How to solve PC products cracking?

Thanks to the excellent physical and chemical properties of PC material,  polycarbonate crystal sheet has been widely used in electrical accessories industry for making household switches and socket products. It can not only meet required safety standards, but also has high forming speed, bright appearance and various advantages.

But it also has weakness --- large residual internal stress. When improperly used, polycarbonate sheet might crack, ultimately affecting the use of the product. Product stress cracking, will not only affect the appearance, but also the performance of the product, so PC product cracking problem is one of the most urgent problems facing current polycarbonate sheet manufacturers.

Why does stress cracking occur?

Stress cracking refers to a complex failure of plastic products, which occurs when under the simultaneous action of stress and exposure to chemical media. It is related to environment, material properties, molding methods and other factors.

Research have shown that microcracks can be eliminated by heat treatment when there is no gap formed by splitting. Under the influence of environment and external force, if microcracks grow and develop continuously, resulting in cracks and voids, such damage is irreparable.

The solution to the cracking problem

1. Raw material control

An important index of PC material is the height of molecular weight, and its detection index is melt flow rate (MFR). The higher molecular weight polycarbonate molecules have longer chain length, more interchain entanglements, and greater intermolecular force, so their cracking resistance is stronger. The higher molecular weight, the lower melt flow rate and flow performance, and the higher the strength of the molded parts. Melt flow rate not only represents the mechanical properties of the material, but also directly affects the price of the material and the efficiency of plastic molding.

2. Recycled materials control

In the process of injection molding, the manufacturer will add the waste and nozzle generated in the process of production into the raw materials. It should be noted that the added recycled materials must be treated clean and no oil can be mixed with them.

3. Product design and assembly process improvement
In order to eliminate the stress cracking phenomenon of plastic products, engineering and technical personnel when designing products should take the injection molding parts assembly into consideration, to prevent improper size and reduce deformation risk; If this problem cannot be completely avoided, it is necessary to strengthen the structure of the parts under stress for a long time (such as reinforcing or chamfering the transition part).

4. Design improvement of injection mold

(1) Optimize the location and quantity of the gate
When designing the mold, the position and quantity of the gate should be taken into consideration so as to ensure that the melt can fill the mold cavity evenly and quickly. Make sure weld wiring and weld marks appear in the insensitive area, so as to reduce the risk of cracking of injection parts.
(2) avoid right-angle connection of plastic parts, increase chamfering and arc transition

In addition, increasing flow channel, and canceling the unnecessary transverse stiffener, increasing the longitudinal stiffener, increasing the draft, and using less metal embedded parts all have obvious effect to solve the problem of stress cracking.

At present, PC material has a wide application area, especially in the electrical accessories industry. But because of the structural characteristics of polycarbonate materials and injection molding process restrictions, it is impossible to completely avoid internal stress; And the stress release process is slow, its cycle can be as long as one year, which may finally lead to product cracking. Therefore, in the production process, the manufacturer should try to reduce the internal stress or avoid stress concentration to reduce the stress cracking problem.