The rotocasting of a plastisol consists of pouring a given amount of plastisol into a mold, closing the mold, and rotating it in two planes simultaneously while heating. The vinyl fuses in to the shape of the mold. Rotocasting produces a totally enclosed hollow part. This process is used for making such things as vinyl beach balls, boat bumpers, squeeze toys, bicycle seats, doll parts, etc.
1. The mold surfaces should be coated with a mold release such as MRV 1000 IPA. See MRV 1000 IPA instructions.
2. Pour a predetermined amount of plastisol into the mold cavity. The amount of plastisol used depends on the thickness of the part desired and the surface area of the part. Ten ounces of plastisol per square foot of mold surface will yield approximately 1/8 " (125 mils) thickness.
3. Close the mold and begin the rotocasting process. The sections must fit snugly and be securely fastened or the mold will leak. Generally, an oven temperature range of 450° to 550°F is used for 5 to 15 minutes depending on the the molds size, weight, material, type of plastisol, and shape of the molded part.
4. After a cure period which is determined experimentally, the mold is cooled with water or oil. A bath or generous spray to all mold surfaces at the same time is essential for a uniform part because plastisols lower than 60 Shore A durometer will tend to hot flow causing a deformed part.
Common Problems in Rotocasting Plastisol
1. Part is weak or crumbly Under cured. increase heat or cure time.
2. Uneven inner Surface There will always be some runs or drip marks inside a rotocast part. Jelled or lumpy plastisol can be strained and reused (would also indicate insufficient cure).
3. Blisters or bubbles Moisture or oil contamination if using oil bath to gel plastisol. Requires lime to dry out plastisol / scavenge water. Oil contamination ruins the plastisol.
4. Off color burn spots Plastisol is over cured. Decrease temperature or cure time.
5. Sticking in the Mold Plastisol is burnt in the mold or molds are dirty. Clean mold and reapply mold release.
6. Off color, Hardness Variation Plastisol separation / settling. Stir before using.
7. Pin Holes Mold may be porous and gassing. Seal or repair mold, or use a new mold.
8. Some Areas Heavier than Others (plastisol> 60SA) Uneven heating, mold rotation, or mold of uneven thickness. Check heat source and rotational ratio. Check mold thickness.
9. Problem #8 Softer than 60SA Same as # 8 or cooling may be uneven. Check cooling system to make sure even cooling is taking place.