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The test is very simple: plastic is put into a heated pot and then pushed out of this pot through a small hole via a piston with weights pushing on it. The melt flow test measures how much plastic can be pushed out of a small hole in a given amount of time. One of the most common ways of measuring viscosity is MFI, or melt flow index. Note that the peak injection pressure correlates inversely with cavity pressure. Next time a solid process turns into a headache, you won’t have to guess whether the viscosity changed, or by how much.įigure 1: An example of a viscosity shift and its effect on cavity pressure. Tracking viscosity is a vital step in solving a great deal of your molding problems.
#DEFINE VISCOSITY DRIVERS#
Since variations in cavity pressure influence most of our issues (dimensional variation, shorts, sink, flash, warp, etc.), tracking one of its drivers is tremendously valuable. Notice the clear correlation between viscosity and cavity pressure-molders who are already using instrumented molding are likely familiar with this correlation. Figure 1 shows an example of data over time. More often than not, short shots correlate with higher viscosities, and flash correlates with lower viscosities. As contradictory as it may seem, higher injection pressures typically indicate lower cavity pressures. When viscosity rises, pressure in the cavity drops, which yields smaller parts or parts with sinks and shorts. If we track viscosity changes over time, we can often be alerted to problems well before quality has a chance to inspect the parts. Why Is It Important to Monitor Viscosity? There are clear part performance issues with weaker material, and lower viscosity plastic will tend to give us issues, such as out of spec dimensions or flash.
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Two important things happen when polymer chain degradation occurs: the material becomes weaker and the viscosity lowers. Plastics can degrade in different ways, one of which is polymer chain degradation, a simple breaking of the polymer chains. Or worse, shipping bad parts.Īnother cause of viscosity change is plastic degradation. Without knowing it’s viscosity, this can be quite confusing and lead to a lot of unnecessary button pushing and scrap. As you can imagine, a viscosity change could greatly affect how your process runs, leading to poor quality parts when nothing else has changed. Viscosity often changes from lot to lot of the same exact material. Higher viscosity means more complexity and friction (beads), so it is more difficult to inject. Which necklace would go down the drain easier? The plain chain, right? That’s the same idea with viscosity-with low viscosity, the molecules are lined up in a plain, simple chain. Picture a plain chain necklace and a necklace with a lot of large beads on it. The higher the viscosity, the more friction that exists, so the more pressure you need to inject it into a mold. Viscosity is a fluid’s resistance to flow, or the amount of friction that exists in the material.
#DEFINE VISCOSITY HOW TO#
This article covers the definition and characteristics of viscosity, why it’s important to monitor viscosity, and how to measure viscosity. If, by the student’s estimation, 90% of our issues come back to viscosity, it sure would make sense to track it. If the viscosity of the material was either consistent or predictable, this whole injection molding thing would be easy. We like asking why things happen, and quite often viscosity is indeed the answer. A student once joked that, “9 out of 10 times when you ask a question, the answer is viscosity.” While it’s probably not quite 9 out of 10, the student did have a point.