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Wet versus dry preforms: knowing the difference is critical

PTI preforms SQ
Sometimes the root cause of a problem isn’t what you first thought it might be. Take plastic blow molding bottles, for example.

Have you experienced issues blow molding bottles from inventoried preforms? Have you spent hours trying to tweak your blow molding equipment settings, without really getting to the root of your problem? You just might be looking in the wrong place.

The issue could very well be with the length of time your preforms have been stored and not the preform design or problems with the resin from which they were molded. 

In this day and age of trying to maximize profitability, companies may be scheduling large preform runs thinking that that is the best way to improve production efficiencies. This approach may work well for a wide variety of products, but not so much where preforms are concerned. It all boils down to how much moisture those stored preforms have absorbed.

The bottom line is that older preforms do not process the same. This means that the blown containers don’t meet performance specs unless the processing conditions are tweaked sufficiently or have a reasonable factor of safety. For example, the preform would have to be overdesigned for the intended bottle. It could have excess weight or orientation than what’s needed for the application.

Drilling down even further, preforms stored three, six or 12 months, respectively, will each process differently. The older the preform, the more difficult it will be to process so that the container meets performance criteria. The time of year the preforms were injection molded, along with the environmental conditions in which they are kept, will also impact performance.

Dry vs. wet preforms

Here's some terminology you should know.

The term “dry preforms” refers to freshly injection molded preforms that haven’t had the chance to absorb much moisture from the atmosphere. Because PET is a hygroscopic material, it absorbs moisture continually throughout its life at different rates depending on humidity of the environment. A “wet preform” is created when it is exposed to atmospheric moisture. This makes the preform behave differently during reheating and blow molding, which also means the bottle’s performance attributes will not be the same had it been blown from a dry preform.

Why is it important that we understand performance differences?

Increasingly, containers are blow molded from light- and optimal-weight preforms. This implies that when material is distributed over the bottle surface optimally (and with right orientation) it leads to the best properties and good performance.

But it’s important to point out that there is not much excess material in a preform. A slightly thinner or less oriented sidewall may lead to additional growth and deformation that would not meet the specifications. If the material distribution shifts due to different material stretching and blowing characteristics, then the blown container may have poor performance.

Examples include a lower top load or higher creep expansion during pressurization. For carbonated beverage packages, this leads to a more distended look and a high-fill point drop along with other problems associated with shorter shelf life. For household chemical oval containers, this could be thin corners more prone to denting and poor material distribution.

For additional information and detailed graphs on a study we conducted on wet vs dry preforms, please download our white paper.

Author: Sumit Mukherjee is the chief technology officer of PTI. He has 25 years of experience in preform and container design, materials characterization, process simulation and modeling, and finite element analysis (FEA) for package performance prediction.

About PTI

PTI is recognized as a leading source for preform and package design, package development, rapid prototyping, pre-production prototyping, and material evaluation engineering for the plastic packaging industry. For more information:

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