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List of TOPAS® COC grades

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6. Product design

6.6 Stress crack

(1) Stress crack

When a molded product is made of an amorphous resin and is non-uniform in wall thickness or when molding distortion remains, or when great external stress is exerted on the molded product, it may be crack. This is called stress crack.

Stress crack occurs in places where force is exerted in the tensile direction and stress is concentrated. In order to prevent it, wall thickness should be made as uniform as possible to reduce differential shrinkage on large molded products. On thick-walled parts, walls should be thinned to make thickness uniform. Also, in order to prevent stress concentration, sharp corners should be avoided, and corners should be rounded or chamfered. However, there are cases where stress concentration is unavoidable due to some product shapes. In such cases, reinforcing ribs should be provided. Rib thickness should be 1/3 to 1/2 with respect to product base.

Cracks do not occur immediately after molding but several days later. In such shapes as mentioned above, make observations over several weeks. Also, contact us at design time or when cracks occur because measures to be taken differ with cracking situations, shapes, etc.

(2) Insert shapes

Extra care is needed in performing so-called insert molding, in which some part is placed in a mold and molded integrally. For example, if the insert is notched to be a whirl-stop or retainer, round off corners as much as possible to prevent stress concentration.