Food Contact Declarations for Custom Molded Plastic Components
Food contact declarations for custom molded plastics require finished part migration testing to cover processing degradation and additive changes beyond raw resin certificates.
Polymer Shear Friction
Meaning
Resistance encountered when layers of molten plastic slide against each other or against metal surfaces during extrusion generates the heat required for melting the resin. Polymer shear friction is a fundamental physical process in the operation of an extruder or an injection molding machine. As the screw rotates, it shears the solid pellets and the molten plastic, converting mechanical energy into thermal energy.
This internal heating is often the primary source of melting, with the heater bands on the barrel providing only a secondary source. The amount of friction depends on the viscosity of the polymer, the speed of the screw and the clearance between the screw and the barrel. Managing this heat is essential for preventing the degradation of the polymer and for maintaining a stable process.
Viscous Dissipation
Converting mechanical work into heat through the internal resistance of the fluid is a key technical concept in polymer processing. In polymer shear friction, this is known as viscous dissipation. The rate of heat generation is proportional to the viscosity of the melt and the square of the shear rate.
High viscosity resins, such as those used for blow molding, generate more heat through friction than low viscosity resins. If the shear rate is too high, the temperature of the melt can quickly exceed the safe limits for the material. This can lead to the breakdown of the polymer chains and the formation of black specks or off odors.
Engineers must carefully choose the screw design and the processing parameters to balance the need for melting with the risk of overheating.
Screw Interaction
Controlling the forces between the moving parts of the machine and the plastic material is essential for efficient operation. In polymer shear friction, the interaction between the screw flights and the barrel wall is where the highest shear rates occur. This region is responsible for a large portion of the heat generation and the mixing of the melt.
The geometry of the screw, including the pitch and the flight depth, determines how the material is sheared. A well designed screw ensures that the heat is generated uniformly throughout the melt, preventing hot spots and cold slugs. Wear on the screw or the barrel can change the clearance and alter the friction, leading to changes in the process over time.
Regular inspection and maintenance are required to keep the friction within the desired range.
Thermal Management
Balancing the heat generated by friction with the heat removed by cooling is the goal of process control. In polymer shear friction, the temperature of the melt is monitored using sensors in the barrel and the die. If the friction generates too much heat, the cooling fans or the water jackets on the barrel are activated to remove the excess energy.
In some cases, the screw speed may be reduced to lower the shear rate and the heat generation. Accurate thermal management is necessary for producing parts with consistent dimensions and properties. It also helps to prolong the life of the machine and the polymer by preventing thermal stress.
The use of advanced control algorithms allows for the real time optimization of the thermal balance.