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Home / How Can Residual Moisture Cause Splay Marks in PC Injection-Molded Parts?

How Can Residual Moisture Cause Splay Marks in PC Injection-Molded Parts?

2026/11/11 By le zhan

How Can Residual Moisture Cause Splay Marks in PC Injection-Molded Parts

Residual moisture in PC resin can cause “silver streaks” (commonly known as “air lines” or “flow marks”) during the injection molding process. PC is hygroscopic, which means it absorbs moisture from the surrounding air during storage and material handling. When moisture-containing pellets enter the heated barrel, the absorbed moisture turns into steam. It becomes trapped in the molten polymer, forming visible streaks on the surface of the PC injection-molded parts. To prevent this defect, simply increasing the drying temperature is not enough; manufacturers need to install a suitable drying and dehumidification system to control residual moisture and prevent the resin from reabsorbing moisture before plastification.

How Does Moisture Absorbed by PC Injection-Molded Parts Cause Silver Streaks?

Moisture-related “silver streaks” (splay) defects typically occur in two interconnected stages: First, PC pellets absorb moisture from the air prior to processing; subsequently, when the resin is heated inside the injection molding machine, this moisture turns into steam. The steam bubbles disrupt melt flow and leave visible “silver streaks” on the surface of the part. To understand the causes of this phenomenon, it is first necessary to understand the moisture absorption characteristics of PC and analyze how residual moisture behaves during the plasticization and cavity filling processes.

How Does Moisture Absorbed by PC Injection-Molded Parts Cause Silver Streaks

Why Is PC So Sensitive to Residual Moisture?

PC is a hygroscopic engineering plastic capable of absorbing moisture from humid air. Even if the surface of the resin pellets appears dry, moisture may still be present within their internal structure. During storage, the amount of moisture absorbed depends on ambient humidity, duration of exposure, and material handling conditions. If manufacturers feed PC pellets into the injection molding machine without adequate pre-drying, the residual moisture will cause processing issues.

The impact is not limited to surface appearance. If the pellets are not sufficiently dry, a reduction in molecular weight may occur during processing, leading to silver streaks, surface bubbles, material brittleness, and a decline in mechanical properties. This is noted in the technical documentation provided by Covestro for its Apec® copolycarbonate series. (Reference: Covestro Apec® Processing Guide)

Therefore, during PC processing, your plastic resin drying system must maintain an appropriate drying temperature and a sufficiently low dew point, while ensuring adequate residence time to thoroughly remove moisture before the pellets enter the injection molding machine.

Why Is PC So Sensitive to Residual Moisture

How Does Moisture Cause Silver Streaks During Plasticization?

When moisture-containing PC pellets enter the heated barrel, the temperature rises rapidly, causing the absorbed moisture to vaporize within the molten material.

These bubbles become entrapped in the polymer melt. As the screw pushes the material toward the nozzle and mold cavity, the bubbles move along with the leading edge of the melt flow. Some bubbles reach the melt surface and expand; when the melt comes into contact with the cooler mold walls, these bubbles become trapped or deformed.

Once the polymer solidifies, these defects appear as silver streaks extending along the direction of material flow.

This defect is not merely the result of residual moisture on the surface of the finished product, but rather stems from the interaction between moisture and the melt during the plasticization and molding processes.

For PC injection-molded parts: the residual moisture content must be strictly controlled before the material enters the screw and barrel; this is because adjusting process parameters alone cannot effectively compensate for problems caused by insufficient material drying.

How to Identify and Prevent Splay in PC Injection-Molded Parts?

It is important to note that splay in PC injection-molded parts is not necessarily caused by residual moisture. Thermal degradation, trapped air, and other volatile contaminants can also lead to similar surface defects, but each situation requires different corrective measures. Therefore, effective prevention should begin with identifying the true root cause of the defect, followed by testing the resin for residual moisture and verifying that the drying and handling processes ensure the material remains in optimal condition at all times. The following sections will explain how to distinguish between these defects and establish a more reliable moisture control system.

How to Identify and Prevent Splay in PC Injection-Molded Parts

How to Distinguish Between Defects Caused by Moisture-Related Splay, Thermal Degradation, and Trapped Air?

These three types of defects appear similar but require different corrective actions.

Moisture-related splay: This typically manifests as elongated silver or white streaks extending along the direction of melt flow. The primary cause is often excessive residual moisture in the pellets. The first steps in troubleshooting should include verifying the material’s moisture content and the effectiveness of the drying process.

Thermal degradation: Excessively high melt temperatures, prolonged residence time in the barrel, or excessive shear forces can all lead to polymer degradation, resulting in similar streaks. Depending on the material type and degree of degradation, defects may appear as coarser streaks, dark spots, or discoloration. Corrective actions should focus on checking barrel temperature, screw speed, material residence time, and the presence of localized hot spots.

Trapped-air defects: Air trapped within the mold can cause localized air marks, bubbles, or scorch marks. These defects typically appear in the last-filled areas of the mold cavity, in grooves, or in other locations where air is difficult to vent. The key to resolving such defects lies in optimizing mold venting and adjusting injection speed and filling conditions, rather than simply increasing the drying temperature.

The diagnostic logic is to determine whether the defect originates from the resin, the plastification process, or the mold before selecting corrective measures.

How to Measure and Control Residual Moisture?

The most reliable method to confirm whether PC resin is sufficiently dry is to measure its residual moisture, rather than relying solely on drying temperature or air dew point.

The Karl Fischer method is the recognized method for measuring moisture content in plastics. For PC pellets, since volatile substances may interfere with direct measurement, the oven-heated Karl Fischer method is more appropriate.

Operators should collect representative samples after drying and before injection molding, and store them in sealed containers to prevent moisture absorption during handling. However, even if operators confirm that the resin is dry at the sampling point, they cannot guarantee its dryness until processing begins. If humid shop air exposes the pellets to moisture during storage or conveyance, they may reabsorb it. Therefore, operators must maintain dry environmental conditions throughout the entire material conveyance process.

This requires a reliable drying and dehumidification system that combines low-dew-point air, adequate airflow, sufficient residence time, and protected material conveyance. Topstar’s TCDH and TCDHE series three-in-one dehumidifiers utilize honeycomb rotor dehumidification technology to provide stable, low-dew-point air. Their integrated drying and conveying configuration not only maintains the material in optimal condition but also minimizes secondary moisture absorption during conveyance.

How to Measure and Control Residual Moisture

Reducing Silver Streaks to Improve the Molding Quality of PC Injection-Molded Parts

If moisture is present in the material, it will vaporize during the plastification stage, thereby disrupting the flow of the polymer melt and causing silver streaks to appear on the surface of PC injection-molded parts. To prevent such defects, adjusting injection molding process parameters alone is insufficient. It is essential to understand the resin’s moisture sensitivity, distinguish between streaks caused by moisture and other defects, verify the resin’s actual moisture content, and prevent the material from reabsorbing moisture during handling. By employing appropriate drying conditions, reliable moisture measurement methods, and a closed-loop conveying system, manufacturers can reduce silver streaks, protect the performance of PC materials, and thereby achieve more consistent injection molding quality.

Specific solutions should include: measuring residual moisture after drying, comparing the results with the resin supplier’s requirements, and using a reliable drying and dehumidification system to ensure the material remains protected throughout the entire process of being conveyed to the injection molding machine.

 

 

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