Souce: http://www.21cp.com/
“Why TPE rubber overcoating on PC cracks?” This question has been interpreted differently. Which part cracks, the TPE rubber coating or the PC part? We think most customers have met with PC parts failure.
Those who are familiar with engineering plastics surely know that cracking on PC (polycarbonate) products are quite common. PC products may easily develop cracks, mainly due to the internal stress of the product. During molding of PC products,
molecules are forced to certain orientation which causes internal stress. After the molded parts cool down, the poor flexibility of benzene rings in the molecular structure do not move or reorient themselves easily. In such situation, stress will be retained. Therefore, during molding of PC products, improper measures of stress release such as setting of a too low mold temperature and insufficient cooling time could deter the molecules from releasing their stress, in other words, the molded products still retain internal stress; when the stress goes up to certain level (exceeding the cracking threshold), the PC products crack.
When TPE or TPR is overmolded on PC products, the destruction of internal stress in the PC molded parts may surface. Particularly when overmolding in a wide area and/or on thin PC parts, the extensive shrinkage of the overmolding rubber will trigger the stress to develop cracks on PC parts.
Solution: Use higher mold temperature, properly extend the time of molding pressure and cooling shall reduce or eliminate internal stress of the PC parts.
In the case of cracking on the overmolding layer of TPE/TPR, the problem usually appears some time later after overmolding. Cracking of overmolding layer is mainly due to poor aging performance of TPR and TPE. Overmolding materials may age and crack with time passing by.
Solution: Choose TPE and TPR products with better aging performance.
