Some plastics are sensitive to stress and easily generate internal stress during molding, resulting in brittle and easily cracked parts. Cracking may occur when the parts are subjected to external forces or solvents. To avoid this issue, additives can be added to improve the anti-cracking properties of the raw material. It is also important to pay attention to drying the raw material and selecting appropriate molding conditions to reduce internal stress and increase the resistance to cracking. Furthermore, choosing a suitable shape for the part and avoiding measures that concentrate stress, such as embedded parts, can also help to minimize stress concentration.
When designing the mold, increasing the mold release angle, selecting appropriate feed ports and ejection mechanisms, and adjusting the temperature of the material, mold, injection pressure, and cooling time during molding can all help to reduce the risk of parts becoming too fragile and cracking during demolding. After forming, TPO thermoplastic can also be treated to improve their resistance to cracking, eliminate internal stress, and avoid contact with solvents.
Each plastic has different thermal properties, such as specific heat, thermal conductivity, and heat distortion temperature. Plastics with a high specific heat require more heat during plasticization, so injection machines with higher plasticization capabilities should be used.
Plastics with a high heat distortion temperature can have shorter cooling times and be removed from the mold earlier, but extra care must be taken to prevent cooling deformation after demolding. Plastics with a low thermal conductivity, such as ion polymers, have a slow cooling speed and thus require adequate cooling and strengthened mold cooling effects. Hot runner molds are suitable for plastics with lower specific heat and higher thermal conductivity. Plastics with high specific heat, low thermal conductivity, low heat distortion temperature, and slow cooling speed are not suitable for high-speed molding and require appropriate injection machines and strengthened mold cooling.
Different plastics require appropriate cooling speeds based on their characteristics and part shapes. Therefore, molds must be equipped with heating and cooling systems as required during molding to maintain a certain mold temperature. When the material temperature causes the mold temperature to rise, it should be cooled to prevent deformation after demolding, shorten the molding cycle, and reduce crystallinity.
If the remaining heat of the plastic is not sufficient to maintain a certain mold temperature, the mold should be equipped with a heating system to control the cooling rate, maintain flowability, improve filling conditions, or control the part to be slowly cooled to prevent uneven cooling of thick-walled parts and improve crystallinity, etc. Therefore, molds should have corresponding cooling or heating systems as needed.