Multi-shot moulding is a process where two or more materials are injected in sequence usually involving a hard / rigid polymer along with a soft elastomeric thermoplastic. The injection moulding machine has two or more barrels for each polymer type and comes in many configurations:
The process has many benefits and can eliminate secondary operations like the fitment of elastomer seals, introduce multicoloured components eliminating secondary painting process and more. For this process to work, it is usually best to work with polymers that chemically attach to each other so that they cannot become separated in application.
Both Crastin® and Hytrel® happen to share chemistries that are extremely compatible, so when it comes to multishot moulding, these two brands can be an excellent combination choice. Crastin® is a PBT and the start point of Hytrel® is actually the same PBT, but modified with ether glycol soft segments to provide the softness and elasticity. Similar chemistry usually leads us to good adhesion for two component moulding.
To prove this two polymer combination, DuPont™ has run a number of moulding trials with Crastin® and Hytrel® using different combinations of grade selection to see which ones have the best adhesion properties. This is done on a two shot moulding machine, moulding a rigid portion as shown below with the soft Hytrel® elastomer overmoulded in the middle. This finished moulding then provides a way of testing the adhesion in a tensile testing machine.
Under the test conditions above, we pull the parts until failure and note the result. Cohesive failure is the preferred result, as this demonstrated that the bond between the Crastin® and Hytrel® has remained intact and it is the elastomer portion that has physically torn itself in two as demonstrated below.
Under the same test conditions mentioned, an adhesive failure is where the adhesion between the elastomer and rigid substrate fails, resulting in complete adhesion loss at the interface. This indicates that the adhesive strength is not as strong as cohesive failure.
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