Why you see flash when staking plastic and how to optimize it

The moment is here, the process is set, the machine has cycled. How does it look?  “GASP!  My staked part has FLASH!!! If you are not familiar with flash, I’m referring to the displaced plastic material from a staking or welding process that extrudes outwards and does not contribute to the strength of the joint. Flash may be observed as fine dust surrounding the joint, thin strands of plastic material in a disorganized clump, or even smooth rounded material adjacent to the weld joint. So why does flash occur in plastic staking and how can you optimize stake appearance?

During the staking process, the plastic material in the boss or stud is heated and then formed within the tool cavity until the molten material exceeds the volume of the cavity and is squeezed outward around the perimeter of the stake.

In most cases, having a small and uniform amount of flash on the perimeter of a stake is a good characteristic. Don’t view this as a flaw or mistake. First, visible flash indicates that the volume of the forming tool is completely filled and that the components are held tightly together. Second, it indicates the forming tool is centered properly on the boss, which is ideal for aesthetics and optimum strength. Finally, it gives a quick, visual verification that the melt process was sufficient and that the assembly is not at risk of a loose rattle or weak joint between the components.

Despite the benefits, flash is not acceptable in every application. In some cases, loose flash poses a risk of contamination to the final assembly. Do not worry; a stake without flash CAN BE ACHIEVED! The optimized stake joint that is high-strength, holds components tight, and has a clean, flash-free perimeter can become a reality.

Here is how to optimize the stake for no flash.

  • Match the volume of the plastic boss to the appropriate volume of the tool cavity. For smaller boss volumes, the cavity should be 5-10% larger than the boss volume, but for larger bosses, the cavity should be 15-20% larger than the corresponding boss.
  • Center the forming tool over the plastic boss. Alignment between the forming tool and the plastic boss is critical for a flash-free stake because it promotes even heating and symmetrical stake forming.
  • Optimize the staking process parameters to appropriately soften the material, form, and resolidify it. Parameters such as pressure, temperature, amplitude, cooling air, and others can all have an effect on the final appearance of a finished stake.
  • Dry components with high moisture content prior to staking. Some materials absorb moisture and melt well above the boiling point of water. These materials tend to have a near-violent release of water vapor and can expel material as they melt. When staking a material with this characteristic, it is ideal to dry the components prior to staking or to process the assembly very soon after the molding process.

In conclusion, flash in plastic staking is common, expected, and can be a useful indicator of a good stake. However, for those applications that require a flash-free process to mitigate the risk of contamination, strong, tight stakes without flash are possible. Extol has 30+ years of plastic joining experience to guide you through the process setup with confidence and ease. Please contact us to learn how you can leverage our expertise on your next product launch.

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