Determining Intrinsic Viscosity Loss in Polyethylene Terephthalate Processing
Intrinsic viscosity loss in PET processing is determined by measuring dilute solution flow time or capillary melt viscosity to quantify polymer chain cleavage.
Melt Shear Rate
Meaning
Measurement of the internal friction experienced by a polymer during injection. Processing speed determines the melt shear rate, which governs how the molecular chains align and slide past one another as the plastic moves from the barrel into the cavity. Higher speeds lead to greater alignment, which reduces the effective viscosity of the material through a phenomenon known as shear thinning.
Maintaining this variable within the limits specified for a particular grade prevents permanent damage to the polymer structure.
Tooling Geometry
Design of the runner system and the gate size dictates the speed at which the material enters the mould. A narrow gate increases the melt shear rate because the same volume of plastic must pass through a smaller cross section in the same amount of time. If the gate is too small, the friction generates excessive heat that can lead to material degradation or discoloration.
Proper sizing ensures that the flow remains stable without causing localized overheating.
Processing Step
Control over the injection speed is the primary method for managing this variable on the factory floor. While the melt shear rate is set by the machine settings and the tool design, the actual value experienced by the polymer can vary across the melt stream. It is highest near the walls of the channel where the friction is greatest.
Engineers use flow simulation software to predict these gradients before the steel for the tool is cut.
Quality Outcome
Physical properties of the final part are often tied to the stress experienced during the fill phase. When the melt shear rate exceeds the critical limit for a specific resin, the molecular chains can break or become overly oriented. This leads to defects such as splay or brittle failures in the field.
Consistent monitoring of the injection profile helps to maintain part integrity across long production runs.