PETpla.net Insider 08 / 2017

BOTTLE MAKING 34 PET planet Insider Vol. 18 No. 08/17 www.petpla.net injection tools, it is outright daunting in its scope. A more efficient method is to check only the so-called “hot cavi- ties.” In Chapter 8 I have explained the mechanics of viscous heating, the fact that the resin heats up unevenly as it moves through the barrel and hot runner channels. While this has undesired effects in injection stretch- blow moulding with respect to the bottles, in injection moulding the effect is that the preforms in some cavities are always warmer than the majority of the others. This higher tempera- ture also increases the AA level in the affected cavities and fortunately it is always the same cavities that show this behaviour. After establishing the performance of all cavities, lab personnel can con- centrate on the “hot cavities” and still make valid assumptions about the AA level of the entire tool. 3 Reheat stretch blow moulding machines 3.1 Overview In polyethylene terephthalate (PET) bottle manufacturing, reheat stretch blow moulding (RSBM) machines are the second part of the so-called two-stage process, the first part being the injection moulding of preforms. All RSBM machines use injection-moulded preforms, heat them up, and stretch and blow them in a blow mould. However, besides these rudimentary basics, there is now a variety of machine types avail- able for all output and quality require- ments. A potential buyer should be aware of all the various differences in order to make an informed decision on what machine to purchase. In the single-stage process, both preforms and bottles are manufac- tured in the same machine. The single-stage process is quite different from the two-stage process in many respects. There are also machines referred to as integrated two-stage machines, which fall between the two categories. These machines inject mould preforms, transfer them to mandrels, and rotate them in front of a short section of infrared lamps or other device before blowing. See also Chapter 8 for a discussion of single- stage machinery, integrated two- stage machinery, and a comparison between these processes. 3.2 Semi-automatic machines Most semi-automatic machines are manufactured in Asia. They usu- ally comprise a stand-alone oven section, set next to a blow clamp. An operator places one or two preforms on mandrels that spin through the oven section. He also takes one or two preforms that have gone through the ovens off the mandrels and places them into the open blow mould. Typically, closing of the blow clamp requires a combination of two buttons to be pressed in order to protect the operator from injury. The mould closes and stretchblows the bottle(s). The operator subsequently takes the bot- tles out, adds preforms to the man- drels, and the process starts all over again. Outputs vary from 60/h for 20l (5 gal) water bottles in single cavity machines to 800 half liter bottles/h in dual cavity machines. Most machines are sold with at least one mould, a compressor, and possibly a small chiller. Quality varies between manufacturers, and buyers should vet each supplier or purchase through a distributor to avoid disap- pointment. This is discussed in detail in Chapter 11. 3.3 Linear shuttle-type machines In these machines, blow moulds are mounted together and all move with a common cylinder. Machines with up to eight cavities have been built (see Fig. 3.1), but the most common models have one to four cavities. Thus, these machines cover the lower output range of applications, often competing with the single-stage process. Preforms travel from a common hopper to an unscrambler via an incline conveyor and fall between two rotating and inclined rollers so that gravity forces them to slide down the rollers sup- ported by their transfer ring (Fig. 3.2). A rotating flap prevents unscrambled preforms from reaching one (or more) rail(s) where preforms now hang and glide down toward the pick-up station. Preforms are still right side up and are usually turned upside down via a pick- up device that consists of the appropri- ate number of grippers driven by four pneumatically-driven cylinders. Figure 3.1 Layout of linear, five-cavity machine. Diagram courtesy of Amsler Equipment Inc. Figure 3.2 Most preforms are blown upside down on this type of blow- moulding machine. Photo courtesy of Chumpower Machinery Corp. These linear machines index, and grippers deliver, the appropriate number of preforms in one motion to the waiting mandrels (Fig. 3.3). Most mandrels are made of aluminium, with a diameter just below the minimum inside diameter of the preforms.