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.
Viscoelastic Sag
Meaning
Time dependent deformation of a polymer sheet under its own weight occurs when the material is heated above its glass transition or melting temperature. Large part thermoforming is particularly affected by viscoelastic sag. This phenomenon combines the viscous flow and the elastic response of the polymer chains as they lose their rigid structure.
Managing this effect is necessary to ensure that the material remains workable and does not thin out excessively before the moulding tool is engaged.
Molecular Strength
Longer polymer chains provide greater resistance to gravity, a property often referred to as melt strength. Viscoelastic sag is less pronounced in high molecular weight resins because the entangled chains take longer to slide past one another. Conversely, recycled materials with shortened chains often show increased sagging at lower temperatures.
This difference requires the processor to adjust the heating profile to prevent the sheet from drooping too quickly.
Process Control
Automated systems use infrared sensors to monitor the depth of the sheet as it softens in the oven. If the viscoelastic sag exceeds a pre-set limit, the system can trigger the start of the forming cycle or inject air to support the sheet. This control prevents the plastic from touching the machine components and causing a defect.
Precise timing of the vacuum application ensures that the part has a uniform wall thickness.
Product Integrity
Excessive stretching during the heating phase can lead to internal stresses and thinning in the finished product. Because viscoelastic sag is permanent once the material cools, any unevenness in the initial droop will be reflected in the final dimensions of the part. Moulders must balance the need for a pliable sheet against the risk of losing control over the material geometry.
In heavy gauge applications, the weight of the sheet itself can cause the edges to pull away from the clamps if the sag is not managed through careful temperature ramping and physical support. Understanding this behaviour allows for the production of complex shapes with consistent structural strength.