The importance of pre-drying injection molding materials
Many Injection Molding defects, such as bubbles, silver streaks or insufficient strength, often result from improper handling of moisture in the material.
- Why is pre-drying so important?
Plastic materials, especially engineering plastics such as nylon (PA), polycarbonate (PC) and PET, often absorb moisture from the air due to hygroscopicity. This moisture will vaporize during high-temperature injection molding, causing bubbles, voids, silver streaks or surface defects in the product, and even trigger hydrolysis reactions, reducing mechanical properties and dimensional stability. Imagine an automotive or medical part failing due to moisture issues. The consequences go beyond rework.
The main goal of pre-drying is to remove moisture and volatiles to ensure uniform material consistency, thereby improving processing stability and product quality. For non-hygroscopic plastics such as PE and PP, although the impact is smaller, drying can also optimize appearance and performance. In 2025, environmental regulations require the use of more recycled materials, which tend to have higher moisture content, making pre-drying a necessary step.
- Pre-drying methods and equipment:
There are various pre-drying methods and the appropriate method should be selected according to the hygroscopicity of the material and the scale of production. Common methods include:
(2) Dehumidification and drying: First use a dehumidifier to remove moisture in the air, and then use dry hot air to heat the particles. Suitable for PA, PC, PET and other highly hygroscopic plastics. The equipment costs more but provides excellent results, especially in humid environments.
When choosing the right drying equipment you should consider:
- Flow rate = CFM
- Drying temperature
- The capacity of the storage barrel is related to the amount of material used per hour and drying time
- Dew point = moisture content of air stream
- Drying conditions for specific materials:
Different materials have specific requirements, please refer to the material property table requirements. Here are common examples:
(1) Nylon (PA): Drying temperature is 80-120°C, time is 4-6 hours, to prevent hydrolysis and dimensional instability.
(2) Polycarbonate (PC): 100-120°C, 2-4 hours to avoid silver streaks and surface defects.
(3) PET: 150-170°C, 4-6 hours to ensure smooth surface and mechanical strength.
- Moisture content detection:
Use a moisture analyzer such as a Karl Fischer titration or an electronic moisture meter to quantify the moisture content and ensure it is below a specified threshold.
(1) Karl Fischer titration method: high measurement accuracy, high cost and complicated to use.
(2) Electronic moisture meter: lower measurement accuracy, lower cost, easier to use
- Best practices and considerations:
For efficient pre-drying, the following are practical suggestions:
📌 (1) Pre-storage conditions: Store materials in a low-humidity environment and use sealed packaging to prevent moisture reabsorption.
⚙️ (2) Monitoring parameters: Track temperature and dew point in real time to ensure consistency. Excessive drying may cause thermal degradation, oxidation or color changes. The dew point value of the dryer will increase and change with the use time. A dryer with a dew point that doesn't meet the requirements cannot fully dry the material, so the maintenance of the dryer is very important.
🧪 (3) Test results: Check the moisture content of the dried material through a moisture analyzer to ensure sufficient drying.
🔍 (4) Dew point detector: When the drying equipment itself doesn't have a dew point display, it can be detected by a handheld dew point meter.
Material pre-drying isn't a simple matter that can be generalized. Smart injection molding companies will not blindly follow the extreme practices of "always dry" or "never dry", but instead establish a data-driven decision-making process.