Materials:
Theoretically any thermoplastic material can be used for a blow molding operation. but practically only those material which show high hot strength and good stretch property at extrusion temperature are suitable for blow molding. The most common materials which show these properties are HDPE, LDPE, PP, medium impact PS, PVC, and PC. Acetal, nylon, ionomers and acrylonitrile styrene are other materials which may be used. But because of their cost and/or sharp melting point or other properties, they have not been proved to be as suitable as the materials mentioned earlier.
HDPE:
Advantage:
1. It has greater rigidity of the material which permits the use of thinner bottle walls and thus provide light weight, low cost packaging.
2. It has lower water vapor permeability
3. It has ability to withstand higher working temperature.
4. HDPE pack can be decorated with high speed silk screen printing method.
Disadvantage:
Considerable knowhow is required for the correct design of HDPE bottle. On an average a bottle requires approximately 2years development period and 30% of its production costs are usually accounted for R&D.
LDPE:
It is used for more flexible items.
PP:
It is used for high gloss, heat resistance, stiffness and good strength.
PS:
It exhibits good color, high glass like clarity, decorative finish pack and it will not damage sinks, glass shelves and other delicate bathroom fittings and it will not rust. Also it has a moderate cost.
PVC:
With the advances in the understanding of PVC compound chemistry, new blow molding equipments are developed; which can manufacture bottles and packs of very light weight and at a high production rate. These bottles are rigid, strong and of comparatively low permeability. It can possess glass like clarity. Thin walled PVC products have extremely competitive material cost. Main problem with PVC is its toxicity when used for the packaging of edible and potable substances. To overcome this thicker walls are necessary. This results in the increased cost.
PC:
It is used where transparency and strength are required. But this is availed at somewhat higher cost.
Other thermoplastics materials which are now coming into use for blow molding and which increase their application as blow molding material are:
Phenoxy material is used for eye wash bottles.
Methyl pentene polymer used for experimental bottles.
Cellulose acetate butyrate used for Sun Tan oil bottles.
ABS for pressure containers used for spray.
This material though expensive is used due to technical consideration like freedom of corrosion, light weight etc.
