PA6 nylon: causes of discolouration and how to avoid it.

PA6 (polyamide 6) nylon has excellent mechanical strength and abrasion resistance and is used in many industries. However, if the material is not dried properly before molding, problems can occur, such as yellowing of the molded product and reduced physical properties. In particular, over-drying and thermal degradation due to high temperatures are major causes of molding defects. This article provides detailed information on the precautions to be taken when drying PA6 material and how to avoid thermal degradation.

Properties of PA6 nylon and the importance of drying

Hygroscopicity of PA6 nylon

PA6 is a highly hygroscopic material. If it absorbs moisture from the air, hydrolysis is likely to occur during molding. This can lead to problems such as reduced strength, dimensional changes and poor appearance. It is therefore essential to dry the material appropriately before molding.

Reasons why drying is necessary

Problems caused by insufficient drying include

Gassing → silver streaks and voids

Hydrolysis → reduced mechanical strength

Cloudy or matted surface effects

However, if the drying process is too dry, thermal degradation will occur, resulting in discolouration and a reduction in physical properties, which requires appropriate control.

Main causes of discolouration due to thermal degradation

(1) Drying temperature too high

The general guide is to dry PA6 at around 80°C for 3-5 hours. However, prolonged drying at temperatures above 100 °C will lead to oxidation and yellow discolouration.

🔹 Measures:.

Strictly adhere to the manufacturer's recommendations of 80°C for 3-5 hours.

Regularly check the temperature setting of the dryer.

(2) Cylinder temperature too high during molding

Nylon is often molded at temperatures in the 230-270°C range, but if the temperature is too high, the material will decompose, causing discolouration and gas emissions.

🔹 Countermeasures:.

Adjust temperature in the rear zone to the correct value (230-250°C).

Shorten residence time and prevent degradation.

(3) Long residence time in the cylinder

If the shot size is too small, the resin stays in the cylinder for a long time, leading to thermal degradation.

🔹 Countermeasure:.

Set shot size in the range 20-80% of cylinder capacity.

Shorten the molding cycle and reduce unnecessary heating time

(4) Material oxidation

Contact with oxygen during drying and molding causes oxidation and discolouration. Drying at high temperatures or leaving the material for long periods of time will result in a more pronounced deterioration.

🔹 Measures:.

Regularly clean dryer filters to reduce oxidation.

Introduce nitrogen displacement drying (especially when drying at high temperatures).

(5) Use of regrind material

Regrind material (regrind material) has already undergone a thermal history and is therefore more prone to discolouration.

🔹 Measures:.

Limit the use of regrind material to 20% or less.

Exclude regrind materials that have deteriorated

Key points to prevent thermal degradation.

Strictly adhere to drying temperatures and times (80°C, 3-5 hours as a guide).

Set cylinder temperature within the appropriate range (230-270°C)

Adjust shot size appropriately to reduce residence time

Ensure air venting (degassing) to prevent oxidation

Reduce the proportion of regrind material used

In particular, optimising the drying process and molding conditions is key to preventing PA6 degradation.

Conclusion.

PA6 nylon is a material that is prone to discolouration and loss of physical properties due to thermal degradation if not dried properly. In particular, factors such as excessively high drying temperatures, excessive cylinder temperatures and long residence times are the main causes of yellow discolouration.

As a countermeasure, it is important to observe appropriate drying conditions (80°C, 3-5 hours) and to optimise the molding conditions. In addition, material control (limiting the use of regrind materials, introducing nitrogen displacement drying, etc.) can also be considered to further stabilise quality.

If you have problems with discolouration or degradation of PA6, check the points introduced in this article and take appropriate measures!