Issue: Varying closure dimensions and removal torque for the same cap mold depending on the color pigment.
Solution: Nucleating Agent
The color pigment can be a source of variation in dimensions and removal torque values. Color pigment in a plastic is acting as a heat sink. Different colors influence the rate at which plastic is cooling. Pigments affect the closure dimensions differently in PP and HDPE, in large part because they nucleate (crystallize) at different temperatures. Nucleating agents can be used to address these issues by increasing the crystallization rate. This means that the resin will freeze more quickly and enable improvements in the part quality in most conversion processes.
According to an article on www.plasticsnews.com written by David Hutton, “nucleation has traditionally been used to improve physical properties of PP but now it is bringing PE to the next level of performance. Nucleating agents can reduce cycle times, improve warpage and aid dimensional control. Milliken’s Hyperform HPN-20E, a nucleating agent for PE, allows for consistent processing, improved quality and cycle time reduction in the production of caps and closures. An organic salt, HPN-20E nucleates both PP and PE and has gained major food approvals across the globe.”
Should you have any questions about nucleating agents, color pigments or closure torque, please do not hesitate to contact us.
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