Analysis weld seam weak in blow molding large parts made of commodity plastics

Blow-molded parts tend to present failure problems on its welding line. This problem has been studied on three parts with different morphologies. Cooling rate directly affects semicrystalline thermoplastics crystallinity, so an increase in the crystallinity degree means an increase in polymer fragility. Differential scanning calorimetry (DSC) allows us to analyze the crystallinity degree of polymers, showing different crystallinity degrees on different areas of the blow-molded parts. polyethylene blow-molded parts tend to present failure problems on its welding line while being used. This problem has been analyzed on three different parts with a general failure. To determine the possible cause, the crystallinity of the part has been analyzed on the failure area and also on areas without problems. By means of this technique, there have been found appreciable differences in the cooling process of different areas of the part. Material in the failure area presents a lower crystallinity (it has a lower fusion energy on the first cycle fusion peak), thus having cooled down more quickly. On the other hand, fusion energies in the non-failure area are quite similar on both peaks, so it has cooled down more slowly. The different cooling rate can be explained mainly due to the thickness variation between different areas of the part. Factors such as the cooling system and the parison flow direction may also influence. Specific software has been used in this study to simulate the areas with a higher stress concentration during the process, thus being able to analyze part failure on service.