Laser welding system and method using cooling mask to control the width of the weld

A laser welding system comprising:

• a laser beam source configured to emit a laser beam having a wavelength of about 2 microns;
• a first workpiece made of thermoplastic material that is optically transparent and partially permeable to said laser beam;
• a second workpiece made of thermoplastic material that optically transparent and partially permeable to said laser beam;
• a pair of clamping plates positioned and configured to engage opposite sides of said first and second workpieces when said first and second workpieces are adjoining each other with contacting surfaces, said pair of clamping plates being positioned relative to the laser beam source such that both said first and second workpieces partially absorb said laser beam to cause heating and melting in respective portions of said first and second workpieces;
• an actuator configured to urge at least one of said pair of clamping plates toward the other one of said clamping plates to press said first and second workpieces together;
• a transparent glass plate;
• a mask on the opposite side of said first workpiece from said second workpiece, said mask being impermeable to said laser beam and forming a slot for passing said laser beam to the portion of said first workpiece exposed by said slot, so that heating and melting of the material of said first and second workpieces is limited to the width of said slot, said mask being positioned on said transparent glass plate to absorb heat from said first workpiece to cool the portions of said first workpiece not exposed to said laser beam;
• and
• a drive unit configured to move said laser beam to illuminate said slot.