Stress may be introduced into a part during the thermoforming operation, or during fabrication operations such as machining or polishing. Stress may sometimes be held to a minimum by altering the processing conditions when forming parts, and by using sharp tools of the recommended types when cutting or machining. For machining instructions, see page 11.
The possibility of crazing may be greatly reduced by minimizing internal stresses by annealing the parts before cementing. For annealing instructions, see page 30.
Low joint strength may be due to a number of causes. Improper fit of parts, inadequate mixing of the cement, excessive clamping pressures, or poor technique may all help weaken cemented joints. The best way to avoid problems is to follow the recommendations in this manual.
Preparation of the joint is very important when cementing Plexiglas® MC acrylic sheet. The parts must fit accurately and smoothly without force. If the parts do not match or fit well, the edges to be cemented should be machined smooth to fit, but should not be polished, since polishing tends to round edges. Proper preparation of edges is necessary to ensure that little or no internal stress is present in the material. Such stresses will cause crazing on contact with solvent. To prevent crazing, avoid flame polishing and dry belt or disc sanding where solvent contact may occur.
Cementing Plexiglas® Acrylic Sheet to Plexiglas® Acrylic Sheet
Cementing provides a versatile and simple method for joining pieces of Plexiglas® acrylic sheet. Properly cemented assemblies of Plexiglas® acrylic sheet have nearly the appearance of a single piece; however, cemented areas of colored material, especially white translucent colors, may deviate from the original color when the joints are viewed under certain reflected- or transmitted- light conditions.
Cementing permits the manufacture of parts that cannot conveniently be made by other methods because of process limitations (e.g. part size, undercuts), low production volume, or cost. Good cemented joints are also air- and water-tight.
The two types of cement used for Plexiglas® acrylic sheet are solvent cements and polymerizable cements. The solvent cements may be used as supplied, or may be thickened with Plexiglas® acrylic sheet chips or Plexiglas® acrylic molding resin. Polymerizable cements, such as PS-30™ and Weld-On 40™ or 42™, are viscous as supplied.
Solvent cements work by softening and swelling the Plexiglas® acrylic sheet, permitting actual cohesion of the parts. After assembly, the solvents evaporate or dissipate through the material, leaving a hard, clear joint. Thickened solvent cements work the same way, but allow for longer solvent action due to slower evaporation, limit capillary flow between two closely fitted surfaces, and provide limited gap-filling capability. The polymerizable cements have little or no solvent action, but actually form new polymer in the joint, thus holding the parts together.
Both Plexiglas® G and MC acrylic sheets may be cemented with either solvent or polymerizable cements. Generally, however, a polymerizable cement, such as PS-30 or Weld-On 40 or 42, provides better joint strength and appearance.
Any of the unthickened cements may be used if the parts will not be used outdoors. The choice may be governed by the availability of the solvent or by previous experience with a particular cement. (Cement PS-30 or Weld-On 40 or 42 is best for outdoor use, however.)
For names of manufacturers of cements discussed in this manual, call the Altuglas International Polymer Technology Center at 800-217-3258 your local Plexiglas® acrylic sheet distributor, or one of the Sales Offices listed at the back of this manual.