Tech Tips

Questioning the Science in Injection Molding: What are you actually getting in a “Pressure Loss Study”?

Written by John Beaumont.

This article was originally published in SPE’s Plastics Engineering Magazine


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Determining Flow Groups in Multi-Cavity Molds, Part II

Determining the Regions of the Mold

In the last tech tip, we discussed determining patterns within the mold in order to analyze “random” filling patterns and how to determine flow groups.  This tech tip will discuss how to determine regions in the mold.


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Determining Flow Groups in Multi-Cavity Molds, Part I

Often times when we hear that there is a random filling imbalance related to a random part quality issue, we are skeptical that the problem is truly “random”.  A majority of molds that we are asked to look at often times have a pattern to the data, and therefore, a common root cause behind the associated problems. These patterns may be a result of either non-uniform shear heating variations or steel related issues that affect a certain set of cavities. In order to first determine if there is a pattern, we need to break down a mold into flow groups and regions. By determining flow groups in a mold, not only can a person more easily find the true imbalance but it is also easier to determine the source(s) of the imbalances.


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Beware of the “Naturally Balanced” Runner System

For years now the standard method designing a melt delivery (runner) system has been the naturally balanced runner system. A naturally balanced runner system is a runner configuration where there is an even flow length from the tip of the sprue to the end of each cavity with symmetrical cross-sectional shapes. While the naturally balanced runner system looks good from a visual standpoint, it is what lies underneath that leads to molding issues. This tech tip will explain the pitfalls of the naturally balanced runner system and how to avoid them.


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Viscosity, Part 3- Therma-flo™

In the previous two tech tips, we began a three part discussion on how our industry looks at the viscosity of a polymer. We reviewed capillary rheometers and melt flow index machines up to this point. We pointed out various benefits and flaws for each method. Now we are going to take a look at a newer approach to evaluating a polymer’s viscosity called Therma-flo™.


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Viscosity, Part 2: Melt Flow Index

In our last tech tip, we started a three phase discussion about viscosity. The first phase talked about how capillary rheometers are used to characterize the viscosity of plastics at a wide range of shear rates and temperatures. However, due to a variety of reasons most people in the injection molding industry prefer to use a simpler device when talking about the viscosity of a plastic – the Melt Flow Index machine. One should be aware that even though MFI is the dominate method used to characterize how plastic will flow in a mold, it does not actually provide a measure of viscosity.


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Viscosity, Part 1: Capillary Rheometers

Viscosity is a term that people in the plastic injection molding industry talk about quite often, so it is something we will discuss in more detail during the next three tech tips. Viscosity measures the thickness of a fluid. The thicker the fluid, the higher the viscosity. In other words, viscosity is resistance to flow. There are various methods used to measure viscosity and all three will be talked about in the upcoming tech tips, but this tech tip will start with capillary rheometers.


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Artificial Balancing – What Exactly Are You Balancing?

Artificial balancing is the practice of restricting and/or enhancing plastic flow to one or more cavities in order to have all the cavities in the mold fill at the same time. The methods most commonly employed in cold runner molds include changing gate diameters and/or runner diameters. When restricting flow in the runner system it is unfortunately too common to see people using restrictive pins, set screws, or other items that protrude into the runner system (Figure 1). In hot runner molds, adjusting the nozzle or tip temperature is the most common practice.


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