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!
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:
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.
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.
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.
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.
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.
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.
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:
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!
Brandau, Ottmar. 2022. Solve Four Common Problems in PET Stretch-Blow Molding. 06 26. Accessed January 30, 2023. https://www.ptonline.com/articles/solving-four-common-problems-in-pet-stretch-blow-molding.
Cheway. 2021. Blow Molding Troubleshooting Guide - 6 Problems. August 23. Accessed January 30, 2023. https://www.chenway.com.tw/en/news-Blow_Molding_Troubleshooting_Guide_Top_6_Problems.html.
Microdyne Plastics Inc. n.d. Types of Plastic Blow and Injection Molding. Accessed January 30, 2023. http://microdyneplastics.com/2015/07/plastic-blow-injection-molding/.
The Cary Company. n.d. PET Bottles, Polyethylene Terephthalate Plastic Containers. Accessed January 30, 2023. https://www.thecarycompany.com/containers/plastic/bottles/material/pet.