Fixing Four Major Issues in PET Stretch-Blow Molding

In the world of PET stretch-blow molding, perfection is a journey, not a destination. As manufacturers strive to create flawless PET(polytehylene terephthalate) bottles, they often encounter a handful of recurring issues. From unpredictable breakage to structural inconsistencies, these challenges can compromise the quality of the final product. In this blog, we're diving deep into the top four major problems in PET stretch-blow molding and, more importantly, how to address and prevent them. Let's ensure every bottle you mold not only meets but exceeds industry standards. Ready to troubleshoot? Let's dive in!

Pearlescence & Haze

Two issues, one similar outcome - a whitened bottle.

Pearlescence is often termed stress-whitening. It's like when you stretch a rubber band too much and see white lines. For PET, when overstretched, its tiny structure can fragment, creating a pattern resembling "pearls." This is tied to both wall thickness and temperature. For example, a thin wall at a high temperature might be okay, but a thick, colder wall might show pearlescence.

This issue typically occurs inside the bottle. Why? Because the interior of the preform has to stretch more. If you spot a thin affected area, cool it down and heat the area beneath. This draws more material to the bottle's whitened part. If the affected area is thicker, simply increase the temperature, allowing for easier stretch.

For two-stage molding, adjust the lamp or ventilation. For single-stage, modify the overall heat by tweaking the holding and cooling duration. Additionally, injection speed changes can influence the preform's temperature.

Haze is another challenge. If PET gets too hot (around 115°C or 240°F) and cools briefly, haze can form. This happens because PET molecules restructure themselves for lower energy at this temperature. To fix it in two-stage processes, find the lamp causing the overheating and turn it down, or boost ventilation to keep the preform below the threshold. Remember, haze affects the preform's exterior, and increasing ventilation cools this more than the interior. For single-stage methods, increasing cooling or hold time can help reduce haze.

Reduced Top-Load Strength

In the quest to cut down on resin usage and costs, bottles are now lighter than ever. This makes the top-load strength a central concern. For the uninitiated, top-load is the measure of a bottle's ability to withstand weight from above. It's typically gauged on empty bottles using a device with variable speed. After taking this measure, it’s compared to the static load (think of the weight from bottles stacked above).

This figure is then multiplied by a safety margin to factor in unexpected forces—like when a truck hits a pothole and the cargo jostles.

However, simply multiplying the static load by two or more isn't always accurate. Some firms have tested actual loads in moving trucks, leading to some unexpected findings.

The main influencers of top-load performance? Bottle design and wall thickness. The bottle's weakest point—whether that's the shoulder, bottom, or main body—determines where it will give in. It's crucial for processors to pinpoint this weak spot to make the necessary design or material adjustments.

Here's a quirky fact: The top-load values for empty bottles rarely matter. Why? Because they don't typically collapse under top-loading. Some bottles maintain their strength, whether full or empty. However, certain bottles might stand up to four times the pressure when filled. It's essential for brand owners to understand this. And if such a disparity exists, processors ought to discuss the required top-load values with their clients.

Addressing the "Rocker Bottom" Issue in Bottles

A bottle's base often experiences a phenomenon known as "rocker bottom" when it's too hot post-molding. This issue causes the base to contract, resulting in the creation of a central leg. Consequently, the bottle wobbles, much like a rocking chair, giving the problem its name.

At the heart of every bottle, there's a feature called a push-up. This is the inward curve in the center of the bottle's base. Its primary function is to make sure the bottle rests on its outer edge. For bottles containing fizzy drinks, this push-up ensures they stand on multiple feet, usually five in number. However, excessive heat after molding causes this push-up to extend outwards, sometimes beyond the outer rim, causing the bottle to be unstable.

Although rare, there are times when the air trapped inside the bottle pushes this center outward as the mold opens. Some machines employ cooling jets that blow air to the bottle's base post-molding. This approach prevents excessive outward expansion. If not, adjusting the heat or increasing the cooling duration becomes essential.

Taking a 15 L container as an example, one can observe a pearlescent effect, especially in areas that are highly stretched, like the feet.

For carbonated beverages, base clearance is critical. It refers to the gap between the bottle's central bottom and its feet. Since carbonated drinks can apply up to 70 psi pressure from the inside, the center disk of the bottle base needs enough space to flex without going past the depth of its feet. To maintain this balance, it's suggested to check the base clearance frequently, perhaps every 2 hours. Often, this clearance requirement dictates the production speed due to the cooling time needed.

Another point to note is that bottles tend to have thin spots due to insufficient orientation, especially in single-stage molding compared to the two-stage process.

Surviving the Drop Test

Bottles need to endure a fall from heights of 4 to 5 feet, especially when filled with water at a temperature close to 40°F. This mimics the environment inside a home fridge. Unlike polyolefins, PET bottles don't crack at their slimmest point. Even if they get a dent, they won't shatter. This is because PET is strongly structured in these thin areas. Failures usually happen when there's a lack of this structure. Custom bottles, particularly those with limited stretch, and bottles from single-stage molding are more prone to this issue.

For two-stage molding, keeping the preform cooler can enhance the material's structure and boost its resilience. However, in single-stage molding, you might need to tweak the cooling time or the holding period, which can make the process longer. It's better to adjust the hold time since the preform moves away from its core as it cools. Overheating preforms can make certain parts brittle and prone to breakage, but using the aforementioned solutions can prevent this.