Have you ever picked up an injection molded part and seen what appear to be faint lines in the part, or maybe they were cracks? Most likely they were just knit lines caused by the molding process. If you’re unfamiliar with what causes them, and ways to avoid them, we’ll dive into the subject of knit lines to shed some light on the common occurrence within plastic injection molding.
What Are Knit Lines?
Knit lines appear to be a faint line or a crack in the finished part after it comes out of the injection mold. They appear around holes or seams of an injection molded part, and some may call them “flow lines” or “weld lines”. These knit lines are formed as molten plastic in the injection mold flows around an obstruction like a feature in the part, a hole for say a button on an electronic case, or a seam between the halves of the mold. As the flow fronts move around the obstruction, they will meet and at that joining point they begin to melt together. This can also be thought of an “knitting together” to give it the “knit line”. These knit lines are formed from molten resin flowing around an obstruction because the two flow paths are slightly lower in temperature than other areas of the mold.
Are Knit Lines a Problem?
It is hard to answer the question of a knit line being a problem without knowing what the function of your molded part is. Knit lines naturally occur in all molded operations that have obstructions inside the mold. The knit lines will slightly reduce the strength of the part locally where they occur, but that slight change in strength may not cause a problem with durability. If your part requires a high visibility standard on the raw molded part (no secondary process to cover the surface), you may have to find a way to reduce the knit line appearance. You can’t complete eliminate them, but there are ways to reduce them to avoid detection.
How Can Knit Lines Be Avoided?
We often hear this question of how to avoid knit lines, but the more appropriate question is concerning what to change to reduce or avoid the detection of a knit line. Your resin, resin temperature, and filling speed can have a positive effect on the prevalence of a knit line around a hole or feature in your molded part. Choosing the best resin for your application is a great starting place. Once your optimal resin has been selected, your molding partner can adjust the resin fill speed into the mold and the molding temperature to reduce the appearance of the knit line. With some resins at optimum fill and temperature, knit lines are virtually invisible.
Mold parameters are one way to reduce the knit lines, but the mold design itself can also affect the prevalence of the knit line. The geometry of the part can positively or negatively affect the knit lines, and the gate placement within the mold can help or hurt the situation. The knit lines are influenced by resin temperature, so a hole near a gate location may not have a significant drop in temperature. Having multiple gates into the mold will also cause a knit line on the part as the two resin flow fronts from the gates must meet somewhere.
One thing to keep in mind is that everything with injection molding is about balance. Temperature and resin fill speed can be adjusted to reduce knit lines, but the changes can also cause issues in other areas of the part or with other molding operations. The goal should be to choose the best resin for the application and find the balance in parameters to meet your goals.
Conclusion
Troubleshooting knit lines isn’t a complicated process, but it does require certain considerations before you start the process. Starting with the best resin is a good first step, and then the adjustments can start to be made. If you need a partner that has the knowledge of creating the perfect injection mold or how to fine tune the operating parameters, look no further than SEA-LECT Plastics. At SEA-LECT Plastics we pride ourselves in tool and die manufacturing that allows us to deliver high quality products free from defects. We have multiple options for manufacturing to help your business succeed. Our expertise is in a wide range of molding disciplines. SEA-LECT Plastics has an elite team that can discuss the available options for your new project, we produce world-class prototypes and products, and we have decades of experience with plastic injection molding operations. Our tooling experts can create molds from 3D designs and make fine adjustments to ensure smooth molding operations on a daily basis. We can offer support to determine what type of tooling you need, what materials to choose, and can support the generation of documentation to win new business in your industry. Give us a call at (425) 339-0288 or email us at mattp@sealectplastics.com.
Matthias Poischbeg was born and raised in Hamburg, Germany. Matt moved to Everett, Wash., after finishing his bachelor’s degree in business in 1995 to work for Sea-Dog Corporation, a manufacturer, and distributor of marine and rigging hardware established in 1923.
In 1999, Matt took over the reins at Sea-Lect Plastics Corporation, a sister company of Sea-Dog and a manufacturer of plastic injection molded products with an in-house tool & die shop. Matthias Poischbeg is also a contributor to Grit Daily.