Cold runner injection molding is a common defect where when a small amount of molten plastic solidifies at the nozzle tip of an injection molding machine, it breaks off and is injected into the mold cavity along with the remaining melt. This can cause problems with the appearance and functionality of the molded part.
During the cold runner injection molding process, when the molten plastic enters the mold cavity through the nozzle, a small part of the plastic will solidify at the tip of the nozzle due to temperature differences or changes in flow rate. As the injection process proceeds, this portion of the solidified plastic breaks off and is injected into the mold cavity. This cold runner phenomenon can lead to uneven defects such as cold spots, cold joints or cold gates on the surface of molded parts.
These cold runner defects not only affect the appearance quality of the molded parts, but may also have a negative impact on their functionality. For example, cold spots may cause uneven surface gloss of the part, affecting the aesthetics of the product; cold joints or cold gates may reduce the strength and sealing performance of the part, thereby affecting its service life and reliability.
In order to solve the problems of cold runner injection molding, some measures can be taken to improve the molding process. First, the injection speed and temperature can be adjusted to ensure that the molten plastic does not solidify prematurely at the nozzle tip. Secondly, you can use a heated nozzle or increase the length of the nozzle to extend the residence time of the molten plastic in the nozzle and reduce the possibility of solidification. In addition, the mold design can also be optimized to reduce the difference in cooling time and cooling rate to reduce the probability of cold runner occurrence.
In conclusion, cold runner injection molding is a common defect that can cause cosmetic and functional problems in molded parts. By taking appropriate measures to improve the molding process, the occurrence of cold runners can be reduced and the quality of molded parts can be improved.
What Causes Cold Slug Injection Molding?
Cold slug injection molding is caused by a temperature difference between the nozzle tip and the mold cavity. During the cooling phase of the injection cycle, the nozzle tip is in contact with the cold mold steel, which causes a small amount of plastic to solidify inside the nozzle tip. This solidified plastic, called a cold slug, acts as a plug that prevents drooling or stringing during mold opening and ejection.
However, during the next injection cycle, this cold slug has to go somewhere, and it usually gets pushed into the mold cavity along with the rest of the melt. Depending on the size and shape of the cold slug, it can create a visible mark on the part surface, or it can get trapped inside a subgate or pinpoint gate, partially blocking the flow.
Some factors that can increase the likelihood of cold slug injection molding are:
- Low nozzle temperature
- High mold temperature
- Long cooling time
- Large nozzle diameter
- Small gate size
- High injection speed
- High viscosity resin
How to Identify Cold Slug Injection Molding?
Cold slug injection molding can be identified by inspecting the part surface for silver or white streaks that are usually located near the gate area. These streaks are caused by air bubbles or voids that are trapped inside the cold slug as it enters the mold cavity. The cold slug can also cause a depression or sink mark on the part surface due to its lower pressure and density compared to the rest of the melt.
Another way to identify cold slug injection molding is by examining the runner system for signs of cold slugs. The runner system is where the molten plastic flows from the nozzle to the mold cavity, and it consists of sprues, runners, and gates. A cold slug well is a feature that is designed to capture and store the cold slug at the end of each injection cycle, preventing it from entering the mold cavity. A cold slug well is usually located at the base of a sprue or at a runner drop.
If there is no cold slug well in your runner system, or if it is too small or too shallow, you may find cold slugs in your runners or gates, which can affect your part quality and consistency.
How to Prevent Cold Slug Injection Molding?
The best way to prevent cold slug injection molding is to minimize or eliminate the temperature difference between the nozzle tip and the mold cavity. This can be achieved by following these tips:
- Increase nozzle temperature: This will reduce or prevent plastic from solidifying inside the nozzle tip during cooling. However, be careful not to overheat your resin, as this can cause degradation or burning.
- Decrease mold temperature: This will reduce or prevent plastic from solidifying inside the mold cavity during injection. However, be careful not to undercool your resin, as this can cause incomplete filling or poor surface finish.
- Decrease cooling time: This will reduce or prevent plastic from solidifying inside both nozzle tip and mold cavity during cooling. However, be careful not to shorten your cooling time too much, as this can cause warpage or shrinkage.
- Reduce nozzle diameter: This will reduce or prevent plastic from solidifying inside both nozzle tip and mold cavity during cooling. However, be careful not to reduce your nozzle diameter too much, as this can cause excessive pressure drop or shear heating.
- Increase gate size: This will reduce or prevent plastic from solidifying inside both nozzle tip and mold cavity during injection. However, be careful not to increase your gate size too much, as this can cause flash or poor gate appearance.
- Decrease injection speed: This will reduce or prevent plastic from solidifying inside both nozzle tip and mold cavity during injection. However, be careful not to decrease your injection speed too much, as this can cause short shots or poor weld lines.
- Choose low viscosity resin: This will reduce or prevent plastic from solidifying inside both nozzle tip and mold cavity during injection. However, be careful not to choose a resin that is too low in viscosity, as this can cause jetting or sagging.
Another way to prevent cold slug injection molding is to add a cold slug well to your runner system, or to modify your existing cold slug well to make it larger or deeper. A cold slug well is a simple and effective solution that can capture and store the cold slug at the end of each injection cycle, preventing it from entering the mold cavity and affecting your part quality.
Conclusion
Cold slug injection molding is a common defect that occurs when a small amount of molten plastic solidifies inside the nozzle tip of the injection molding machine, and then breaks off and is injected into the mold cavity along with the rest of the melt. This can cause aesthetic and functional problems for the molded part, such as reduced gloss, increased stress, poor dimensional stability, reduced mechanical strength, increased risk of contamination and degradation.
To prevent cold slug injection molding, you need to minimize or eliminate the temperature difference between the nozzle tip and the mold cavity. You can do this by adjusting your process parameters, such as nozzle temperature, mold temperature, cooling time, nozzle diameter, gate size, injection speed, and resin viscosity. You can also add or modify a cold slug well to your runner system, which can capture and store the cold slug at the end of each injection cycle.
