By Shell Polymers on Nov 12, 2020
Valued at $258.2 billion in 2019, the injection molding industry has been manufacturing products in the same way for a long time.1 However, recent research has disproved some of the most common methods and resulted in science-based best practices. That’s why it’s crucial to continue refining operations and realize that defects and processing issues are often the result of multiple variables combined.
According to our injection molding expert, Elliot Carnevale, there are four common defects that occur during this production process. He's shared the latest, research-backed ways to troubleshoot them below:
Warping is a common molding defect operators in the plastic industry are all too familiar with. Occurring when different component parts cool unevenly, warping usually results in unwanted twists and bends in the product’s structure that prevent it from sitting properly.
To tackle warping, Elliot recommends addressing it as much as possible in the design phase. Using advanced technology like simulations to understand how differences in part thickness will affect the way the product cools is one of the best tools to prevent warping.
Learn more about How To Enhance Your Plastic Injection Molding Operation In 3 Steps
Beyond attempting to solve it up front, converters employ popular but often impractical solutions including extending hold times in the molding process, post mold fixtures, and adding ribs all over the part are some solutions that are used in the industry today. From a processing perspective, varying the temperature of the moving platen and the fixed platen may help. But, ultimately, designing a part with warpage in mind up front is one of the best solutions.
2. Flow lines
Flow lines show up as discolored streaks, patterns, or lines in the final product. These can happen if molten resin changes direction inside the mold or if cooling speeds vary as it’s traveling through the mold. While this deformity doesn’t impact the structural integrity of a product, it’s still unsightly and can impact consumer satisfaction.
Especially in high-tech industries such as automotive, flow lines on plastic parts can be a deal breaker for a product’s usability.
Especially in high-tech industries such as automotive, flow lines on plastic parts can be a deal breaker for a product’s usability. This puts added pressure on processors to find a solution. When encountering flow lines, the processing solution generally lies in how the part is being filled. The resin is often cooling and ‘setting up’ at different points throughout the process. Oftentimes, increasing the injection speed to fill the part faster will help offset this issue. If filling faster doesn’t solve the issue, increasing the mold pressure and the melt temperature of the resin are also potential solutions. Converters should aim to get processes to a point where the part cools as uniformly as possible.
3. Air Pockets and Gas Traps
True to how it sounds, air pockets or gas traps occur when trapped air bubbles appear in a finished molded component. These can show up as a short shot, a burn mark, or other aesthetic defects. Many industries are particularly sensitive to final part aesthetics, so any defect caused by trapped air can result in wasted product.
Addressing air pockets or gas traps generally comes down to optimizing the gate location and venting. Leveraging simulation to study the flow of your resin in the mold can be a great tool to predict where you might see these issues in the design phase. Once the steel mold is cut, trying to process around air pockets can be extremely challenging. As the polymer flows through runners, flow channels, and the part, it displaces the air and gas that is present in the mold when it is empty. This air has to go somewhere, otherwise pressure will build up and prevent the polymer from flowing.
Leveraging simulation to study the flow of your resin in the mold can be a great tool to predict where you might see these issues in the design phase.
Typically, the two best solutions are either to add a vent(s) in a place that is not aesthetically critical to allow the gas to escape. Alternatively, moving or adding gates to fill out the part in a different way could address the issue. However, keep in mind that moving the gate will only move the issue, not solve it. But, if you can move the issue to a place where you can add a vent that is not going to impact aesthetics, that would be a net win.
4. "Short Shots"
Short shots, a reference to the incomplete filling of a part, occurs when the mold cavities don’t fill completely with molten resin. Short shots are almost always deemed “off-quality,” forcing converters to toss the piece and start over. While short shots can happen for a variety of reasons, mold design and resin selection are two of the most common causes.
This challenge often starts in the steel. Designing in flow channels that force the mold or part to fill from thin to thick can cause major issues, as the resin can “freeze off” in the thin section before the thick section is completely filled. Adding to the challenge, this design flaw can also cause sink marks. Sometimes these types of designs are unavoidable, and moving to a resin with a higher melt index or melt flow rate could help improve the overall quality of the part.
Selecting a resin with a higher melt index, or lower viscosity, will allow the resin to flow more easily and potentially fill out the part despite a less than ideal design. When making this change, it is important to consult with your resin producer, as lower viscosity almost always means lower molecular weight, and this can negatively impact the other physical properties of the resin.
Want More Injection Molding Tips?
The injection molding industry has been manufacturing products in the same way for a very long time. However, recent research has disproved some of the most common theories and resulted in science-based best practices. Stay up-to-date with the latest technology advancements by partnering with Shell’s team of Polymer Experts, such as Elliot, to experience more profitable production that produces high-quality products.