Plate Geometry
Dual circular plates positioned parallel to each other enclose a test specimen to measure dynamic shear moduli and loss factors under controlled thermal conditions. In rheological testing of hot-melt adhesives and polymer coatings, parallel-platen DMA applies torsional oscillation across a specified gap height. The geometry accommodates solid discs and high-viscosity liquids that cannot be clamped in tensile fixtures.
Temperature sweeps executed on parallel plates characterize melting transitions and modulus drops during heat activation.
Gap Calibration
Thermal expansion of metal test fixtures during temperature sweeps alters the precise gap dimension between upper and lower plates. Advanced instrument control in parallel-platen DMA utilizes automated normal force tracking to adjust gap distance continuously, preventing thermal expansion from squeezing polymer samples out of the testing zone. Correct gap setting ensures reliable calculations of shear strain and storage modulus across wide temperature ranges.
Oversized gaps introduce thermal gradient errors, whereas undersized gaps induce normal stress artifacts that distort viscoelastic damping measurements. Edge trim inspection guarantees that sample perimeter matches plate boundary exactly.
Strain Uniformity
Torsional shear strain increases linearly from zero at the central axis to a maximum at the outer radius of the circular plate. Operating parallel-platen DMA within small strain limits prevents edge fracture and non-linear strain response during rheological characterization. Boundary overflow alters effective plate area and invalidates stress calculations.
