Many of the challenges faced in molding can be addressed by returning to the basics of how a material moves through the nozzle, gate, and mold. In the first part of this two-part series, we take you through the physics of resin behavior. Stay tuned for part 2 next month, when weâll give you the not-so-secret formulas for calculating shear rates.
The process of molding thermoplastics requires shearing the resin to make it melt and to get it to flow into the mold. Shearing occurs throughout the process and, done correctly, makes for successful molding. Done incorrectly, it can lead to resin degradation, part failure, poor cosmetics, and excessive mold corrosion, among other things.
A great number of molding problems can be avoided with a basic knowledge of plastic resin shear. The gate is generally the highest-shear area and many problems start there. From sizing gates to selecting proper machine nozzle sizes, a little understanding and a few calculations can save you a lot of grief.
How often are you relying on experienced people to pick the gate sizes? How often do you sample molds only to find the gate size is incorrect? The purpose of this two-part article is to provide you with the knowledge needed to troubleshoot this straightforward area and help size the gates right the first time.
Back to basics
Letâs start with a few definitions:
What is shear?
Shear in injection molding occurs when layers of molten resin flow relative to each other. During injection, molten plastic generally flows through the melt delivery channel (nozzle, sprue, runner, and gate) and then into the mold like a fountain. Fountain flow means the material moves through the flow channelâs center and then outward to the walls. The flow channelâs outer layers flow more slowly in relation to the material in the center. For visualization purposes, imagine the plastic separating into many different layers as it flows, each layer moving at a different rate relative to an adjacent layer, or shearing.
What is shear rate?
How fast shear occurs is referred to as the shear rate. Shear rates are important to the design and molding of plastic products. As shear rates rise, the molecular chains that make up the polymer are stressed more. If a polymerâs shear rate is too high, molecular chainsâor the glass fibers they might containâcan be torn apart. This can reduce the mechanical properties of a product.
The highest shear rates occur where the relative movement between layers is the greatest. This is generally near the wall and at the gate. The shear rate at the center of flow is much lower because there is little difference between the center layerâs speed relative to the other center layers. The units of shear rate are reciprocal seconds, also shown as sec-1. Maximum suggested shear rates can range from acetal at 20,000 sec-1 to PP at 100,000 sec-1. For our purposes, the higher numbers simply mean more cubic inches of material can go through a gate in a certain time.
What is shear stress?
Shear stress is