A method of sealing parts of a plastic capsule by forming a weld seam in an overlapping region of the parts of the capsule, wherein the capsule comprises a capsule cap having an open end and a capsule body having an open end, the method comprising:

• (a) holding the capsule cap and the capsule body in a capsule holder comprising a first holding part and a second holding part which can be guided synchronously with one another, wherein the first holding part interlockingly surrounds the capsule cap and the second holding part interlockingly surrounds the capsule body;
• (b) closing the capsule holder holding the capsule cap and the capsule body so that the open end of the capsule cap and the open end of the capsule body form a sealed cavity therebetween and form an overlapping region, wherein the overlapping region is not covered by the capsule holder, to obtain a closed capsule; and
• (c) welding the closed capsule using an energy beam of hot gas or laser light on the overlapping region by forming the weld seam thereon. The capsules produced by the process according to the invention are disposable capsules and preferably contain a single dose of a pharmaceutical formulation in the form of a powder or liquid suitable for inhalation and are suitable, by their form and function, for use in powder inhalers or liquid nebulizers for producing aerosols. Aerosols thus produced can be inhaled, for example, in order to administer a pharmaceutical formulation to the lungs.