How Surface Finish Can Improve Manufacturability

By Meghan DeChello, Applications Engineer

Most of us would assume that a higher polish in an injection mold would improve part ejection. After all, it makes perfect sense. A smooth surface should allow the plastic part to slide right out of the mold. Right?

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That usually proves true. But what if that assumption were wrong? What if a higher polish on an injection mold hindered the part’s ejection from the mold? This can happen, and figuring it out does not necessarily come easily.

That is why a solid problem-solving process makes a big difference during product development. The scientific method offers a reliable discovery process. By forming an initial hypothesis, testing it, and interpreting the results, you can methodically solve many manufacturing problems. You can read more about the four stages of lean product development here.

When the manufacturing process includes injection molding, ejection issues sometimes present the most challenging mysteries. Sometimes those solutions cause us to update our body of knowledge.

Application

A medical client manufactured a polypropylene oropharyngeal tube at high quantities from a four-cavity mold. Medical professionals would insert these tubes into the patients’ mouths to maintain an open airway. The curved shape of the device allowed for ease of placement for medical professionals and safety for patients by following the natural curve of the mouth into the pharynx.

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From a user standpoint, any form of part deformation could prove detrimental. Any deformations within the internal diameter would disrupt air flow to the patient. Deformations around the external diameter could cause oral trauma during insertion and even bleeding in the airway. It would also make insertion much more difficult for nurses and doctors.

A curved core with an A3 finish within the injection mold formed the arced shape of the tube. This increased the complexity of the mold because the mold could not eject the part linearly. Before the part could eject, hydraulic slides used linkages to translate linear motion into rotational movement. This revolved the curved core around a theoretical center point to pull the core from the part.

Problem

Over time the mold experienced normal wear. Parts started sticking onto the core, so the client turned to our engineers for support. Our team followed the process of hypothesize, plan, test, and interpret to search for a solution.

Hypothesis 1

They initially hypothesized that too much friction between the part and the core prevented movement. To reduce the friction, they tried a release coating on the core. This offered a way to test the hypothesis at low cost with little downtime.

The team ran the experiment and the part still would not move off the core. The release coating had not improved the ejection. This meant their initial hypothesis failed, so the team had to form a new hypothesis.

Hypothesis 2

Friction, undercuts, and vacuums between plastics parts and molds can cause sticking. The team eliminated friction and undercuts as causes. By process of elimination they then hypothesized that a vacuum prevented the part from moving off the curved core.

To test their hypothesis, the team gave the core a glass bead finish. This texture should reduce the vacuum that could form between the part and the core by allowing air to flow around the texture ridges.

The team molded parts with this textured finish. Parts no longer stuck to the core and ejected smoothly. Over time, the core must have worn down to a smoother finish increasing the risk of forming a vacuum during ejection.

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Conclusion

In certain cases, polypropylene can tend to eject better from a slightly textured surface. This can seem counterintuitive because more texture might give the plastic part small undercuts to get caught on. However, the texture also serves to reduce the ejection risk introduced by vacuums.

Sometimes initial assumptions prove wrong. By forming, testing, and analyzing a hypothesis, you can devise quick and easy tests of your assumptions. And oftentimes, true learning depends upon our ability to shatter our own assumptions.

As your Applications Engineer, I support your product development through design transfer. I will help you understand your options, navigate the risks, and make the right choices for your project.

Call me to advance your design for manufacture. (631) 580-3506 ext. 233