How to Address Uneven Wall Thickness in Stretch-Blow Molding
Among the beneficial properties of PET is its self-leveling ability. This means that as one part of a preform starts to stretch first, the very act of stretching induces strain-hardening, making it tougher to stretch as a result. This forces adjacent parts that were maybe a little cooler to stretch, and in this back-and-forth of stretching and pausing, the preform develops into a bottle that may show as little as 0.001 in. difference in wall thickness around the circumference of a round bottle.
However, many bottles show much greater differences than that. As with all plastic processes, there are a number of conditions that must be maintained in order to achieve the described result. Following are the various issues that can skew wall thickness, in order of relevance:
This is by far the most common cause of wall-thickness problems. The vestige of the injection molding process must be centered to the blow mold and held there firmly when high-pressure air inflates the pre-inflated preform bubble into a bottle. Otherwise, wall thickness will go where the gate goes and no amount of self-leveling can prevent this from happening.
Here are the most common causes for this defect:
• Stretch rod does not pin the preform down enough. Typically, the distance between stretch rod and blow mold bottom should be adjusted to 0.040 in. less than the preform gate wall thickness. This will ensure that the preform cannot slip during high-pressure blow. As preforms have become thinner, these gaps have to become smaller.
• Preblow pressure is too high and/or too early. The stretch rod has to connect with the preform before preblow pressure has a chance to blow the preform off-center. When it is connected, the preblow cannot be so high as to blow the preform off the rod. The amount of preblow pressure required depends on the preform thickness and temperature.
Therefore, operators may have to experiment with different pressures to test their effect. The same is true for when to engage the preblow. In two-stage (reheat) stretch-blow molding this is most often controlled by position rather than time, and often delaying the onset of pressure keeps the gate in the center.
• High pressure is too early. High pressure may not be energized before the stretch rod has firmly pressed the preform against the blow mold bottom. Otherwise, even small differences in temperature or preform wall thickness will invariably move the gate off-center. Again, in most machines this is controlled by position, and the operator has to make sure this is adjusted correctly. Rotary machines do this automatically but many linear machines do not.
• Blow mold bottom is not machined correctly. In order to assist the stretch rod in pinning down the preform, a small well is machined into the blow mold bottom. This allows room for the small injection vestige at the end of the preform and prevents the preform from slipping.
• Bent stretch rod. Stretch rods are typically very sturdy at diameters of around 0.5 in. and do not easily bend. But lightweight water bottles require stretching out the entire base, and often rods are slimmed down at the ends to allow for a smaller contact area and cooling effect on the preform. Or, in the case of bottles with necks as small as 20 mm for cosmetic applications, the thick rods just won’t fit into them. Thin rods can bend much more easily, and even thicker rods have been known to bend occasionally. In any case, a bent rod will easily be recognized as it will skew the gate always in the same direction while other defects occur more randomly.
• Preform is bent before entering the blow mold. This is an issue that occurs more frequently in single-stage stretch-blow molding and has a different cause there. In two-stage it may happen when the preform wall thickness is uneven by more than 0.004 in. This leads to uneven heating—i.e., the thinner side gets hotter, and this side may then shrink more than the cooler side between the preforms leaving the ovens and the blow mold. In that case, the stretch rod hits the preform off-center and transports it to the blow mold in the same way.
In single–stage stretch-blow there may be another problem besides possible wall-thickness differences—non-uniform heat distribution in the preform. This is because viscous heating creates a ring of hotter material inside the molten plastic. When the runner is typically divided into two streams, hotter material is pushed more to the back than the front and this can often be measured in uneven wall thickness.
UNEVEN HEATING OR COOLING
It often baffles processors when the gate is in the center of a round bottle but the walls show differences of 0.004 in. or more. This usually indicates that one side of the preform is cooler than the other. The warmer side stretches more and so thins out. In single-stage processes, this is quite common, as noted above, but can also happen in two-stage stretch-blow. It could be that air is blowing on the preforms after they stopped rotating. I have also seen that when preforms do not spin while in the oven system, heat from the oven metal heats up the side of the preform that is turned towards it. A thermal camera is helpful here to detect heat differences and locate their sources.
SMALL STRETCH RATIOS
For the self-leveling effect described above to work, preforms must be stretched in both the vertical and hoop direction. Minimum ratios are 2:1 in the vertical and 4: 1 in the hoop plane. But design limitations, especially for small bottles below 12 oz, or the process itself (these numbers are already at the realistic maximum for single-stage) may prevent designers from implementing large enough stretch ratios. As a result the material cannot fully stretch out the cooler parts and they stay thicker.
In many cases preforms are purchased that have the right neck finish and weight but are not necessarily designed for the particular application they are used for. This can also lead to improper stretch ratios in all or part of the bottle.
Although this continent has been slow to follow Europe’s example, market forces in North America are finally shifting in favor of three-dimensional blow molding.
Today’s industrial blow molding machines are highly efficient and predictable and generally can be relied on to produce sophisticated parts from the first shot.
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