Stephen Barter from the Australian Institute of Packaging sheds light on the best ways to approach blow moulding and the influence it can have on a package’s effectiveness.
The strength and success of a bottle design, regardless of its contents, is in every case a direct descendant of the bottle shape. The shape – which includes the corners, handles, necks and panels – governs both the physical performance in filling and the degree of difficulty in moulding the bottle; the two are delicately connected and cannot be separated.
Useful pointers and guidelines when blow moulding:
One of the important facts about moulding plastic is it shrinks as it cools; the longer it takes to cool the more it shrinks. The mould acts as a heat transfer mechanism removing the heat from the plastic at 190 degree celsius down to less than 70 degree celsius at which point the plastic is a solid state and no longer liquid.
As the plastic takes the shape of the mould, the plastic becomes variable throughout the bottle, thicker cross sections shrink more than thinner sections inducing stress within bottle features and creating issues like buckled label panels. The best method to reduce this effect is to design contours to reduce thicker sections.
Features can be added to the shape to improve mouldability and function and do not detract from the appearance. Features like corners act as hinge points for slumping and top-load weakness. Using variable radii in all the corners will increase structural strength; fixed radii will reduce structural strength.
Neck – orientating the thread start over the part line (opposite the handle) will improve the capping process, the bottle is able to handle downward pressure from the capper when the thread start of the cap lands on top of the thread start of the bottle. In these conditions the bottle is able to bounce back after being hammered by the capper.
Shoulder – making the bottle strong through the shoulder by keeping surfaces in the form of an upside down funnel and compound radii allows the top load to be dispersed down trough the bottle into the base. Compound curves reduce plastic and ovality in the neck finish, especially on oval shaped bottles.
Handles – the closer to square or round for the handle crossection the easier the handle will be to mould. As the depth of the handle increases in relation to the handle width the risk of webbing increases, a major quality complaint.
The above rule applies to blending the handle out into the bottle; try to avoid blending the handle by increasing the depth of the handle only without increasing the width in similar ratios.
Panel features: 2D or conical curves in label panels make labelling easy. 3D curves in panels will lead to the labels lifting and bubbling. The reason for a conical shape in particular is the increase in the bottle topload strength.
Bulging in the panel is difficult to control, especially with thin walled bottles. Start the design with bulge in the shape and work back by changing only the label panel back to a 2D curve.
Imagine the bottle shape you have in mind and then partially blow up a balloon inside this bottle shape. Take particular note of the corner radii and then factor these into the shape of the bottle. These will not only improve slump resistance, it will also make the bottle a lot easier to mould.
- Base corners: base controls the topload strength and stability of the bottle on-shelf. Using a simple radius in the base corner will result in poor topload, slump resistance, base rock as well as difficult moulding. Compound curves are essential here.
It is paramount the bottle designer is aware that most design flaws cannot be ‘processed out’ by the moulder. Quite often an over-confident moulder can underestimate these details, which can significantly delay, and add substantial cost to new projects and the finished bottle weight – costs that may be carried for the life of the bottle.