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Article | November 30, 1995
Single-stage is on stage at Coke plant
The great debate
According to SIPA one advantage a single-stage system has over two-stage reheating and blowing is that in single-stage production the temperature profile of the preform can be closer to the ideal stretching profile. In this ideal profile the internal temperature of the preform is higher than the external temperature so that the crystallinity induced by the stretching ratio in the internal material matches the crystallinity induced in the external material. This warmer-on-the-inside temperature profile is produced in single-stage production because when the preforms leave the molds the outside and inside surfaces are cooler than the center of the wall due to the cooling effects of the core and cavity. In this state the preforms proceed immediately through conditioning and blowing. In reheat and blow production the ambient-temperature preform cools completely and must be warmed up. Despite the existence of two-stage reheating devices designed to produce preforms that are warmer on the inside at blow SIPA contends that no such system can produce preforms as close to the ideal stretching profile as a one-stage system. Thus bottles with higher crystallinity can be produced by means of the one-stage process says SIPA and higher crystallinity usually translates into better mechanical properties including CO2 retention and top load strength. The firm goes on to claim that because SIPA-produced bottles have higher crystallinity they offer lightweighting opportunities compared to bottles produced on a two-stage system. Not surprisingly the leading proponents of the two-stage method see it differently. Among those suppliers are of course Husky and Sidel the former a maker of preform injection molding systems and the latter a maker of stretch blow molding machines. In a paper given at Bev-Pak Americas '95 last April Husky's Ken Halsall and Sidel's Jean-Guy Delage insisted that two-stage systems are the ones that produce bottles with better crystallinity and allow cost savings through lightweighting. "One-step systems rely on a conditioning station to achieve heat profiling [of preforms]" Halsall and Delage told their audience. "Since bottle output is determined by the time for injection and temperature conditioning there is usually only 15 to 20 seconds to do this. Since this isn't long enough for temperature profiling adequate material distribution is almost impossible to achieve with short thick-walled preforms. Therefore the one-step preform must be a relatively long thin-walled design which reduces bi-orientation and bottle performance." Halsall and Delage showed their Bev-Pak audience how the shorter thicker preforms used in two-stage technology yield greater bi-orientation. They also described the quartz lamps of the reheat ovens used in the two-stage process as the best means of achieving optimum preform temperature at blow. Temperature distribution in the preform during one-stage production they claimed is inferior. "Even after conditioning" they told their audience "the temperature profile is not yet optimum. There just isn't enough time available to optimize the temperature profile especially if the cycle is pushed to try to improve the economics of the process." So who does Coca-Cola Amatil's Nick Tilley side with in the great preform profile debate? Neither. "I believe the potential for lightweighting is available through both single-stage and two-stage equipment" says Tilley. Bev-Pak Americas '95 was sponsored by Directions 21 Inc. (Sarasota FL).
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