Determining Commodity Resin Grades for High Volume Injection Molding
Determining commodity resin grades requires multi-point shear viscosity validation and strict Certificate of Analysis contract limits to secure part performance.
Gate Shear Rate
Meaning
Velocity gradient of a polymer melt as it passes through the narrowest point of the feed system defines the level of mechanical stress on the material. High gate shear rate values can lead to localized heating and molecular degradation if the material is pushed too fast through a small opening. This measurement is critical because it represents the peak stress the polymer experiences during the entire injection cycle.
As the melt is squeezed through the gate, the internal friction between molecular chains generates heat, which can be used to lower the viscosity and improve flow. However, if the stress exceeds the limits of the resin, the chemical bonds can break, leading to loss of physical properties and surface defects like splay.
Thermal Degradation
Excessive friction within the melt can raise the local temperature of the plastic far above the set point of the barrel. This rapid heat spike often leads to burning or discoloration of the part near the gate area. Sensitive materials like PVC or flame-retardant resins are particularly prone to this type of damage.
If the gate shear rate is too high, the polymer chains may shorten, resulting in a part that is weaker and more prone to failure under load. Processors must carefully calculate the gate size to ensure the material can be injected at the required speed without reaching these damaging temperatures. Adding cooling to the gate area can help, but the primary solution is usually adjusting the geometry or the injection velocity.
Flow Resistance
Small gates create a bottleneck that requires higher injection pressures to overcome the resistance of the melt. As the material accelerates through the gate, the apparent viscosity drops due to the shear thinning behavior of the polymer. This allows the plastic to fill the cavity more easily once it has passed the constriction.
However, the energy required to force the material through a tiny opening can be substantial, limiting the size of the part that can be molded on a specific machine. If the gate is too small, the pressure drop will be so large that the machine may hit its pressure limit before the cavity is full. Designers use rheological simulations to find the optimal gate dimensions for a given material and part volume.
Viscosity Change
Shear thinning is a temporary effect that allows for easier processing, but it must be managed to ensure consistent part quality. The gate shear rate determines how much the viscosity will drop during the fill phase. If the injection speed varies, the viscosity also changes, leading to fluctuations in the pressure required to fill the mold.
This can result in variations in part dimensions and weight. Operators often set a specific injection speed profile to keep the shear rate within a safe and predictable range. The boundary of this effect is reached when the shear is so extreme that it causes melt fracture, where the surface of the plastic becomes rough and distorted.
Understanding the relationship between gate size and shear is fundamental for successful molding of complex parts.