Environmental Stress Cracking (ESC)
Blow moulding is a conversion of thermoplastic material into a hollow article using air pressure in a mould.
Blow moulding is accomplished in two steps:
- the making of a starting tube from molten plastic , called the parison in the case of an extrusion moulding process, and preform in the case of an injection moulding process; followed by
- inflation of the parison or the preform to form the desired final hollow article
Various types of polymers are used to make plastic bags, like HDPE, LLDPE, and mixtures of LDPE and LLDPE. To process PP flims a significantly higher cooling is needed than for PE, due to its slower crystalization rate. Even the die position is turned vertically downwards to enable water cooling.
image source: http://www.silganplastics.com/capabilities-technologies/extrusion
In the extrusion blow mouldingprocess, the rotating screw will force the molten plastics through a die head, equiped with a blow pin,to form the parison. The parison is extruded vertically between two open mould halves, so they can enclose the parison and blow the pin. Relatively hot compressed air flows through the blow pin to inflate the parison, which will fill the mould wall. The article can be removed and trimmed for the final result. The process is shown as follows:
In the injection blow moulding process, the preform (or parison), is injection moulded rather than extruded. The first step is to make the preform with an injection moulding machine. In the secondstep, the preform is transfered to a blow mould. In the third step , the soft preform is inflatedwith hot air from a blow rod to fill the cavity of the blow mould. In the last step , the blow mould is opened and the blown product is removed. No trimming needed.
image source: http://www.sinotech.com/blowMoulding.html
One variation of injection moulding is called injection stretch-blow moulding. After producing the preform by injection moulding, the blowing rodsextend to the lower end of the preform, stretching the preform, and after that the air is blown, giving the polymer a bi-orientation condition, resulting in better clarity and rigidity of the article while reducing material consumption. The most widely application of this technique is in the production of PET bottles for drinking water. The low permeability of the material is much improved with the injection stretch blow moulding process. Carbonated beverages now donot need any more heavybottles such as glass bottles.
As a packagingmaterial, PE, especially HDPE, at a service temperatureof up to 60oC, is highly resistant to a wide variety of reactive chemicals such as inorganic acids, alcohols, fatty oils, food chemicals and detergents. However, bottles or jerrycans made from PE can crack when exposed to the same chemicals if they are streched, bent or stressed. Thus if conditions are unfavourable, various oils or even common cleaning agents may cause PE tanks, drums, bottles, jerrycans to crack after only a short period of time. Bottles or other articles, designed to contain detergents, solvents, acids or bases must be manufactured from material that exhibits a high resistance to 'Environmental Stress Cracking', ESC.
The failure of a container to resist ESC can originate from the residual stress acquiredduring the extrusion or moulding process . The 'agressive' liquid seepsthrough the minute cracks of the PE macromolecule chains. This minute crack is a result of the macromolecular breakdown in the case of agressive chemicals, and wetting phenomena in the case of surface active chemicals. In either case the surface tension of the solid plastic layer is reduced. ESC accelerates at higher temperatures and high external stress such as high top load storage.
Higher crystallinity, higher density and higher average molecular weight of the container material will reduce the ESC.