As bottles become ever lighter and thinner it is crucial that processors know how to transform preforms into bottles with as even a wall thickness as possible. This article describes the most common methods to do that. They are quite different between single and two-stage stretch blow molding as we will see.

There are two major and two minor elements that processors can use to control wall thickness in the vertical axis:

  • The heating lamps in the oven system, their number, voltage, and position in two-stage
  • Injection speed and preform design in single-stage
  • The pre-blow setting, both onset and pressure
  • The oven fan setting
  • The mold temperature, especially for oblong bottles

In a typical two-stage machine there are typically 9 lamps organized in rows in one oven. Distance between these lamps varies between 13 and 19 mm. The entire oven bank can be adjusted in height to suit different neck finishes. Many machines feature as many ovens as blow cavities and some have more. Lamps are controlled by voltage regulators often as percentage value of the maximum of 220 V. In large cavity machines with respective many oven modules it is often better to run a smaller number of lamps at higher voltage as this results in an emitted wave length that is able to penetrate the preform better. Lamps should be adjusted that the first lamp is just above the cooling shield that protects the necks from the emitted radiation. Operators must know which lamps controls which part of the preform and they get to know this by scratching preforms in the lamp positions, then observing the result in the blown bottle. Once an operator has decided to move material for example out of the neck area and into the center of the bottle via the lamps he/she always should add heat to the preform areas that need to be thinned while at the same time reducing heat by the same amount in an area that should be thicker. This avoids a situation whereby the preforms become ever hotter or colder and other problems ensue. Of course, this does not apply if the purpose is to change the preform temperature.

In single-stage the operator has control over the overall preform temperature by adjusting injection speed, hold and cooling time, as well as cooling water temperature and flow. However, temperature within the preform escapes the same type of control as we have in two-stage. It is governed by preform wall thickness, cooling water flow in the core and cavity sections, and viscous heating. The latter has a tendency to create sections in the melt stream that are hotter than the surrounding material and that are often pushed to one side of the preform so that heat differences exist in both the vertical axis and circumference. Injection speed can be used to influence this behavior to some degree. In order to avoid hot preform gate areas this area is machined thinner than the side wall thickness by a percentage to suit the application.

The pre-blow setting is another important setting to move material. As a general rule, the earlier the pre-blow pressure comes on while the stretch rod is moving the more material is trapped in the shoulder of the bottle. However, if it comes on too early the stretch may not engage firmly with the preform and the preform center (the gate) would not be centered to the blow mold. This would cause wall thickness unevenness in the circumference that must be avoided. Some machines energize the pre-blow pressure by timers, others by position. Operators can use either setting to manipulate the distribution of the material.

The oven fan speed is only available in two-stage molding and can be used to cool the outside of the preform when the oven temperature is below the final preform temperature. This can change the overall heat profile in the preform and is especially important to raise the temperature of the inner preform beyond that of the outer surface because the inner parts have to stretch up to 50% more than the outer. In single-stage this is already a given. Because cooling surface and water flow in the core is mostly smaller compared to the cavity side the inside of the preform is always warmer than the outside.

When running oblong bottles warming the mold cooling water to up to 120°F is in many cases of great advantage. As the round preform inflates evenly it touches the short side of the mold first. If that mold is cooled to 50°F, as is common, the material freezes off. That leaves a thick area (a “spine”) in the center of the label panel that shrinks after de-molding causing voids. In two-stage some machines are able to preferentially heat the preform area that will form the short side and/or cool the area 90° to it. This results in the cooler areas to stretch less and so pull the warmer sides apart for a more even distribution. In single-stage machines with a conditioning station aluminum conditioning rods are machined to only touch (and cool) the areas of the preform that will form the far sides of the bottle for the same purpose and with good effect.

In conclusion, many conditions have to be met in order to get the best wall distribution in a PET bottle. Skilled operators are, as always, indispensable.

Ottmar Brandau has been working in the Plastics industry since 1978, and is the president of Pet All Manufacturing Inc. His latest book “The Rapid Guide to Perfect PET Bottles” describes 31 common defects and their solutions. It can be found Here

- Ottmar Brandau -