| 1 |
Parts are positioned and clamped before welding. |
Uneven support or excessive clamping can introduce initial stress into a thin wall before heat is applied. |
Misalignment, local gaps, wall flattening, or a part that is already slightly bowed. |
Flexible geometry, unsupported spans, uneven fixtures, excessive clamp force, and dimensional variation between parts. |
Use full-surface support where possible, control clamp force, verify the initial fit, and avoid forcing the parts into alignment. |
| 2 |
Heat is introduced at the weld interface by vibration, a heated tool, infrared radiation, laser energy, or another welding method. |
The weld zone expands as its temperature rises. Because the surrounding plastic is cooler and stiffer, thermal expansion is restrained and internal stress begins to build. |
Small local bulges, softening, gloss changes, or movement of the wall near the joint. |
High heat input, concentrated heating, long heating time, poor heat distribution, and low wall stiffness. |
Use a validated heat or energy setting, keep the interface uniformly engaged, and avoid concentrating heat outside the intended weld area. |
| 3 |
The plastic reaches its softening or melting condition and a weld bead or molten layer forms. |
Softened material loses much of its load-bearing stiffness. Pressure, gravity, and fixture forces can then change the wall shape more easily. |
Wall indentation, local collapse, excessive bead formation, or material squeezed away from the joint. |
Excessive pressure, excessive displacement, insufficient support, high melt volume, and thin sections with low bending stiffness. |
Control weld pressure and collapse distance, support the wall close to the joint, and set energy input according to material and joint design. |
| 4 |
Molten material flows while the parts are compressed or moved relative to each other. |
Material flow can be asymmetric. One side may experience more displacement, friction, or pressure, creating an uneven weld bead and an unbalanced force field. |
Offset joint lines, uneven bead width, corner distortion, or a slight twist along the weld path. |
Unequal part stiffness, nonparallel surfaces, inconsistent contact pressure, misalignment, and variable friction. |
Improve joint alignment, maintain parallel contact surfaces, balance fixture support, and use a joint design that guides material flow consistently. |
| 5 |
Heating stops and the joint begins to cool under pressure. |
The weld zone contracts as it cools. The cooler surrounding material restricts this contraction, leaving residual stress and sometimes pulling the thin wall toward the joint. |
Post-weld bowing, sink-like areas near the weld, flange pull-in, or a change in flatness after release. |
Rapid or uneven cooling, large temperature gradients, thick-to-thin transitions, sharp corners, and insufficient hold time. |
Maintain stable holding pressure during solidification, allow adequate cooling before release, and reduce abrupt section changes in the design. |
| 6 |
The fixture is released after the joint has partially or fully solidified. |
Elastic recovery and the release of residual thermal stress can cause the part to spring back into a different shape. |
Sudden dimensional shift, opened gaps, angular change, or a part that no longer meets the fixture datum. |
Early demolding, excessive clamping, high residual stress, poor datum control, and insufficient cooling uniformity. |
Define a consistent release temperature or cooling time, use repeatable datums, and measure parts after thermal stabilization. |
| 7 |
The welded part continues to equilibrate at room temperature and may undergo post-weld stress relaxation. |
Residual stresses gradually redistribute. The final shape may differ from the shape observed immediately after welding. |
Delayed warpage, dimensional drift, long-term twisting, or a gradual change in flatness. |
Residual stress, anisotropic molecular orientation, uneven wall thickness, environmental temperature changes, and moisture-sensitive materials. |
Inspect after a defined conditioning period, control storage conditions, and evaluate the complete assembly rather than only the freshly welded part. |
| 8 |
Deformation is evaluated against functional and visual requirements. |
Not every shape change is a failure. The critical issue is whether deformation affects sealing, fit, strength, appearance, or dimensional tolerances. |
Rejectable leakage, interference during assembly, reduced weld area, cosmetic distortion, or acceptable nonfunctional warpage. |
Unclear specifications, inconsistent measurement location, unsuitable gauges, and failure to distinguish local variation from functional failure. |
Set measurable limits for flatness, angle, gap, bead size, and leakage; use the same measurement method and stabilized inspection conditions. |
Heping Mechanical
