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In injection molding, voids are a type of molding defect where cavities form within the thicker sections of the molded product. As the resin filled in the mold cools, it shrinks. During this time, the surface of the mold product that touches the mold (skin layer) may shrink and sink, which is referred to as sink marks. Conversely, while the skin layer may solidify, the interior may shrink and form vacuum cavities, which are called voids. Voids become a defect issue for two main reasons:
Appearance Defects: The appearance is unsatisfactory.
Functional Defects: There is a lack of strength in the void area.
Voids that are hidden when fitted may not be a problem appearance-wise, but if they appear on the surface, they become very noticeable and need to be addressed. Typically, voids occur in the thicker parts of the product. These are functional parts that need strength, and voids here can lead to a lack of strength, making it a critical point of management.
In injection molding, voids generally occur in the thicker sections of the molded product. The causes of void formation are as follows:
Ideally, the thickness of the molded product should be uniform. However, for various reasons, conditions arise that necessitate varying thicknesses. Voids tend to form in these thicker sections. Factors that cause uneven thickness include:
One cause of voids is low holding pressure. If sufficient pressure is not applied to the thicker sections during the filling and holding stages, the material cannot compensate for the shrinkage, leading to the formation of voids inside.
If the cooling time is too short, shrinkage begins before the surface skin layer solidifies, leading to sink marks. Voids may also form inside.
If the water channels inside the mold become clogged, it can lead to insufficient cooling in parts of the mold, causing increased shrinkage. Excessive shrinkage can lead to the formation of voids.
Various factors can cause voids, as mentioned above. Specific measures to prevent their occurrence include the following:
Fundamentally, voids occur in the thick sections of the mold. For design and functional reasons, these sections are unavoidably thick, so it is not feasible to reduce this thickness. As a countermeasure, replace the thick mold sections with metals that dissipate heat well or add cooling water channels to improve this. However, modifying the mold can be costly and time-consuming, so it's considered a last resort after easier adjustments to molding conditions and equipment have been attempted and failed.
To address voids in thick sections, increase the holding pressure or extend the cooling time. This reduces the amount of shrinkage in the thick sections, improving void issues. However, as a side effect, increasing holding pressure can affect other parts of the product by making them heavier, and extending cooling time can make shrinkage difficult, resulting in larger dimensions.
By isolating the mold water pipe circuit where voids occur, the heat exchange efficiency increases, making it harder for the material to shrink. Also, if voids suddenly occur during molding, it could be due to a clog in the mold's internal water pipes. The diagnostic method is to stop molding immediately, touch the relevant area by hand to check if it's hot. If it's too hot to touch, the internal water pipes are clogged. Connect air to the hose of the clogged water pipe and remove any debris from the water pipe (air purge). Doing air purge from both the IN and OUT sides sequentially can more effectively remove debris inside the water pipe. When restarting, after a few shots of molding, stop molding again, manually check the area to confirm that the debris has been removed from the water pipe.
Insufficient holding pressure is one of the causes of void occurrence. If gas venting is poor, it can lead to insufficient filling. Regular gas cleaning of the parting line alone is not enough to remove accumulated gas dirt inside the mold, so be careful. As a countermeasure, regular overhauls after tens of thousands to hundreds of thousands of shots are effective.
We have discussed the main causes of voids and their countermeasures. However, completely eliminating void occurrence through measures alone is challenging. Pay particular attention at the following times to accurately detect void occurrence:
At startup, it is important to thoroughly conduct an initial inspection to ensure compliance with quality standards. Voids occur in specific areas. Focus on these areas and compare carefully with the standard samples and defect limit samples. If it is difficult to make a judgment, leave the decision to the quality control department and wait for approval before starting up.
Voids generally worsen proportionally with the cumulative shots of the mold due to clogged gas vents and worsening events. Additionally, sudden occurrences can also be caused by debris clogging inside the mold water pipes, resulting in continuous defects with every shot thereafter. It is important to take time samples and conduct regular quality checks.
We have summarized the points to be aware of when considering defect countermeasures and productivity improvements in injection molding factories, particularly concerning voids. Voids are a defect event where the inside of thick sections shrinks to form vacuum cavities.
By controlling the causes of occurrence and ensuring that no voids are overlooked, aim to establish higher molding processing technology through effective countermeasures.
