Plastic rheology or the science of behavior of plastics is the science of flow and deformation of flows, which describes the interaction between forces, deformation and time. Rheology is adapted from the Greek word Rheos, which means flow, and is the study of all fluids in nature, from gases to liquids. This science has a history of about a century, which was first proposed due to the need to describe the properties of fluids.
The difference between rheology and fluid mechanics is that fluid mechanics examines simple Newtonian fluids such as water and oil, but rheology describes the behavior of complex non-Newtonian fluids such as polymers, adhesives, food materials, etc. It goes without saying that talking about plastic rheology is very detailed and long, a small part of which is examined in this article, and the rest of the topics related to this issue will be discussed in future articles.
Eddy currents:
Fluid flows can be divided into two groups: Laminar and Turbulent. The measure of smooth or turbulent flow is the Reynolds number. The Reynolds number is the ratio of inertial force to viscous force. Laminar flow is a flow in which the fluid moves in an orderly manner and under certain layers and paths, but in turbulent flow, the fluid is subject to flow fluctuations and strong mixing processes.
In general, it can be said that in laminar or slow flow, the hydraulic fluid moves through the pipe in the form of cylindrical layers in layers, and each layer has a different speed, so that the inner layers move faster than the outer layers. does If the speed of the hydraulic fluid flow exceeds a certain limit known as the critical speed, the fluid particles will no longer move in regular layers. In this case, the fluid particles in the center of the tube rotate around and as a result affect each other and prevent movement. The result of this is the creation of a vortex movement, which eventually causes the flow to become confused. Creating a turbulent flow in the fluid causes power loss.
In general, it can be said that in laminar or slow flow, the hydraulic fluid moves through the pipe in the form of cylindrical layers in layers, and each layer has a different speed, so that the inner layers move faster than the outer layers. does If the speed of the hydraulic fluid flow exceeds a certain limit known as the critical speed, the fluid particles will no longer move in regular layers. In this case, the fluid particles in the center of the tube rotate around and as a result affect each other and prevent movement. The result of this is the creation of a vortex movement, which eventually causes the flow to become confused. Creating a turbulent flow in the fluid causes power loss.
One method to determine the type of flow is to calculate the Reynolds number, in which:
Due to the relatively high viscosity of commercial polymers, it is generally expected that the flow formation during the injection process will be laminar even when the melt passes through small cross-sectional areas. Although the term turbulent flow is often used to describe the cause of defects in polymer products, the reality is that true turbulent flow occurs very rarely in plastic injection processes.
Turbulent flow usually occurs at a Reynolds number of 2300 or higher, and the value of this number also depends on the cross-sectional shape of the fluid passage channel. Calculation of this number is often less than 10 for most polymers during processing in plastic injection molds, even at the high flow rates generated after passing through small sections of the feed system. The performed calculations also confirm that turbulent flows in polymers rarely occur during injection operations, and the flow can always be considered slow in design considerations.
Streams of the spring:
As shown in the figure below, during the injection process, regardless of the type of channel system used in the mold, plastics exhibit a specific behavior known as “Fountain Flow”. When materials enter the Quetta channels, the flow front is diverted towards the Quetta wall and remains there. In such a way that at the same time as the newer materials enter the channel, the older materials are pushed towards the mold wall and the newer materials move forward, and the repetition of this process causes the spring flow. This flow is caused by the pulling force from the side of the channel wall on a layer of polymer and causes the central layers to have a higher speed than the layers adjacent to the channel walls.
The same speed difference causes the shear rate between the polymer layers and also causes the appearance of a parabolic shape at the tip of the material flow. Materials near the channel wall experience near-zero shear velocity, and this creates the conditions in cold channel systems for the formation of frozen layers of materials next to the channel walls. But in hot channel systems, due to the high temperature of the channel wall, ice layers do not form on the side of the walls despite the spring flow.
