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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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.
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.
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.
The Tech Tip about Tips
For this tech tip the focus will be on the actual hot tip itself which will include those tips used in full direct gated hot runner systems and hot-to-cold runner systems (one or more hot drops feeding a cold runner system).
Sizing Up Your Runner System
The topic of runner sizing always comes up during our plastics education courses. Everyone wants a magic formula for sizing every runner for every mold regardless of machine capabilities, pressure to fill the part, cycle time requirements and packing. So let’s take a look at various methods people use for sizing runner systems.
If you work in the injection molding industry long enough, at some point in your career you will be sure to hear someone say “turn the mold 90 degrees (or 180 degrees) to see if the problem changes”. And when asked why on earth would that person suggest such a thing, the answer usually is “because gravity is pulling the plastic down to the bottom of the mold”. At this point everyone gets a good laugh and makes fun of the person who suggested such an absurd solution. But are plastic materials immune to Isaac Newton’s Law of Universal Gravitation? No. Then why is it considered absurd to think that gravity could not influence the filling of a plastic part made by the injection molding process?
At one point or another, we have all been in the position of standing beside the molding machine with a new mold hung in the press and it’s time to make that first shot. The process technician or engineer builds up a shot size and away it goes. As the screw moves forward, everybody is holding their breath wondering if too much or too little plastic was injected. Is the mold going to flash (Figure 1)? Will the parts come out of the mold? What if something else breaks and the mold must be pulled after only one shot?