TABLE OF CONTENTS It was stated previously that blow moulding is the preferred method of making hollow containers, this requires the designer and engineer to think in different terms. Generally most have been accustomed to thinking in terms of shells because plastic parts are predominantly one-dimensional objects which are injection moulded or thermoformed. Now the consideration is three-dimensional.
The understanding of the blow moulded structure, its advantages and disadvantages and how the configuration can best be utilised is the first step to making a successful part. When looking at a part being considered for the blow moulding process the following considerations are to be made: where the mould separates (part line), allowing for draft of the part (removal from mould), layout, blow ratio, corners, finishing and appearance. The following is a list of these terms with definitions:
Draft of the Part all surfaces which are parallel to the direction of the movement of the mould should have taper or draft for easy part removal.
Layout of the Part is dependent on its outer shape and configuration.
Blow Ratio All parts are shaped by the process of blowing air into the parison. To understand blow ratio one needs to think of a balloon, which when it is blown up, has a much thinner wall.
When the parison enlarges it fills the mould space and the wall thins out. The mould therefore determines the outer diameter (OD) of the part. The difference between the...
