what is Rheology?

Rheology deals with the deformation of matter. In dealing with plastic materials we are only concerned with its flow characteristics. This flow is of interest in cylindrical or rectangular channels. Understanding this flow we will then be able to understand how a plastic part is formed, starting with the screw, going through the barrel, the nozzle, sprue bushing, runner system, gates, and then into the mold itself. In short, we must understand every course the plastic material passes to form it's final configuration, the finished, molded product. We are primarily interested in not disturbing or degrading the molecular configuration that has been incorporated during the polymerization process of the plastic material.

Plastics conform somewhat to the laws of Newtonian Flow. What this law of viscous and laminar type of fluids means can be explained as follows in a simple Rheometer.

A Rheometer is used to measure flow rate and establish Melt Index information for plastic materials.

1. When water is forced through an orifice the stream coming out the other side is the same as the opening.

2. When rubber is forced through the same orifice, under the same conditions, the material comes out and springs back to its original shape. This is due to the memory capabilities of rubber.

3. When melted plastic is forced through this same orifice then the flow on the outside of the orifice is greater in diameter than the orifice exiting diameter. The plastic acts to some extent like rubber and partially like a fluid, water. This is known as Viscoelastic.

If the conditions of temperature, pressure and orifice diameter are kept constant then the amount of volume of plastic that comes through in 1 second will depend upon the internal resistance to flow of this polymer.

All of this is very important in processing because the molded product will not achieve its intended physical specifications if the flow of the plastic is not understood. It is also necessary to perform a rheology study on a specified material and its intended mold. In analyzing the rheology curve the variation in melt index from lot to lot of the material will have very little implication on the molded part.