Knowledge Base

What Causes these Four Common Issues in the Blow Molding Process?

Feb 15, 2023, 16:24 PM by The Stoner Molding Solutions Team
When you work in the blow molding industry for long enough, you realize that this molding method has many advantages. It can produce a large volume of hollow plastic parts at a relatively low cost.

PET bottles in an automatic blow molding machine.When you work in the blow molding industry for long enough, you realize that this molding method has many advantages. It can produce a large volume of hollow plastic parts at a relatively low cost. However, this molding process isn’t without its issues. You can sometimes experience leakage, rocker bottoms, surface wall defects, and uneven wall thickness, among other problems.

By learning more about the blow molding process, understanding how this process molds thermoplastic materials, and the common issues and resolutions, your molding process can be more efficient. Another way to keep your molding process efficient is by using the right products for your molding equipment and raw materials. Mold sealers, releases, and cleaners are all necessary steps in blow molding. Not sure which products are right for you? Talk to one of our experts today!

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What Is the Blow Molding Process?

The plastic blow molding process is like the glass-blowing process. The blow molding cycle begins by heating a plastic tube, then filling it with air, called the parison. A mold is then clamped around the tube to trap the plastic as the air continues to fill the parison to form the shape of the mold. It’s ideal for molding a high-volume production of hollow objects and creates uniformly thin-walled objects at a relatively low cost.

It’s sometimes preferred over the popular injection molding process because the blow molding station typically costs less than the injection molded machinery and can create shapes that injection molding cannot.

Additionally, there are three types of blow molding processes:

  1. Injection Blow Molding: The injection blow molding process uses a blow or core rod throughout the process. The parison, shaped like a test tube, is injected inside a split mold cavity around the rod. The core pin moves the parison to the blow molding machine, where forced air creates the final product.
  2. Plastic bottles with a screw neck in the shape of a test tube before going through the blow molding processExtrusion Blow Molding: The extrusion blow molding process can either be continuous or intermittent. In continuous extrusion blow molding, the parison is constantly inserted into the mold, and each product form will be cut off with a blade as it forms. The machine releases the plastic part from the cooled metal mold in intermittent extrusion blow molding. The parison is back into the mold only after the previous part is released.
  3. Injection Stretch Blow Molding: This plastic blow molding combines the first two and is primarily used for manufacturing bottles. The plastic resin is first molded into a solid preform to create a threaded bottle neck. Once the preform mold’s plastic cools, it’s fed into a stretch-blow molding machine. It is then reheated using an infrared heater and blown into a plastic bottle with forced compressed air.

What Is Blow Molding Used For?

Blow molding is used to create hollow objects. Some examples of blow molding objects include containers, plastic bottles, drums, and tubing. A popular blow-molded item is a Polyethylene Terephthalate (PET or PETE) bottle. These bottles are semi-rigid plastics with durable, scratch- and shatter-resistance for products like spice jars, vitamin bottles, water bottles, and energy shot bottles.

Another popular blow molded product is High Density Polyethylene (HDPE) products. This plastic material is molded into shampoo pottles, motor oil, coolers, industrial drums, fuel tanks, and play structures. It’s easy to color, chemically inert, and FDA-approved.

What Common Issues Does Blow Molding Experience?

Now that you know how blow molding works to create the desired shape and some of its common products, it’s time to look at some of the issues that can be experienced during the blow molding process and what causes them.

1. Leakage

Leakage occurs in the blow molding process when the gap between the blow pin nozzle and the mold is too wide. It doesn’t allow the blow pin to push enough material inside the mold, creating leaks. Leaks can also be caused by a tear in the product’s wall, contaminated plastic resin, or poor welding of the molding plastic. To prevent leakage, be sure that your blow molding manufacturer must make the blow pin nozzle and the mold a perfect match.

2. Rocker Bottoms

A plastic water bottle to the right with the bottom of two different bottle types to the left.Rocker bottoms occur when producing bottles in the blow molding process. All bottles have a feature on the bottom known as the push-up, where the center of the bottle curves inward at various degrees. This feature helps ensure plastic bottles, especially those for carbonated beverages, sit on the base's outer rim (feet). A rocker bottom occurs when the plastic products are too hot after molding. The plastic then protrudes outward instead of inward and causes the bottle to rock instead of standing straight.

Rocker bottom can also occur, though rarely, when the residual air pressure inside the bottle pushes the center of the bottom out. To prevent rocker bottom, ensure you allow for adequate exhaust and cooling time, your blow molding vent is clean, the melt temperature is not too hot, and the cycle time is not too fast.

3. Surface Wall Defects

Sometimes the defects on the surface wall of the molded product are barely noticeable, like tiny black spots. Other times, it’s quite obvious: horizontal ring-like lines, vertical streaking lines, and bubbles. These defects can be caused by foreign matter, recycled resin, old resin stuck inside the extruder die head, excess moisture, the parison thickness controller moving the die too fast, or the parison wall coming in contact with the cold mold surface twice instead of once.

4. Uneven Wall Thickness

Uneven wall thickness happens when one part of the preform begins to stretch first, making it harder for the rest of the product to stretch to the same thickness. This problem can occur because of an off-center gate, uneven heating and cooling, and small stretch ratios.

How Can I Prevent These Issues from Happening?

While all these issues have different origins, there are some things you can do overall to prevent them from happening. Some of those ways include:

  • Ensure Proper Training: Sometimes, issues occur because an operator doesn’t have enough experience or proper training. If you find that one operator tends to have more issues than others, refresh them on their training or pair them with a more experienced blow molder.
  • Use the Right Mold Release: If the molding plastic sticks to the mold surface, then it isn’t going to release cleanly, causing defects. Using a high-quality mold release product specifically designed for the blow molding industry, such as the ones we sell, you can release more products in less time.
  • Adjust Cycle Parameters: Sometimes defects in the finished product happen because the cycle time is too long or too short, the temperature isn’t right for the raw plastic pellets, or the cooling time isn’t long enough. Ensure your cycle parameters are correct for the process you’re performing.
  • Clean Your Molds: A clean mold is a productive mold, so using the right mold cleaners can make all the difference in the world. It can remove excess resin contaminating your raw materials or excess water from the blow molding machine. You may face more defects during your cycle time if you don’t have proper cleaning procedures and products.

Need More Ways to Improve Your Blow Molding Process? Talk to an Expert!

The blow molding process is a great way to create hollow objects quickly. But without proper training, cycle parameters, and molding products, you could face defects that hurt your productivity. That’s why we advocate for proper training and using our molding products.

We specifically design the products to work for the blow molding industry so that you can count on us for high-quality, custom products. Don’t see something that will work for you or need more help improving your blow molding efficiency? Talk to one of our experts today!

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Brandau, Ottmar. 2022. Solve Four Common Problems in PET Stretch-Blow Molding. 06 26. Accessed January 30, 2023.

Cheway. 2021. Blow Molding Troubleshooting Guide - 6 Problems. August 23. Accessed January 30, 2023.

Microdyne Plastics Inc. n.d. Types of Plastic Blow and Injection Molding. Accessed January 30, 2023.

The Cary Company. n.d. PET Bottles, Polyethylene Terephthalate Plastic Containers. Accessed January 30, 2023.

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