Volumes of production
Unlike injection blow molding (see p.129), which offers the possibility of production runs extending into the millions, extrusion blow molding can be used on much shorter runs, sometimes as low as 20,000.
Unit price vs. capital investment
Although lower in cost than injection blow molding (by about a third), it is still an expensive setup.
Speed
As with other, similar methods, the production rate is determined by the weight of the part: a typical gallon container can be produced at a rate of 1,000 per hour (using a single machine, with four molds running concurrently). Blow-molded milk bottles of the sort found in supermarkets can be made at a rate of around 2,000 units per hour.
Surface
Excellent finish.
Types/complexity of shape
Extrusion blow molding is suited to the production of larger and more complex shapes than injection blow molding, notably the integrated handles on plastic milk containers, or on the large fuel containers you can find at gas stations.
Scale
Although it is capable of producing products such as shampoo bottles, extrusion blow molding is also suitable for small runs and can be used to make products at the larger end of the blow-molding scale, typically more than about 20 fluid ounces.
Relevant materials
Polypropelene (PP), polyethylene (PE), polyethylene terephthalate (PET), and polyvinyl chloride (PVC).
Typical products
Extrusion blow molding is best suited to larger products, which might typically include toys, oil drums and car fuel tanks, and large detergent bottles.
Similar methods
Injection blow molding (p.129) and rotational molding (p.137).
Sustainability issues
A small amount of excess plastic produced from runners where the molten plastic was fed in is trimmed from each mold. This waste can be heated into a molten state again and recycled back into the process to reduce material consumption and prevent waste. Additionally, at the end of the product’s lifespan the plastic can be recycled to reduce the use of raw and non-renewable materials.
Further information
www.rpc-group.com
www.bpf.co.uk
www.weltonhurst.co.uk
Dip Molding
Product |
balloon former (far left) and balloon (left) |
Designer |
Michael Faraday created the first rubber balloon in 1824 |
Materials |
earthenware ceramic former; latex balloon |
Manufacturer |
Wade Ceramics Limited (balloon former) |
The simple ceramic former perfectly illustrates the principle behind dip molding, showing how hollow products—such as this party balloon—are made.
Dipping a shape into a material that has been melted (or is in an otherwise liquid state) is possibly one of the oldest methods of forming shapes. It is also one of the simplest techniques to understand, and, in terms of tools and molds, it is one of the cheapest methods of producing plastic products.
To be presented with a ceramic former, such as the one illustrated here, is to be given a gem from the usually hidden world of manufacturing. Artists (particularly Rachel Whiteread in her award-winning 1993 concrete sculpture House) have often explored the negative spaces within our environments. In a similar way, these little gems give us a unique view into the world of production from an angle that is rarely seen. The bulbous shape triggers a small flash of recognition, but you cannot quite put your finger on what it is until someone tells you that the shapes are ceramic formers for making balloons.
In principle, the process of dip molding is incredibly straightforward. As the name suggests, you simply dip a former into a liquid polymer bath, let it cure, and peel it off. In reality, it is a little bit more complex than that, because dip molding is a process that can be adapted to many different materials and setups, although the basic idea stays the same.
An automated production line showing the dipping of ceramic formers to make rubber gloves.
A vat of sky-blue latex being used to produce party balloons.
– Highly cost-effective for short production runs.
– A prototype former and sample moldings can be produced in a matter of days.
– Limited to simple shapes.