For many plastic molding companies, how to dry material is an inevitable problem and engineering plastics especially need drying. The drying purpose is to make good quality products. The main problem caused by the change in the performance of the product is not directly detected by the naked eye, but can only be found through testing its appropriate components, such as mechanical properties and dielectric strength.
When selecting the drying process, the drying properties of the material are of key importance. Plastic material can be divided into two kinds: hygroscopic and non-hygroscopic. Because of its physical and chemical structure, the hygroscopic material would absorb moisture from the surrounding environment, and constrain them in its internal structure. Non-hygroscopic materials cannot absorb moisture from the environment. For non-hygroscopic material, any moisture present in the environment remain on the surfaces, being “surface water”, it is relatively easy to remove. Particles made of non-absorbent material can be hygroscopic because of the effects of additives or fillers, and absorb moisture from the environment.
1, how to confirm the excessive moisture
For several common plastic materials (crystalline engineering plastics), too high moisture content of plastic material would have following impacts on the product and the performance:
|plastic||Phenomenon during injection||Defects of surface||Impact on performance|
|PA||Nozzle droolingPossible bubble inside barrel||Jet on flow directionflash||Reduce the impact and tensile strength|
|PET||No obvious phenomenon||No surface streak||Reduce the impact and tensile strength heavily|
|PBT||No obvious phenomenon||No surface streak||Reduce the impact and tensile strength heavily|
|POM||Possible bubble inside barrelPossible dirt on nozzle||Possible flow jet||Nonobvious|
2, maximum moisture content, drying temperature and drying time technical requirements for plastic raw materials
Most plastics can absorb moisture from the atmosphere and the surrounding environment; the adsorption varies because of the different types of plastics material. Too high moisture content in the raw material grains (even just the accumulation on the surface of grains) will cause many of the quality problems during injection molding; seriously affect the smooth production of injection molding, too frequently adjust the production process of the abnormality and result in poor appearance and deduce the mechanical strength of the workpiece.
In general, the moisture of raw materials can not directly be checked by eyes, the following table adopted by topworks plastic mold company provides maximum moisture content, drying temperature and drying time technical parameters of plastic raw material
|Plastic||Moisture content||Drying temperature||Drying time||Remarks|
|PA||0.2%||80℃||2-4 hrs.||Drying if material exposed to air directly|
|PBT||0.05%||120℃||3-4 hrs.||Drying by plastic dehumidifying dryer|
|PET||0.02%||130℃||3-4 hrs.||Drying by plastic dehumidifying dryer|
|POM||0.05%||80℃||1 hr.||Drying if dubiosity moisture accumulation of surface|
3. Drying plastic raw materials
Before injection molding, most plastic raw materials need its moisture contents drop to a certain level. The drying time depends on the water-sensitive plastic material. At same time it should take into account of packaging way, storage duration and different usage.
PA (nylon) mostly is sealed by multi-foil packaging bags, so you can directly use that after open the bag. If the raw materials will be used up within one hour, drying is not necessary.
PET and PBT (polyester resin) are particularly sensitive to the surrounding environment, and they must be dried to ensure a smooth production and quality injection molding parts before processing. The material could rapidly absorb the surrounding moisture after drying, so it should avoid the use of open hopper, material on the delivery system or inside the hopper should be dried. Under greater humidity climate (such as rain day), PET will absorb large amounts of moisture in 10 minutes, and the moisture content would exceed the allowable content of 0.02%.
Reground materials and fully hygroscopic particles (if material containers always open) must be fully dried. On this situation, the drying time is not adequate. Fully absorbent nylon may need more than 12 hours in the drying process, and may cause yellowing of the raw material.
As to plastic injection molding, it could take the following precautions to reduce the moisture content of the raw material:
A, store the reground and runner material in closed containers;
B, the container or bag with infrequent use should be sealed;
C.cover the hopper.
4, new drying method – dehumidifying air dryer system
To get a good quality plastic injection molding parts, it is very important to follow the correct procedures. Using a traditional hot air dryer to dry plastic materials is not enough. Dehumidified air dryer system can improve this situation; this system can provide a stable and qualified raw material drying, free of affection by environmental and climatic conditions.
Dehumidified air dryer system in use should follow:
A, maintain the proper drying temperature;
B, confirm that the dry air is maintained below the dew point -20 ℃;
C, and for different filling height and the particle density containers, to ensure an adequate circulation air inside each container;
5, Measurement of moisture content of engineering plastic
The moisture content of the plastic raw materials can be measured by instrument. For example, use a pressure gauge or Karl-Fischer method.
In order to eliminate deviation caused by the sample, the drying sample should be taken from the bottom of heating device and stored in a suitable container. Glass containers for laboratory samples are also acceptable.