Analysis of The Causes of Poor Demolding Defects (2)
Successful injection molding depends on a delicate balance of multiple factors, including material selection, mold design, and process control. Any disruption to this balance can lead to defects, with part adhesion to the mold and poor ejection being particularly challenging. These issues, often referred to as “sticking” oder “dragging,” can cause production delays, damage to the mold, and compromise the quality of the final product.
Improper Process Control
If the injection molding machine is oversized, with a screw speed that is too high, injection pressure that is too great, and injection hold time that is too long, it will result in overpacking. This leads to a lower than expected shrinkage rate, making ejection difficult.
If the barrel and melt temperatures are too high, and the injection pressure is too great, the hot melt easily enters the gaps between the mold inserts, creating flash and leading to poor ejection.
Additionally, a nozzle temperature that is too low, insufficient cooling time, and interrupted material flow can also cause poor ejection.
Therefore, when troubleshooting adhesion and ejection problems, the following actions should be taken:
• Reduce the injection pressure.
• Shorten the injection time.
• Lower the barrel and melt temperatures.
• Extend the cooling time.
• Prevent material flow interruptions.
Inadequate Material Quality
If impurities are introduced into the raw material during packaging and transportation, or if different grades of materials are mixed during pre-drying and pre-heating processes, it can lead to part adhesion to the mold. Additionally, the presence of foreign objects in the barrel and hopper can also cause this issue.
Inconsistent or overly large particle sizes of the raw material can also contribute to adhesion to some extent.
Therefore, it is crucial to properly clean and screen the molding materials before use.
Improper Use of Release Agents
The purpose of using release agents is to reduce the adhesion between the plastic part surface and the mold cavity surface, preventing them from sticking to each other. This facilitates shorter molding cycles and improves the surface quality of the plastic parts. However, the effectiveness of release agents is influenced by both chemical interactions and physical conditions. Furthermore, since molding materials and processing conditions vary, the optimal type and amount of release agent must be determined based on the specific situation. Improper use often fails to produce the desired release effect.
Regarding the molding temperature, the effective working temperature of fatty oil-based release agents generally should not exceed 150°C and are not suitable for high-temperature molding. Silicone oil and metal soap-based release agents typically work within a temperature range of 150°C bis 250°C. Polytetrafluoroethylene (PTFE)-based release agents can withstand temperatures above 260°C, making them the most effective for high-temperature molding.
In terms of material type, soft polymer parts are generally more difficult to release than hard polymer parts.
Regarding the application method, paste-like release agents should be applied with a brush, while sprayable release agents are applied using a spray device. Since it is difficult to form a regular and uniform layer when applying paste-like release agents with a brush, the surface of the molded part may exhibit wave marks or streaks after ejection. Therefore, it is preferable to use sprayable release agents whenever possible.
Overfilling
When molding with excessively high injection pressure, the shrinkage rate is smaller than expected, making ejection difficult. In such cases, reducing the injection pressure, shortening the injection time, and lowering the temperature of the melt and mold can facilitate ejection. In these situations, using release agents that reduce the friction between the plastic and the mold is more effective. For the mold, increasing the surface finish, eliminating side wall irregularities, polishing, and adding ejector pins are also effective measures. When molding deep parts, blowing compressed air between the mold and the part can further assist in ejection.
Addressing poor demolding in injection molding requires a comprehensive approach, considering process parameters, material quality, release agent selection, and mold design. By carefully optimizing these factors and implementing the suggested solutions, manufacturers can achieve consistent, efficient ejection and produce high-quality molded parts.
