DESIGN FOR ASSEMBLY (PART 3)

In this article we shall look at the bonding of plastics.

ADHESIVE BONDING

As with the materials themselves, adhesives can be affected by environmental factors and time.  Unlike other fastening methods it can take time for the adhesively bonded joint to reach full strength.

Advantages:

  • Aesthetic and design flexibility
  • Uniform stress distribution
  • Can join dissimilar materials
  • Can provide tight seal
  • Flexible adhesives can compensate for thermal expansion mismatches between materials
  • Flexible adhesives can dampen vibration
  • Can be used with thin, flexible substrates
  • Provide electrical and thermal insulation

SOLVENT BONDING

This method can only be used with certain types of thermoplastics and is mostly used with amorphous materials.  This is more of a welding process whereby the solvent softens the surfaces of the two parts to be joined; they are then clamped together while the solvent evaporates.

This is a simple relatively inexpensive method but it does require the parts to be joined not to be warped and be moulded to relatively tight tolerances.

JOINT TYPES

Joint designs tend to be variations of two main types – lap and butt.  A lap joint is where two parts to be joined overlap whereas the butt involves the two parts to be joined end-to-end.

Both types of joints have variations that give improved performance.  Lap joints may be stepped or bevelled and butt joints may have some form of strap applied over the joint.  The use of a strap in this manner adds to the cost but can make for a design feature.

COST

Avoid ‘over-engineering’.  It is necessary to consider likely volumes and life expectancy of the product.  A more simple design could be cost effective whereas a more complex one may be uneconomic.

FREE DESIGN GUIDE

Rutland Plastics produces a free Design Guide to help you with all aspects of designing for plastic moulding.