Injection molding is widely used in mass production because of its consistency, dependability, and short cycle time. However, there are risks involved in the process, just as there are in any design or production method.
Both mold and part designers face fresh obstacles with each new design. Component design, material choice, and mold design are all potential sources of final product defects. Let’s check out five of the most typical issues that crop up throughout the plastic injection molding process and see what you can do to fix them.
Weld Lines
Whenever two or more melted fronts meet in the center of a plastic component, a weld line forms. This may occur when the finished product has a complex geometry such as several injection points, holes, or other features. Weld lines are often the weakest portion of an injection molded object due to the melt fronts partly solidifying upon collision, resulting in considerably weaker molecular bonding.
To repair weld lines you need to raise the resin temperature such that when the weld line is formed, the resin on both sides is liquid. Also, in order to slow down the rate at which the mold is cooling, you should increase the injection rate, and make changes to the mold to eliminate the weld line.
Sink Marks
Uneven cooling during the molding process may cause a plastic component to develop sink marks, which are depressed patches that stick out from the rest of the part. When the melt front reaches the mold wall, it begins to cool and compress, forcing it to retreat from the mold as it moves into the hollow. These flaws are essentially aesthetic, yet they are unappealing.
Common methods for eliminating sink markings include:
- Boost the holding pressure to guarantee the component makes full contact with the mold as it cools;
- To guarantee that the component hardens in the right place, you should lengthen the cooling period;
- Make the walls of your component smaller in design to speed up the cooling process.
Warping
Uneven cooling during the mold-setting phase is another source of warping in plastic components. Uneven stresses, bending moments, and torsion may be introduced into the component when various areas of the plastic cool at different rates, causing deformations that were not accounted for during the design phase. Some precise components may get out of tolerance as a result, necessitating either discard or rework.
If you want to reduce warp you need to make sure the item is cooling evenly so that it contracts consistently and no tensions are introduced. Using symmetry in the part’s design may help maintain consistent temperatures throughout the cooling process. In addition, reduce the temperature at which the plastic is melted to lessen the amount of cooling that will be required.
Air Traps
The formation of air traps in the final product is one of the most dangerous problems that may occur during the injection molding process. Since the air trapped in the mold cannot escape, it is absorbed into the final product, where it may cause structural and aesthetic problems. Both the mold and the end product are at risk of destruction if the air within becomes so heated and squeezed that it explodes.
Standard methods for eliminating air leaks include:
- Reduce the duration of the cycle so that any trapped air cannot get compressed enough to cause an explosion;
- Increase the injection pressure to coerce the air that is trapped within the cavity out of its hiding place;
- Select a lower-viscosity substance if you want to reduce the likelihood of bubble formation.
Short Shots
When working with injection molding, the most problematic issue is the short shot, which refers to an insufficient amount of plastic being injected into the component. Not only do these pieces have aesthetic flaws, but practically all of them need to be trashed and replaced since they were made incorrectly. The mold may need to be recut or modified, the project may be delayed, and significant time and money will be lost if any of these problems arise.
Short shots often need a few typical adjustments. For instance, you’ll need to increase the size of the gates in the mold’s new design to improve fluidity. To keep the resin viscous enough to fill the mold cavity, it is important to keep the mold wall temperature higher. Faster cavity filling may be achieved by increasing the injection pressure.
Final Thoughts
Since there isn’t a one-size-fits-all solution to plastic injection molding challenges, we encourage you to work in close cooperation with your part and mold designers. Once you incorporate their suggestions and input, you will be able to produce consistent parts for the next launch. And just one small change can dramatically reduce the risk of defects.
