Rotational molding cycle times

As expected, the largest portion of the cycle time is the cooling of the blow molded container in the mold cavity. Most machines work with multiple molds in order to increase production. Rotary molds are often used in conjunction with vertical or horizontal rotating tables (Fig. 3.59 [14]). 

Fig. 4 (A) Stress relaxation curve of rotational molding grade ethylene copolymers. (B) Part density, which is indicative of the bubble content, measured after interrupting the rotational molding heating cycle at different heating times. (From Ref.. )...

Gogos, G. Liu, X. Olson, L.G. Cycle time predictions for the rotational molding process with and without mold/part separation. Polym. Eng. Sci. 1999, 39 (4), 617-629. 

In IMA, different types of materials can be injected into a single tool that rotates. For instance, a polyamide material can be injected into injection port 1 injection area 2 can be PBT or some other compound and injection port 3 can be a third material (a different polyamide, perhaps). Cycle times for this process are typically in line with regular injection molding however, a savings in time is realized because a complete part is ejected from the machine. 

The middle seventies saw the advent of high-speed injection molding. For thin wall parts, the production rate was limited by the hydraulic response of the machine. Nitrogen assist accumulators help inject the melt to reduce the overall cycle time. The early seventies also saw the introduction of co-injection molding. In this process a first melt is injected and cooled. The cavity is rotated and the second melt is injected to form the finished part. Usually both injections use the same material but a different color. The process is extensively used for calculator keys where the different colors eliminate a printing step. 

With rotational molding, a closed mold is rotated in two or more planes during fusion or solidification of the fluid polymer system. The action of the rotation deposits an even coating of polymer upon the inside of a "clam shell" type mold. After a predetermined time under heat, the molds are removed from the oven and cooled, and the finished product is removed whereupon the cycle is started again. Conventional gas fired ovens or hot salt bath sprays are used for heat transfer, and cooling is usually effected by spraying or immersion in cold water. This technique is well suited to molding completely hollow objects. 

Typical applications of rotational molding include chemical tanks, automotive and commercial aircraft parts, backyard play equipment, toys, etc. Rotational molding requires long cycle time, which puzzles the overall success of the technology. The cycle times are restricted by the time required to heat up and cool down the mold and the product by the slow heat convection process. 

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