Spin welding is a method of joining thermoplastic parts with circular or cylindrical geometry. The two most common types of joints used in spin welding are butt joints and lap joints.

In a butt joint, the two parts are joined end-to-end, forming a joint with no overlap. This type of joint is commonly used in applications where a smooth, continuous surface is required, such as in medical devices, automotive parts, and consumer goods.

In a lap joint, one part is overlapped onto the other, forming a joint with an overlapping region. This type of joint is commonly used in applications where a strong, leak-proof seal is required, such as in fluid storage tanks, pipes, and valves. The lap joint also provides additional structural support and rigidity to the joint.