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?
There are many suppliers of hot runner systems with each offering different features based on the performance you want to achieve. We touched on this subject in a previous Tech Tip, however, not all topics could be covered in one tip. The purpose of this Tech Tip is to highlight the importance of an aggressive and diligent quality effort and how those checks can contribute to the operational performance, or lack thereof, of the hot runner system.
If it’s the same plastic, why the two different densities?
The first thing we need to understand: What is density? Density can be thought of as how many grams of a plastic will fit into a 1cm x 1cm x 1cm cube as illustrated in Figure 1. When working with plastics, there are two types of densities that need to be considered: solid density and melt density. But why the two different densities? After all, it’s the same plastic we are talking about, right? True, but we need to understand what happens to the molecular chains when we apply heat to plastic. The chains expand and separate, thus increasing the volume that will be occupied.