Dear sea friends: What are the common types, materials, and manufacturing processes of plastic end caps?

The common materials for plastic end caps are polypropylene (PP) and polyvinyl chloride (PVC); they are usually standard elliptical end caps with straight edges of about 50 millimeters； There are two heating methods: far-infrared heating and resistive heating ； PP can be fully formed at diameters below 1000 millimeters, while PVC can be fully formed at diameters below 1200 millimeters; for larger diameters, it is formed in sections, cut, welded together, and then the top circle is welded on.

Types of end caps: End caps include designs such as convex end caps, conical shells, transition sections, flat covers, and reduced diameters. Convex end caps include: hemispherical end caps, elliptical end caps, disc-shaped end caps, and crown-shaped end caps. From a stress perspective, convex heads become less favorable as they move from hemispherical heads, but in terms of manufacturing difficulty, they become easier to produce. 2 Methods of manufacturing end caps a) Small end cap: integrally formed ； b) Large and medium-sized end caps: Splicing first, then shaping – this is the most commonly used type; the requirements in the standards mainly apply to it ； c) Extra-large end caps: Due to requirements related to transportation and opening, they are first formed in segments and then welded together. 3 The splicing position of the end caps: There should be a specified distance for splicing, which must be greater than 3δ and not less than 100 mm. (The welding heat-affected zone is a high-stress area, and the chemical composition in this zone can be altered as a result of welding.) Therefore, it is necessary to avoid high-stress areas, which are related to the thickness. Based on practical experience, the stress decay length should be greater than 3δ and not less than 100 mm. However, refrigeration equipment finds it difficult to meet this requirement due to its specific characteristics. Avoiding splicing at point r of the disc head reduces thickness and increases stress. When splicing, the weld direction is restricted to only radial and circumferential. In the future, this requirement might be removed for large end caps. 4 Weld joint factor for spliced end caps: For end caps that are formed after splicing, the splicing welds must undergo 100% radiographic or ultrasonic inspection, and the acceptance standard shall be determined in accordance with that of the equipment casing. The inspection level and ratio for the finally formed weld are the same as those for the equipment housing; using a higher level is wasteful. Example: If the equipment housing is inspected at 20%, it passes the III level. The header butt welds and the final welds also met the III standard, with a weld joint factor of 0.85 ； If the equipment housing is 100% inspected, it is considered qualified for II. The header butt weld and the final weld also met the II standard, with a weld joint factor of 1. Therefore, although end caps are inspected 100%, their acceptance levels vary and are determined based on those of the equipment casing. But attention must be paid to the manufacturing process: the correct sequence is: material cutting (marking) – assembling small plates into larger ones – shaping – non-destructive testing. It is incorrect to conduct testing before shaping, as this cannot guarantee that the product will still meet the standards after shaping. In other words, non-destructive testing refers to the final non-destructive testing