Thermoforming against Injection for Volumes below Fifty Thousand Units
Thermoforming delivers lower total landed cost below fifteen thousand units by saving upfront tooling capital, beyond which injection piece price efficiency dominates.
Thermal Bow
Meaning
Non-uniform temperature distribution across the thickness of a cooling plastic part causes asymmetric contraction and warpage. This deformation, referred to as thermal bow, occurs when one side of the part cools and shrinks faster than the opposite side. It governs the straightness of extruded profiles and large flat molded panels.
This effect is minimized when both sides of the part are cooled at the same rate.
Gradient Consequence
Asymmetric cooling forces the hotter side to shrink more than the cooler side after the part is ejected or exits the die. This differential contraction results in a curved shape, with the part bending toward the side that cooled last. In extrusion, this thermal bow can be observed when the profile continuously curves as it exits the water bath.
Tool Cooling
Mold temperature control must be carefully balanced between the cavity and core halves to prevent this defect. If one mold half is run hotter than the other, the resulting thermal gradient across the part thickness generates a thermal bow. Adjusting the coolant flow rates and temperatures in the individual circuits allows the molder to balance the cooling.
Material Selection
High-shrinkage resins are more sensitive to temperature gradients and are thus prone to this warping effect. Amorphous resins with low thermal expansion coefficients are less likely to deform under non-uniform cooling.