STRETCH BLOW MOLDING

BY,JITHIN SEBASTIAN

Processing technologies for poly(lactic acid)

Poly(lactic acid) (PLA) is an aliphatic polyester made up of lactic acid (2-hydroxy propionicacid) building blocks. It is also a biodegradable and compostable thermoplastic derived from renewable plant sources, such as starch and sugar. Historically, the uses of PLA have been mainly limited to biomedical areas due to its bioabsorbable characteristics. Although PLA can be processed on standard converting equipment with minimal modifications, its unique material properties must be taken into consideration in order to optimize the conversion of PLA to molded parts, films, foams, and fibers. In this article, structural, thermal, crystallization, and rheological properties of PLA are reviewed in relation to its converting processes.Specific process technologies discussed are extrusion, injection molding, injection stretch blow molding, casting, blown film, thermoforming, foaming, blending, fiber spinning, and compounding.Today, the main conversion methods for PLA are based on melt processing. This approach involves heating the polymer above its melting point, shaping it to the desiredforms, and cooling to stabilize its dimensions. Thus, understanding of thermal, crystallization, and melt rheological behaviors of the polymer is critical in order to optimizethe process and part quality. Some of the examples of melt processed PLA are injection molded disposable cutlery, thermoformed containers and cups, injection stretchblown bottles, extruded cast and oriented films, and meltspun fibers for non wovens, textiles and carpets .

Stretch blow moldingStretch blow moulding is the mass production process for plastic containers replacing the variety.They are used for packaging of oil,wine,spirit..etcBiaxial stretch blow moulding is the method of producing a plastic container from a parison stretched in axial and transverse direction.There are 2 types of stretch moulding.

Injection Stretch Blow Moulding Extrusion Stretch Blow Moulding

SCHEMATIC REPRESENTATION OF STRETCH BLOW MOULDING

Due to the recent consumers’ heightened environmental awareness, there is a sustained interest from the food industry to replace the existing non-biodegradable thermoplasticswith PLA for certain beverage products. Today, PLA bottles are predominantly used for beverages which are not sensitive to oxygen (e.g., flat water beverages, pasteurizedmilk). While barrier properties of PLA bottles may be improved by various technologies (multilayer structures, external coating, internal plasma deposition, oxygen scavenger),their implementation is currently limited due tohigher production costs.The production of PLA bottles is based on injection stretch blow molding (ISBM) technique. This process produces biaxial orientated PLA bottle with much improved physical and barrier properties compared to injection molded amorphous PLA. The molecular orientationinduced during the ISBM process decreases the effect of aging by stabilizing the polymer free volume . The crystallites produced during strain-induced crystallizationalso reduce the aging effect since they can act as physical crosslinks to stabilize the amorphous phase, thereby reducing its brittleness. Similar effects have been reported forsemicrystalline PET . The ISBM process for PLA bottles is depicted in Fig above. It involves first the formation of preform (also known as parison) using an injection moldingmachine. The preform is then transferred to a blow molding machine where it is stretched in the axial direction and blown in the hoop direction to achieve biaxial orientation of the polymer. In the blow molding machine, the preform is heated in front of several banks of infrared heater to temperatures (85–110 ◦C) suitable for blow molding .

Different power settings are usually applied to the infrared heaters to give a temperature profile optimal for stretching the preform into bottle with uniform wall thickness distribution. Frequently, reheat additives, such as carbon black dispersed in a liquid carrier, are added to the resin in the extruder to increase its infrared energy absorption. PLA preforms have a tendency to shrink after reheat, especially regions near the neck and the end cap where the residual injection molding stresses are the greatest. This may be moderated through proper preform design, with gradual transition regions. When the preform has attained the optimal temperature, it is transferred to the blowmold. The blow nozzle is lowered to seal the preform finish, while the stretch rod travels towards the preform,at a typical speed of 1–1.5 m/s, and stretches the preform to the base cup .During the preblow phase, compressed air of 0.5–2.0MPa is admitted to the preform through the blow nozzle to partially inflate the preform to prevent it from touching the stretch rod during the axial stretching. When the stretch rod arrives at the base cup and pins the preform to the mold base, the air pressure ramps up to 3.8–4.0MPa to fully inflate the preform.This forces the inflated preform to take the shape of the blow mold and to imprint the surface details of the bottles. The high blow pressure is maintained forseveral seconds to allow the bottle to cool down sufficiently before discharging the bottle.

Thermal history of PLA polymer during one- and two-stage PLA bottle manufacturing.

As shown, PLA preform made in the one stage process does not go through the aging process during which the polymer tends to embrittle. Thus, PLA preformsintended for one- and two-stage processes may need to be designed and processed differently. The neck finish of the preform is highly amorphous and is quite brittle. Therefore, the neck finish must be designed such that the side wall is thick enough to prevent the neck from blowing out or cracking due to the compression load from the blow nozzle. The blow mold temperature for PLA is typically set at around35 ◦C. Because the base of the bottle tends to be quite thick,the residual heat can cause the base to roll out after the bottle is ejected from the blow mold. This problem can be overcome by incorporating radial ribs to reinforce the base or chilling the base mold insert to a temperature lower than the mold halves.

Schematic representation of PLA preform (left) and bottle (right), showing their key features and main stretch ratios used for preform design

Similarly to PET, PLA exhibits strain-hardening when stretched to high strain. This self-leveling phenomenon is desirable for blow molding of preforms to achieve optimalbottle side wall orientation and minimize wall thickness variation. Since strain-hardening occurs onlywhen the PLA is stretched beyond its natural stretch ratio, the preform must be designed tomatch the target bottle size and shape, such that optimal stretch ratios are achieved during blow molding. In contrast,Over stretched bottles can also result in stress whitening due to the formation of micro-cracks on the bottle surfacesthat diffract light. Preform designs are often proprietary, and therefore there is a lack of information in the open literature. An optimal preform design should meet the minimum required stretching which is above the natural stretch ratio, by varying the shape, diameter, length, blend radius, and transition features, to meet the part weight requirement. Depending on the shape of the bottle, subtle but critical features suchas transition shape (reverse versus standard taper), step changes, and pinch points on the core and cavity may also be incorporated in the preform design. Since the stretchingbehavior of PLA is similar to PET but not entirely the same,conversion of materials using existing PET preform designs may be feasible, although design modifications are oftenrequired to achieve an optimized bottle.