Material Behavior - Hoop Stress and Creep

Posted by Al Elger on Jul 20, 2012 2:00:00 PM

blank.pngInsert molding is a type of overmolding where a hard substrate component or “insert” is placed inside a mold cavity in an injection molding machine and then “overshot” with an exterior layer—typically a thermoplastic elastomer (TPE). The chemical reaction between the insert and the TPE must be fully understood to create the strongest possible bond. The surface of the insert should also be free of contamination, including dust or even skin oil—even the slightest contamination can weaken the bond between the TPE and the substrate, leading to premature failure.  

Inserts are typically hard plastic or metal. Metal inserts should be pre-heated before placing into an injection mold because:

Pre-heating minimizes the quenching effect when the plastic encounters the very thermally conductive metal, which will draw heat from the plastic, increasing its viscosity and interfering with the flow/attachment to the insert

  • The metal inserts are pre-expanded by the pre-heat and shrink with the plastic during cooling, which minimizes molded-in stress levels

  • Knit line strength is improved since the plastic is not quenched

  • Engineering “homework” must be done up front (during part design) to calculate the expected hoop stress due to plastic shrinkage.

What is hoop stress?

Imagine a wooden barrel with metal hoops around the outside. These hoops are added to prevent the barrel from bulging outward. As pressure in the barrel increases, the hoops are stretched circumferentially. Likewise, the melted plastic surrounding a metal insert will shrink as it cools, producing a “hoop” stress in the plastic. This hoop stress will be a constant stress throughout the life of the product and possibly result in cracking if the stress is not minimized during the production process.

If plastic is loaded with a constant force/stress it will stretch, or creep, over time. The larger the force/stress, the faster the stretch/creep will occur. Over a prolonged period of time the plastic molecules simply cannot sustain the load and will rupture. Creep rupture charts are created—based on loading and time to rupture—so that a failure points can be predicted. These are also calculated at various temperatures since plastics behave differently at different temperatures. A creep rupture chart should be always consulted to determine the maximum strain/shrink rate, followed by the design of an insert-surrounding plastic hub that will control hoop stress and creep.