration with a basic surface and features which are attached to it to meet functional requirements.

The actual determination of the wall thickness is based on a number of considerations.


1. Application Requirements

Structural requirements including strength, impact, fatigue or deflection.

2. Moldability 

The size of the part and the ability of the material to fill the furthest point can determine the minimum wall.


Influence of wall thickness

  • Part characteristics 
  • Mechanical performance
  • Cosmetic appearance
  • Mouldability 
  • Economy

Giving wall thickness should be carefully considered in the design stage to avoid expansive mold modifications and molding problems in productions

The optimum thickness is often a balance between opposing tendencies, like:

Strength Vs Weight

Durability Vs Cost




Avoid designs with thin areas surrounded by thick perimeter sections as they are prone to gas entrapment problems














Maintain uniform nominal wall thickness throughout the part








Avoid sudden wall thickness variation that result in filling from thin to thick sections.





Core or redesign thick areas to create a more uniform wall thickness 

 




Wall thickness design for Stiffness:


Corrugation

Corrugation can add stiffness to non cosmetic parts







Curved slide walls

Adding curvature to the sidewalls enhances stiffness and appereance







Wall Thickness Design for Stiffness:



Flexible




Stiffer





Radius

Sharp corners greatly increases the stress concentration. This high amount of stress concentration can often lead to failure of plastic parts.



High molded in stresses
Poor flow characteristics
Reduced mechanical properties 
Increased tool wear 
Surface appearance problems, (especially with blends).







Uniform cooling 
Less warpage 
Less flow resistance
Easier filling
Lower stress concentration
Less notch sensitivity.






What radius should you give?

Standard tables for stress concentration factors are available and should be consulted for critical applications.



As can be seen from the above chart, the stress concentration factor is quite high for R/T values less than 0.5 . For values of R/T over 0.5 the stress concentration factor gets lower.


Radius should be 

  • Radius should be between 50% of the nominal wall thickness.
  • If the part has a load bearing function then the upper end is recommended.
  • A minimum radius of 0.5 mm is suggested and all sharp corners should be broken with at least a 0.125 mm radius.



Internal and external radii should originate from the same point 






Ribs 

Ribs increase the bending stiffness of a part . Without ribs, the thickness has to be increased to increase the bending stiffness. Adding ribs increase the moment of inertia, which increase the bending stiffness.



Function of Ribs

  1. The rib gives stiffness and strength in molded part without increasing overall wall thickness.
  2. Locating and captivating components of an assembly.
  3. Providing alignment in matting parts.
  4. Acting as stops or guide for mechanisms.








Rib design issues

  • Thickness
  • Height
  • Location
  • Quantity
  • Moldability

Consider these issues carefully when designing ribs 














Ribs thickness

Many factors go into determining the appropriate rib thickness. Because thick ribs often cause sink and cosmetic problems on the surface of the wall.






If rib thickness is a not a constraint but cosmetic

  1. The rib thickness should be less than the wall thickness
  2. The thickness ranges from 40 to 60% of the material thickness as per 66% rule 
  3. The rib should  be attached to the base with 0.125 mm thickness radius at the corners and 0.5 degree draft should be given for ejection.












What is the 66% rule? 


The divine 66% rule for ribs

The thickness of ribs should never exceed 66% rule of the nominal wall thickness.If your ribs never exceed 50-66% of nominal wall thickness you will never have a problem with sink.



Sometimes you can get away with 66% to 75% of nominal wall, but it is risky . Do not it unless you absolutely have to. If you do, be certain that the area gets better than average plastic flow.



What happens if you ignore the 66% rule?






Rib height 

Maximum rib height should not exceed 3 times the nominal wall thickness as deep ribs become difficult to fill and may stick in the mold during ejection.



Rib location

The rib location based on providing maximum bending stiffness. Depending on orientation of the bending load, with respect to the part geometry, ribs oriented one way increase stiffness. If oriented the wrong way there is no increase in stiffness.




Rib mouldability

Ribs are preferably designed parallel to the melt flow as flow across ribs can result in a branched flow leading to trapped gas or hesitation. Hesitation can increase internal stresses and short shots.

Position ribs in
the line of flow to
improve filling
and prevent air
entrapment





Boss design standards


The boss height should be 3 times of the wall thickness.





Bosses located at corners can result in very thick walls causing sinks. Bosses can be isolated using the techniques illustrated.





Strengthening a Boss


 The boss can be strengthened by attaching it to nearby walls with connecting ribs.



Gusset design

Avoid sharp corners in your gusset design.




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