There are three types of reheat stretch blow molding machines used to convert preforms into bottles:

  1. Rotary machines
  2. Linear machines with continuous perform motion
  3. Linear machines with indexing perform motion

There are particular challenges with the third type of machine, very common in low and medium-volume applications. They have a low capital cost outlay and are easy to install and operate making them attractive for small companies that want to get into the PET business. The particular issues can be identified in three groups:

  1. Reheating of performs
  2. Control and sealing during blow
  3. Feed and unload
  1. Reheating of performs

    In these machines a set of performs commensurate to the machine cavitation is loaded at one time. Next the transport chain indexes to make room for the next set. Preforms are therefore standing still and moving while they travel through the oven system spinning all the while. On cavitations over 2 it is difficult to make sure all performs stay in the oven system the exact length of time. However, this is actually not the main problem. To better understand how performs are heated we need to look at the output of the infrared lamps. Lamps used in all reheat blow machine are short and medium wave infrareds that emit different wave lengths dependent on the input voltage. What is characteristic for all used lamps is that the emitted radiation power is not consistent along their lengths. Instead, the center of the lamp emits more radiation and is measurably hotter than the parts towards the ends. There are heat differences along the lamp that may result from small inconsistencies of the wire and the halogen gas exchange that actually carries the radiation but the main difference seems to come from the fact that the terminals need to be cooled to prevent them from melting.

    Thermal image of part of an IR lamp in operation

    It is therefore impossible to guide preforms through the oven systems with all of them receiving the same amount of heat and instead temperature differences of up to 8°C are not unusual. These temperature variations make it difficult to have preforms inflate in the same way and transform into bottles with the same wall thickness. This is because even small changes in temperature can influence preform stretching behavior. While this is unavoidable with these types of machines it is paramount that each blow cavity has its own set of blow valves. This allows independent timing and flow control of the pre-blow air that can partially compensate for any temperature variations in the preforms. Unfortunately, many of the less expensive machines use only one set of blow valves for two or even more cavities and make this impossible. Potential buyers should therefore inspect offers from suppliers carefully.

  2. Control and sealing during blow

    Preforms are placed on spindles mostly made of aluminum. As they are inflated the internal air pressure must be sealed in. This is accomplished by holding the necks in from the outside with steel mold inserts while sealing the inside with either O-Rings or by pushing the preforms up against the sealing surface.

    In this situation the force of the blow air is pushing against the inside of the neck increasing with the square of the diameter. This part of the preform is not oriented and therefore weaker especially as the spindles and with them the preform necks heat up in the course of continuous operation. PET loses some of its solidity long before it reaches the official bench mark where it becomes rubbery, the so-called Glass Transition Temperature Tg that is often cited as around 72° C for PET. This becomes especially important for necks larger than 28 mm that can become enlarged or ovalized. Keeping spindles at a low temperature by cooling them between take-out and in-feed and avoiding to cycle the machine without preforms does help. A helpful feature are lamps that turn to a lower voltage when no preforms are present.

  3. Feed and unload

    To load preforms and unload bottles most machines feature devices with 3 cylinders plus one gripper cylinder for each cavity or 2 additional cylinders that move all grippers. One moves the device forward and backward, one turns, one moves up and down, and the cylinders moving the grippers. Each cylinder comes with two flow controls and various grease points. Maintenance is therefore a constant concern as well as operators making flow adjustments that may or may not be necessary. These devices therefore reduce the efficiency of the machine and require additional resources. Keeping them well-maintained by replacing worn parts on a timely basis is critical to run a smooth production.

    In summary, while indexing machines are a low-cost entry point into stretch blow molding careful investigation of machine features before the purchase and meticulous attention to maintenance are paramount for a successful business.