Today's KNOWLEDGE Share: Jetting

Today's KNOWLEDGE Share

At times, the flow front in injection molding appears to chart its own course, making unexpected decisions.For instance, one would anticipate that the shorter and thicker side would fill the flange before moving on to fill the main pipe walls in this example.

However, it's not uncommon to witness a counterintuitive scenario where a supposedly more challenging section fills faster, despite the expectation of higher pressure drop.

Physics dictates that there must be a rationale behind such occurrences.

Upon observing this phenomenon with RPVC, it became apparent that the significant extensional viscosity of the polymer played a crucial role. This viscosity demanded additional energy or pressure to navigate the 90° bend turn. Essentially, in situations where extensional viscosity is notable, it's easier for the flow to progress straight through a slightly narrower channel than to negotiate a sharp turn into a wider one.

Traditional flow analysis software often struggles with simulating this.

Moreover, similar unexpected filling patterns might be due or wrongly attributed to the inertia effect, known as "jetting". When these unexpected fill patterns are observed at low fill rate, inertia can almost immediately be ruled out.

So...it looks like jetting but it is NOT jetting !

It's imperative to delve into the underlying physics of the observed issues. Understanding the principles at play will lead you closer to effective solutions.- and help designing better parts and tools.

source:Vito leo