How to Design Snap Fit Joints in Plastic Parts

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Snap fit joints is one of the best and cost effective way to assemble plastic parts. Most of plastic products in the market are without screws. They are joined using snap fit joints. Therefore screw and assembly cost is saved. In this article we will discuss how to design snap fit joints for injection molded plastic parts.

What is Snap Fit Joint?

A snap fit joint consist of a protrusion in flex plastic wall. When force is applied, Flex part got deflected and goes inside the depression during assembly with mating part.

Advantages of Snap Fit Joints
  • Reduced Number of Parts.
  • Reduced Assembly Cost.
  • Simple and Reliable.
  • Temporary Joints.

Types of Snap Fit Joints

Depending on joint structure, force required for assembly snap fit joints can be classified in following types.

  • Cantilever Snap Joints
  • Annular Snap Joint
  • Torsion Snap Joints

Cantilever Snap Fit Joint

Most of snap joints used to join plastic parts are cantilever snap joints. One end of cantilever snap joint is fixed. Whereas other end is protruded out. They works similar to cantilever beam.

When force is applied joint, flexible length deflects and catches in depression of mating part. 

When force is applied joint, flexible length deflects and catches in depression of mating part. 

During deflection maximum stress acts at the root of the joint. This Stress can be calculated using classical beam bending theory.

Cantilever Snap Fit Joint Design

For smooth assembly and disassembly of cantilever snap, entrance angle is provided at the entry point of the snap. If snap joint is supposed to be disassemble as well. Exit angle is provided at snap resting face.

Entry and exit angle is provided in cantilever snap for easy assembly and disassembly of parts.

If entry and exit angle is constant. Same amount of force will be required to assemble and disassemble snap joint. Generally entry angle value is kept less than exit angle to ensure more force is required to disassemble parts. Exit angle value is kept 90° for one time assembly snap joints.

Maximum snap deflection is equal to the maximum provided overlap. 

Cantilever Snap Fit Joint Calculations

As discussed before, Cantilever snap design works similar to cantilever beam with a point-applied load. During snap design, deflection and strain in cantilever beam is calculated instead of stress acting on material. From strain values force required for snap assembly and disassembly is calculated.

Maximum Possible Deflection in Snap wall can be calculated using beam deflection formula:

Let’s consider a snap of length “L” with Cross section Width “b” and thickness “H”. During assembly when a force “P” is applied, It deflects by a distance “Y”

Maximum Possible Deflection in Snap wall can be calculated using beam deflection formula:

Where “ε” is the maximum permissible strain for the plastic material. 

Snap Mating Force

Snap mating force is the force required to engage snap joint. Mathematically it is equal to the multiple of Deflection Force and Frictional Force.

Snap Mating Force = Deflection Force(P) X Frictional Force

Snap deflection force is multiple of snap width, square of snap thickness, maximum permissible strain, material flexural modulus and divided by six times of snap length.

Where “E” is the Flexural / Bending Modulus of the material.

Where ∝ is Entry angle and μ is the coefficient of friction between materials. 

Permissible Strain Values

Maximum permissible strain value is a property of a material. It can be calculated if value of young modulus of elasticity and max permissible stress values are available. This value is also available in material datasheet. Read this article for more details on strain in mechanical.

Maximum Permissible Strain Values For Reference
Material Acceptable Strain
Polycarbonate (PC) ~ 4 to 8 %
ABS + PC ~ 4 %
Glass Filled PC ~ 2 to 2.2 %
ABS ~ 6 to 7 %
How to Increase Deflection in Cantilever Snap Joints?

Deflection in cantilever joints can be increased by changing plastic material or by modifying snap design.

For example, Deflection can be increased by tapering the snap profile in width or thickness from root to the hook. As shown in image, snap thickness is reduced as it moves from root to the the hook.

From simple beam deflection, maximum snap deflection in tapered beam in thickness can be given by:

Annular Snap Joint

This joint is used to join symmetrical round joint. Depending on annular joint geometry, Joint can be removable, permanent or difficult to remove. Complex plastic tooling is required to make difficult to remove annular joints.

Common Problems in Snap Fit Joints

Stress Concentration

To reduce stress concentration radius is provided at the root of the snap joint.

During assembly maximum stress acts at the root of the snap joint. To reduce stress concentration radius is provided at the root of the snap joint.

Stress Relaxation

During snap design, We need to ensure after assembly there is always a clearance between assembled parts. It ensure snap is always in relaxation state.

During snap design, We need to ensure after assembly there is always a clearance between assembled parts. It ensure snap is always in relaxation state.

Conclusion

To sum up, Snap joints are one of the best way to join two or more plastic parts. It helps in reducing overall part assembly cost. 

Got Questions?  We will be happy to help.

If you think we missed Something?  You can add to this article by sending message in the comment box. We will do our best to add it in this post.


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