Basics The of Blow
Basics The of Blow molding of Bioplastics Blow molding applications abound for polymeric materials and represent significant opportunities for bio- polymers. The blow molding process selected (reheat stretch, injection stretch, single stage or extrusion) depends on a variety of container factors. These include: desired units, performance, size and material properties. It is important to understand the blow molding process from a material perspective, especially as new biopolymers are introduced to help determine their suitability. Reheat stretch blow molding (RHSB) systems were first developed for polyester bottle production, such as PET. Test tube-shaped preforms are injection molded, then transferred to the blow molder and fed through an in-feed wheel which loads the preforms onto spindles that carry them through the heating system. Next, preforms enter the oven where they are heated using infrared (IR) lamps. These are designed so that the maximum wavelength transmission is outside the maximum absorbance for PET. This is important because if too much energy is absorbed on the preform surface, the heat will not penetrate through to the inner wall and it will be too cold to produce a container. Reheat additives may be used to help the material absorb IR energy thus making it suitable for reheat stretch blow molding or broaden the processing window of a temperaturesensitive material. When exiting the blow molding oven, the preforms will be above their glass transition temperature or at the low end of their melting temperature range. The ideal preform reheat temperature depends upon the material choice – polyesters like PET and PLA are typically blow molded 15-25ºC above their glass transition temperature (Tg) while crystalline polyolefins, PP and HDPE, are blow molded closer to their melt temperature. Heated to its ideal temperature, the preform is then placed into the blow mold where it rests upon the support ledge near the neck finish. This support ledge distinguishes RHSB bottles from other blow molded containers as it is not necessary for either single-stage blow molding or extrusion blow molding. Finally, the blow mold closes and the internal action begins. First, the preform is stretched axially with a stretch rod. This distributes the weight properly, keeps the preform centered within the mold by guiding the preform to the bottom, and pins the gate during the high-blow pressure phase. As soon as the stretching starts, low-pressure air is introduced causing it to quickly take the shape of a balloon. The higher pressure air (up to 40 bar) is then turned on after the pre-blow stage. This completes the bottle expansion against the mold which allows the plastic to freeze in place before removing the bottle. The bottle is then removed from the mold by a transfer arm which transports it to an out-feed wheel where it is placed on the line. There are many variables present during the blow molding operation that allow it to be tailored to a specific bottle design (round, square or oval), bottle performance (top Reheat stretch blow molding machine (Photo: KHS Corpoplast) 48 bioplastics MAGAZINE [04/11] Vol. 6
Basics Examples of various materials, preforms and containers. From right to left PLA (Polylactic acid), PP(Polypropylene) and PET (Polyethylene Terephthalate) (Photo: PTI) By Lori Yoder Director, Material Applications Plastic Technologies, Inc. Holland, Ohio, USA load requirements, hot filled or pressurized), and material choice. Blow molding rates of up to 2000 bottles per hour per mold are achievable although the actual rate depends upon the equipment and resin choice, as well as preform and bottle design. Several biopolymers have been successfully reheat stretch blow molded including PLA, PHA and PEF. Each has unique material properties that must be understood to tailor the preform/bottle designs and blow molding conditions in order to produce a suitable container. As a polyester, PLA exhibits strain hardening during the orientation process. However, PLA’s temperature sensitivity can result in a narrow processing window which is frequently offset by incorporating a reheat additive into the preform during injection molding. With a natural stretch ratio slightly lower than PET, PLA may be used in existing PET tools successfully depending upon the preform/ container design. Another unique feature of PLA is its ability to flow into mold details giving very crisp definition to container artwork. PHA has also been successfully reheat stretch blow molded into single-serve containers. The material properties can be tailored to achieve different crystallization rates and mechanical properties as the material exhibits more rubber-like behavior when compared to PLA or PET. Another new biopolymer, polyethylene furanoate (PEF), has proven itself capable of producing acceptable containers through reheat stretch blow molding. Containers were successfully blown using traditional PET preform and bottle tooling with PEF by establishing the process parameters that matched the material’s stretching properties. To compete with existing petrochemical-derived materials in large volume reheat stretch blow molding applications, future biopolymers must reheat efficiently, stretch reproducibly within a short timeframe, and produce a resulting container with satisfactory performance. Injection stretch blow molding is quite similar to reheat stretch blow molding once the preform arrives in the blow mold. However, in injection stretch blow molding both the preform and bottle are produced in a single machine instead of separately. Thus, the machine speeds are dependent upon the injection molding cycle times and production rates per cavity are significantly lower than in reheat stretch blow molding. That being said, injection stretch blow molding has a strong foothold in container production, offering an alternative molding system for custom containers, jars and larger volume packages for bulk foods. Because the preform is not handled, the resulting bottle quality is more pristine than bottles produced through reheat stretch blow molding. In addition, the required space is significantly reduced from two-stage blow molding systems. Injection stretch blow molding (ISM) systems are equipped with a plasticizing screw, preform conditioning, a blow molding station and container ejector. The preforms are first injection molded and the material is cooled until it can be ejected from the mold. The preform’s remaining latent heat Principe of Reheat stretch blow molding (picture: KHS Corpoplast) bioplastics MAGAZINE [04/11] Vol. 6 49
