Although you may not find a blow molded product right in front of you, chances are that you will use one before the day is up. Blow molding creates parts much like roto-molding in that the parts come out of the tool hollow, yet blow molding has a couple distinct differences. Blow molding allows for very large quantities at lower costs, something that is unavailable to roto-molding. Because of this, blow molding is a great choice to save you money when your roto-molding quantities become high enough.
Blow molding creates parts that, like roto-molding, are hollow when removed from the tool, yet the process of making the parts is completely different. In blow molding a complex tool filled with cooling lines, is closed around a paraffin (melted plastic that looks much like someone blowing up a balloon). Once the tool has closed, air is forced into the paraffin at high pressures which blows the plastic out against the inside of the mold. Now the cooling lines come into play as they rapidly cool the part. The tool is now opened and the part removed, where any secondary trimming of the part takes place. The entire process usually takes less than 2 minutes meaning that in a 12 hour day over 1440 pieces can be made, and this is on the low side.
Blow molding becomes a necessity when part numbers start ranging in the thousands per year. Although thousands can be made each day, this doesn’t mean that these high quantities must be met to justify a blow molded part. Fact is that once the cost of a blow molded tool becomes cheaper than the cost of roto-molding the pieces, then blow molding is the way to go. The issue with blow molding is that the tool is usually a large investment, in some cases being 6 or 7 times as much as a roto-molding tool. The good thing about this is that each piece that comes out of the tool is usually much cheaper than roto-molding. This is why once your quantities get high enough to justify a blow molded tool, it’s the way to go.