Ten molding techniques -- moisture in plastic granules

Drying plastic materials
Most engineering plastics require that the moisture content in the particles be lower than a certain maximum value. Whether the plastics need to be dried mainly depends on the sensitivity of raw materials to moisture. Generally, the moisture content of plastic materials is related to transportation conditions, packaging type and storage time.
For example, polyamide (PA) is generally transported in bags. This kind of bag uses aluminum as the waterproof layer, so it can be directly opened to take out the material for use. However, the raw resin of most polyamide (nylon) processing plants must be dried first, although some people believe that if the plastic is used in less than one hour, it does not need to be dried
On the other hand, PET and PBT are very sensitive to moisture, and must be dried to ensure that the impact strength of the product is not affected. In addition, these resins will absorb water again soon after drying. Therefore, in some cases, the processing manufacturer must take special care to avoid using open containers during the transportation and transmission of PBT and PET resins. At the same time, the retention time of the resin in the loading hopper should also be considered.
In adverse climatic environment, the moisture absorbed by PET in 10 minutes can even exceed the maximum allowable value of product moisture by 0.02%. Special attention shall be paid to the recycled material after drying and crushing or the granular material saturated with water absorption (such as when placed in a container under open conditions).
In this case, the recommended drying time often does not meet the requirements. Fully saturated polyamide nylon shall be dried for at least 12 hours. In this dry condition, yellowing of materials is actually inevitable
Therefore, the following guidelines should be followed:
1. The recycled materials in the flow channel shall be stored and re-crushed in a closed container;
2. If only part of the material is used, the container or bag shall be closed;
3. The loading hopper should be covered.
How to dry plastic
To obtain high-quality plastic products, the correct drying procedures must be followed. For example, various types of simple hot air drying methods cannot be used to dry polyester engineering plastics, but air dryers (dehumidification dryers) that remove moisture can be used. Only equipment that can provide stable and sufficient drying under different environmental conditions should be used.
In addition to observing the correct drying temperature, it is also important to ensure that the dew point in the dry air is below minus 20 ℃. When using multi-container devices with different filling height and bulk density, it should also ensure that each container can achieve sufficient air throughput.
Measure water content
The moisture in plastic particles can be measured by some measuring instruments, such as pressure gauges or instruments using the Karl Fischer method. In order to eliminate the measurement error, the sample shall be selected from the lower part of the feed hopper and sealed in a suitable container.
Polyethylene and aluminum multi-layer composite bags can be used for packaging samples with heat sealing method, or sealed glass containers for laboratory use can be used