Plastic injection molding has become one of the leading manufacturing processes to produce everyday products. While you can find excellent quality and competitive prices in the stores, producing these items does have limited drawbacks concerning quality defects. They happen. There’s no way around them, but there are strategies and tips to avoid them. We’ve listed the five most common plastic injection molding defects, and a handful of actions that you can take to limit them in your manufacturing operations.
What Are The Most Common Plastic Injection Molding Defects?
Flashing is the term used for plastic that escapes the injection mold during molding operations. It most commonly escapes the injection mold at the edges where the mold halves meet, but it can also escape around ejector pin locations. The most prominent cause for flashing is due to a lack of pressure holding the mold halves together.
Increasing the holding pressure of the injection mold halves may prevent or limit the flashing that can occur. If pressure won’t 100% correct the problem, you may find that the mold halves have been damaged during use or they have worn enough to allow flashing to occur. Molds typically wear enough at the end of their life cycle to allow flashing to occur and may need to be updated to correct any high wear areas before a new replacement mold is created. Injection speed, mold temperatures, injection pressure, and venting can also affect flashing, so there may be operation parameters and physical mold areas to investigate to limit flashing.
2. Short Shots
Short shots are the result of molten plastic insufficiently filling the mold. It’s basically a missing piece of material. It typically causes the formed part to become scrap as something is missing. Short shots typically occur because of improper settings being used on the molding machine or the resin being used is incorrect for the mold. Improper degassing or venting from the mold can create pressure pockets that won’t allow the resin to fully fill the mold, and a molten resin that is too thick can also only partially fill a mold.
Short shots can be typically fixed with a better flowing resin or a higher operating temperature for the mold to completely fill. Gas pockets causing flow restrictions can be vented from the mold, which should alleviate the missing material.
3. Sink Marks
Sinks marks are small depressions or indentations on the surface of the molded part typically caused by thicker areas cooling and shrinking. The marks are typically caused by insufficient cooling time inside the injection mold, low pressure inside the mold, or too high of a temperature at the plastic gate area.
Sink marks can be removed by adjusting the molding temperature, increasing the holding pressure, and/or increasing the cooling time inside the mold before the part is removed. If these adjustments don’t completely remove the marks, areas on the part with varying thicknesses may need to be changed in the design to equal the varying thicknesses.
4. Flow, Weld, and Knit Lines
Lines in the final part can be called by many names. Some refer to them as flow lines, others say weld lines, and still others call them knit lines. The terms describe discolored or off-color lines in the finished part. The lines are normally caused by resin cooling at different rates as it flows through the injection mold. Causes of the differing cooling rates can be where two or more sections of the mold come together, resin that cools too fast in the mold, or varying wall thickness between differing sections of the mold.
Injection speed adjustments can typically eliminate most of the lines in the finished part. Increasing molding pressure and temperatures can also help with line elimination if injection speed doesn’t correct the issue completely. Design changes including rounded edges, wall thickness adjustments, and flow changes can make major improvements in lines occurring.
5. Burn Marks
Burn marks are caused by overheating the resin against the interior mold surface. They don’t have a particular color, as they can show up as a dark or rusty colored spot. The spot are a degradation of the resin caused by overheating or by injection speed beyond the recommendation for the specific resin being used. The high injection speed causes gas to be trapped inside the injection mold that can burn or degrade the resin during the injection process.
Eliminating burn marks is typically done by adjusting the injection speed and/or temperature to correct the parameters that is causing the resin degradation. Molds are typically designed with sufficient venting for trapped gasses, but in rare cases additional venting can be added to further reduce pressure trapped inside the injection mold.
How Are Plastic Injection Molding Defects Prevented?
Quality defects will occur in injection molding, even if the perfect mold is being used. Resin characteristics can slightly vary batch to batch, and the environment can change throughout the year to cause adjustments to be made. There are four main things that need to be continuously monitored to ensure injection molding operations stay on track: proper injection speed, proper material selection, molding process parameters, and tooling design.
There are thousands of resins available on the market with more engineered resins continually being offered for superior characteristics. Finding the optimum resin for your product and application may require a mentor to narrow the focus on what the final product requirements are to select the proper resin.
One of the best ways to determine which resin to use and what the process parameters should be is to work with an experienced injection molding company with capability and skill to recognize potential future defects and how to prevent them. At SEA-LECT plastics we pride ourselves in tool and die manufacturing that allows us to deliver high quality products free from defects. You should be including discussion points on the function, environment, quantity, and recycling of your product to ensure the best resin is chosen for your product. SEA-LECT Plastics has an elite team that produces world-class prototypes and products, and we have decades of experience with plastic injection molding. We can offer support to determine what type of mold you need, what resin to choose, and how to best invest in your future. Give us a call at (425) 339-0288 or email us at email@example.com. We’ll help to determine the best manufacturing for your next project.
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.