What Is Blow Molding? Producing Bottles, Jerrycans and Drums

Blow molding is the method used to make hollow plastic products: a tube or preform is first formed from molten plastic, the form is enclosed in a two-part mold, and compressed air is blown into it so that it takes the shape of the mold's inner surface. The cooled part is then ejected.

Shampoo and detergent bottles, milk and juice bottles, jerrycans, drums and fuel tanks are examples of products made this way.

This is the most common type. The melt leaving the extruder is shaped into a downward-hanging tube (parison); the mold closes around the tube, pinching its ends shut, and air is blown in. The excess material squeezed out as the mold closes (flash) is trimmed and usually ground and reused.

Polyethylene, especially HDPE, is the dominant material in this method. Grades with high melt strength — that is, a low MFI — are used so that the parison does not sag and thin under its own weight before the mold closes.

In large products (such as 100-200 liter drums), the parison is both heavier and hangs longer, so MFI is even lower. Accumulator-head systems, in which the melt is first accumulated and then pushed out in one shot, are common for such products.

A preform is first injection molded onto a core rod, then transferred to the blow mold and inflated. No flash is produced, and the neck and finish are obtained with high dimensional precision.

It is usually preferred for small bottles — pharmaceutical, cosmetic and personal-care packaging — and is limited to smaller parts than extrusion blow molding.

While being blown, the preform is also stretched mechanically along its length; the material is oriented in two directions, giving a lighter and stronger bottle. PET bottles for water and carbonated drinks are made this way.

Because PET's structure suits this stretching process very well, PET is the dominant material in ISBM; there are also stretch blow molding applications using clear PP (random copolymer).

MFI and melt strength: a low MFI is needed for parison stability. Our catalog includes YY-S0452 for detergent bottles, YY-S0459 for chemical packaging such as shampoo and detergent, YY-S0464 for food bottles and medium-sized jerrycans (MFI 0.36-0.40 g/10 min); YY-S00750 (0.076) for containers up to 100 liters and YY-S00356 (0.04 g/10 min) for containers up to 200 liters.

Environmental stress crack resistance (ESCR): critical in packaging for detergents, bleach and other chemicals, so that cracks do not form over time in contact with the contents. Always check the ESCR value on the grade's technical data sheet.

Density and rigidity: in HDPE, higher density makes the container more rigid, so the same strength can be achieved with a thinner wall; however, ESCR generally decreases as density increases. This balance is set according to the product the container will hold.

Food contact: for food bottles, grades with the manufacturer's declaration of compliance for food contact should be used (see 'What Are TDS, COA and SDS?').

Wall thickness distribution is controlled by parison programming (adjusting the thickness along the length of the tube); thinning at corners and edges is the most common problem in designs with a high blow-up ratio.

Keeping the blend ratio of reground flash (regrind) constant is important for consistent products from batch to batch. Although polyolefins do not absorb moisture, condensation on the surface during storage and contamination can cause surface defects (see 'Do Polyolefins Need Drying?').