What is "Full-Penn" Welding

A full-pen weld is a type of weld joint where the weld seam completely penetrates to the root of the workpiece. This means that the weld goes entirely through the workpiece and connects both sides of the workpiece.

This weld is also often called a "Butt Weld," a term used in certification.

Sketch of a "Full-Pen" weld

A full-pen weld is often used in metalworking, for example, in welding pipes or sheet metal.
The advantage of a full-pen weld is that it is very strong and resistant to high loads.
Moreover, the weld seam is completely filled due to full penetration, leaving no room for imperfections or weak spots in the weld.

Cross-section of a "Full-Pen" weld

In contrast, there is the not fully penetrated weld as shown in the pictures below.
Any type of weld can be either fully or not fully penetrated. This depends on the requirements a company sets for the quality and strength of the weld.
A Full Pen weld is harder and more expensive to make, so it will not be used if not necessary.

Weld is clearly not fully penetrated

Corner welds not fully penetrated