If the part is not used in isolation then consideration must be made of assembly with other plastic or non-plastic parts.  Methods of assembly include adhesive, welding, snap-fit, etc and can be manual or by automated process.  It is possible to combine several features/parts into a single moulding as well as adding threaded or other inserts.  It is also necessary to consider other post-moulding operations such as painting or printing.

For assemblies there are various design features worth considering to cover instances where a mismatch is possible.


The aim of any project is to minimise the number of parts associated with a particular product.  Fewer parts relates not only to lower costs but also easier assembly.  Even eliminating a single screw can result in saving not just the cost of the screw but also the cost of assembly and handling.  It could reduce the need for specialist equipment and scrap levels through damage caused by the incorrect fitting of the screw.

There are a number of assembly techniques:

  • Press Fit
  • Snap Fit
  • Mechanical Fastening
  • Hot Staking
  • Welding
  • Adhesive Bonding
  • Solvent Bonding


Assemblies rely on interference between the parts to keep them joined together.  Annular parts such as gears and pulleys, attached to shafts are a common example.

Although simple there are potential problems.  The degree of interference between the two parts is critical – too little and the joint is loose, too great and assembly becomes difficult and the material overstressed.  Clearly, this type of assembly method is only viable where the associated manufacturing tolerances can be achieved and maintained.

It should also be noted that where different materials are involved then changes in temperature could affect the fit.


This is economical because it is a moulded feature and ideal for recyclability as no metal inserts or adhesives are involved.  All involve the same principle – a protruding feature on one part is deflected briefly on assembly to locate into a recess in the mating part.  This method is not suited to parts requiring repeated assembly operations.  Snap fits can also be damaged due to incorrect handling especially if using a brittle or filled polymer.

There are three main types:

  • Cantilever
  • Cylindrical
  • Spherical


Rutland Plastics produces a free Design Guide to help you with all aspects of injection moulding design.