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Shear Rate Spikes
Meaning
Transient increases in the velocity gradient of a flowing polymer that occur when the melt is forced through small gates or sharp changes in the geometry of the flow path. Managing shear rate spikes is essential for preventing the molecular degradation of the plastic and maintaining the aesthetic quality of the finished part. This phenomenon governs the localized temperature of the melt and the orientation of the polymer chains.
It applies during the injection phase as the plastic moves from the runner into the cavity. The analysis of these spikes is most critical when processing shear-sensitive materials like PVC or polycarbonate.
Flow Dynamics
Molten plastic is a non-Newtonian fluid, which means its viscosity changes depending on how fast it is moving. As the resin passes through a narrow restriction, the layers of the melt slide against each other with incredible speed, creating frictional heat. These shear rate spikes can cause the local temperature to rise far above the set point of the machine barrel.
If the heat becomes too intense, the chemical bonds in the polymer can break, leading to a loss of mechanical properties. In addition to degradation, the high velocity can also cause jetting, where a stream of plastic enters the cavity without contacting the walls. This results in a snake-like defect on the surface of the part.
Careful gate design and speed control are necessary to keep these flow dynamics under control.
Molecular Degradation
When a polymer is subjected to extreme shear, the long molecular chains can be literally torn apart. This molecular degradation manifests as a drop in the molecular weight of the material, which reduces its impact strength and durability. For a moulder, this is a major problem because the parts may look perfect but fail prematurely in the field.
Shear rate spikes are often the hidden cause of brittle failures in parts that otherwise seem to be within specification. Resins like flame-retardant ABS or some types of medical-grade plastics are particularly susceptible to this damage. By using rheological simulation software, engineers can identify areas of the mould where the shear will be highest.
They can then adjust the gate size or the wall thickness to ensure the material remains intact throughout the filling process.
Visual Defect
Aesthetics are often the first casualty of an unoptimized injection process. High shear rate spikes often lead to surface blemishes known as splay or gate blush. This occurs when the overheated plastic releases gases or when the polymer chains are so stretched that they change the way light reflects off the surface.
For consumer products with high-gloss finishes, these marks are an immediate cause for rejection. The blush is usually found right at the gate where the plastic enters the cavity and the shear is at its peak. Reducing the injection speed can help, but it might also lead to other problems like sink marks or cold slugs.
The most effective solution is often to increase the gate size to provide a wider flow path for the melt. This reduces the velocity and the friction, ensuring a clean and consistent appearance.