Radiant Furnace Thermal Absorption and Gravity Sag Calibration for Amorphous Polymer Sheet
Calibrating emitter wavelength to polymer absorption bands and setting optical sag triggers prevents core temperature deltas exceeding five degrees Celsius.
Catenary Sag Profile
Meaning
Suspended polymer sheets undergoing thermal softening adopt a characteristic curve driven by the interaction of gravitational force and internal melt strength. Physical displacement seen in the catenary sag profile indicates material readiness in vacuum forming and heavy gauge thermoforming. If the curve becomes too deep, the plastic may touch the heater elements or the floor of the machine, causing surface defects or fires.
Thermal Relationship
Temperature uniformity across the sheet width determines the symmetry of the resulting arc. A catenary sag profile that leans to one side indicates uneven heating or a draft in the production area. High melt strength resins like polypropylene or certain grades of polyethylene produce a shallow and predictable curve.
In contrast, materials with low melt strength drop rapidly once the melting point is exceeded, narrowing the window for successful forming.
Tooling Interaction
Proper management of the sheet geometry prevents the formation of webbing or chill marks during the initial contact with the mould. The catenary sag profile must be timed so that the vacuum is applied exactly when the material has reached the desired depth. Excessive sag results in the plastic thinning at the corners as it stretches over high points in the tool.
Precise control of this shape is often achieved through the use of photo eyes and air pressure management. In larger machines, automated blow up systems inject a cushion of air to support the sheet, effectively reversing the sag into a dome to prevent premature contact with the cold metal. This technique allows for larger part production without the risk of cooling the plastic too quickly.
Product Quality
Final wall thickness in a deep draw part is heavily influenced by the initial shape of the heated sheet. If the catenary sag profile is not kept within tight tolerances, the finished product will show inconsistent structural integrity. This variance is particularly problematic when using regrind, as the degraded polymer chains provide less resistance to gravity.
Monitoring the sag depth ensures that each cycle produces a part that meets the original design specifications.