TPE encapsulation PC processing method and analysis of its cracking problem!
Characteristics of TPE encapsulated PC material
TPE thermoplastic elastomer is a SEBS rubber modified material with rubber elasticity, which can be easily injection molded or extruded. The hardness range of the rubber material can be adjusted between ShoreA 0-100 degrees. TPE rubber materials commonly used for encapsulating PC have a hardness range of 30-80 degrees
TPE encapsulation PC processing method
TPE encapsulated PC generally adopts the conventional molding method, which uses two sets of molds. One set is used to inject PC products first, and the other set of molds (called encapsulation molds) is used to inject TPE rubber material onto the surface of PC products. This type of molding method is called secondary injection molding, overmolding, or insert injection molding. More advanced two-color injection molding machines are also used for processing, and the results are the same. Through injection molding, TPE and PC are well bonded together. The injection molding temperature for adhesive coating varies depending on the hard adhesive material of the coating. Due to temperature considerations, both hard rubber and TPE soft rubber materials need to soften in order to achieve a certain level of compatibility between the two materials at the contact surface. To achieve the true encapsulation effect, the injection temperature of TPE encapsulation PP material is usually around 170-190 ℃; The encapsulated ABS material is around 220-220 ℃; The temperature of the encapsulated PC material is slightly higher than the injection temperature of the encapsulated ABS material.
Analysis of TPE encapsulated PC rupture problem
Some customers have experienced cracking and deformation of the PC hard rubber inside the product after it was molded. When the product was made and tested outdoors, the joint between the soft and hard rubber cracked and rotted as soon as it was torn off. These issues need to be analyzed from several aspects such as materials and molds.
1. Cracking of TPE/TPR coating layer
TPE encapsulation PC, TPE coating layer rupture (crack), this situation usually occurs after a period of time of encapsulation molding. The main reason for TPE cracking is the poor aging resistance of TPE, which can lead to material aging and cracking over time.
Solution: Adopt TPE product grades with excellent aging resistance to enhance the material's resistance to aging and cracking.
2. Cracks (ruptures) in PC components
Those familiar with engineering plastics are likely aware that cracking in polycarbonate (PC) is a common issue. The problem of cracking in PC components is mainly caused by internal stress. PC materials undergo forced molecular orientation during molding, but after cooling during molding, the presence of a benzene ring structure with poor flexibility makes molecular de orientation very difficult.
Therefore, if the de orientation measures are not handled properly during the molding of PC parts, such as setting the mold temperature too low, insufficient cooling and pressure holding time, etc., the molecules cannot be fully relaxed and de oriented, resulting in internal stress in the molded product. When the internal stress reaches a certain level (greater than the cracking force), the PC part will crack.
When TPE is used for secondary injection molding of encapsulated PC parts, the destructive effect of internal stress is precisely manifested during the encapsulation of TPE due to the large internal stress inside the PC parts. Or when using TPE for encapsulation, the TPE encapsulation area is relatively large, and the TPE coating layer will shrink during encapsulation molding. At the same time, the thickness of the PC part is relatively thin, and the shrinkage of the TPE coating layer will promote the effect of internal stress, leading to cracking of the PC part.
Source: UTPE elastomer portal
