Welding defects are imperfections in a weld that can affect its strength, appearance, or functionality. These defects can occur due to improper welding techniques, incorrect parameters, poor material preparation, or environmental factors.

Thermoplastic materials have proven immensely beneficial for a vast array of industries. For many products and structures, custom fabrication is required to achieve the desired shape and dimensions of the final product. Fabricators in this field often use welding or fusion technologies to accomplish the task. Regardless of the fusion process used, key practices ensure a high-quality fusion, while common pitfalls can result in defects or failure of the finished product.

Welding of thermoplastic materials depends on three primary parameters: heat, time (or speed) and pressure. Using proper temperature, time and pressure, and having the ability to control these parameters, is critical to ensuring a high-quality weld. Failure to use the correct values or to control these parameters — whether through lack of knowledge, experience or improper technique — is likely to result in commonly seen welding defects.

Example of “fish bones” defect in thermoplastic welding, caused by excessive heat and low viscosity leading to material flow under hot air pressure.(left)

Overheated weld bead in hot gas hand welding, showing a too-flat or excessively melted appearance due to high temperature or slow travel speed. (right)

Welding at a higher-than-recommended temperature can tempt many fabricators. The thinking is that increasing the welding temperature heats the plastic more quickly, allowing faster welding. While this can hold true to an extent when moving from the lower to the upper portion of a material’s welding temperature window, this approach often causes more harm than good. Subjecting thermoplastics to temperatures higher than their ideal weldable range risks damaging the material at a molecular level. If this damage occurs, the affected plastic is no longer weldable. Additionally, this thermal damage is not always visibly apparent and may only be discovered through weld failure. This is why it is critical to operate within appropriate temperature windows when welding.

While thermal damage is not always visible, some signs can be observed. Any smoking or burning of the material is a clear indication of thermal damage. Another less obvious sign is what is commonly known as “fish bones.”

“Cold weld” defect featuring a rounded, poorly fused bead resulting from low temperature, fast travel, or insufficient pressure.

“Fish bones” can occur in welding applications that rely on flowing hot air to heat the material. The actual “fish bones” result from the plastic being heated to the point that its viscosity is very low and it begins to flow because of pressure from the hot air moving over it. Typically, when thermoplastic is in a weldable state, it is very viscous and gel-like and would not easily flow due to hot air during normal methods such as hot gas hand welding and extrusion welding.

Another welding defect specific to hot gas hand welding that can arise from overheating thermoplastics is a weld bead that is too flat or too melted. This defect is primarily seen in hot gas hand welding.

This issue can result from welding at too high a temperature, moving too slowly or a combination of both. As discussed previously, time — or speed in the case of hot gas hand welding — is just as critical as welding temperature. As with temperature, there is an acceptable welding speed window for each material. Moving slower than these recommended speeds can result in thermal damage similar to that from higher temperatures and cause a weld bead that is too flat or possibly even “fish bones.”

On the opposite end of the spectrum, a weld bead can be too round, which may be referred to as a “cold weld”. A cold weld is typically caused by low welding temperature, moving too quickly or not applying adequate pressure to the weld bead. In instances where aesthetics are very important, this rounded weld bead style is sometimes preferred over one resulting from proper temperature, speed and pressure. However, this rounded weld bead will not produce a strong weld.

Notch defect in a thermoplastic weld, displaying an indentation at the weld edge or crown that creates a structural weak point prone to cracking.

A poorly shaped weld bead from being too round, too flat or not completely filling the weld joint, can lead to another defect called a notch. A notch in a weld is an indentation on the edge or surface of the weld or where the weld meets the base material. This indentation is a structural weak point. An otherwise perfectly fine weld may form a crack starting at the notch when placed under stress. In some cases, especially when the notch is only present in the crown of the weld, it can be removed with scraping tools. Removing the notch will improve the overall strength of the weld.

Another weld defect that can arise in an effort to speed up production is holes within the weld. This generally results from uneven cooling of the weld area. Thermoplastics are poor conductors of heat, meaning the material heats up, cools down and transfers heat slowly. Because of this, a weld can take hours to fully cool on its own. Therefore, it can be tempting for fabricators to force-cool the weld by pointing a fan at it, spraying it with compressed air or even submerging it in water. This cools the outer portion of the weld quickly, causing it to become a rigid solid. However, the center of the weld will largely remain unaffected by this forced cooling and stay molten because of the slow heat transfer properties of thermoplastics. As the inside slowly cools and shrinks, the already solid outer portion stays fixed, resulting in holes in the weld area.

Internal voids or holes in a thermoplastic weld cross-section, caused by uneven cooling and shrinkage after force-cooling the exterior. 

These defects are not noticeable during a normal visual inspection, as they are under the weld surface. They will only become apparent if a cross section of the weld is examined (which destroys the weld) or if the holes cause the weld to fail, making this defect potentially more dangerous than one like a notch that can be easily observed visually without destructive testing.

These are a few of the more common defects seen in the welding of thermoplastic materials. Most can be avoided when fabricators have proper knowledge of the welding process, operate within appropriate temperature, time and pressure windows for the specific thermoplastic material being used and follow known best practices such as not force-cooling recently welded thermoplastic material. However, defects cannot be completely avoided even by the most knowledgeable and technically sound welders. Thus, being able to identify defects is critical to ensuring consistently high-quality welds in fabricated thermoplastic materials.

by Bryson Nash, Technical Sales, Wegener Welding, LLC