Definition: Drying: to make dry, free from moisture. Tip: to inform
The following are two simple tips that I have used over the years to dry material, when I have been presented with a too large a drying hopper for the material available. Please note that these are tips and should only be used in extreme cases.
In many cases the drying of material is well understood and then again is not understood at all. It should be explain that drying of hygroscopic materials is necessary for many reasons. When certain materials are not dried properly and then processed the residual moisture than reacts, destroying the physical properties of the material. In other cases we are able to re-dry but must understand that the material now has 2 additional heat histories, one via our machine and the other via heat generated by grinding it back up.
Drying small samples of materials;
Ideally we employ a small hopper and fill it to the max but in many cases we do not have a small hopper or unit available. In some literature there are references to using pillow cases and stacking them above the cone in our dryer hopper to dry the material and this solution works fairly well. The following points must be taken into consideration to perform this:
1- The pillow case is a cheap one.
2- This allows for a low thread count thus allowing air to flow through the case.
3- The material is stacked above the cone.
4- The case is not completely full so that it can conform or drape over our cone providing a uniform thickness in the area it covers. This is where multiple cases may be used.
5- This than gives a uniform resistance to flow of air.
6- Multiple materials can be placed in a dryer at same time with no contamination issues.
The results being dry material to a proper moisture level. If this is for short run production it should only be used for limited time with long term goal of getting a smaller hopper.
The 2nd case is the drying of 50 to 100 pounds or more of material but in a 200 to 400 pound dryer. (i.e. do not have enough material to fill hopper, or job calls for low amount of material). This is the typical case, in that the hopper is less than ˝ full. The problem here is that the cone in the hopper may be barely covered, thus the resistance to air flow is less in the center of the cone and thus the air flow may flow more at this point than at other areas covered by material (path of least resistance), resulting in processing of varied material dryness, resulting in poor part quality. In many cases the material never dries.
The solution is to create a resistance to air flow, in the hopper. This can be done I hate to say by placing a piece of cardboard cut to the diameter of the hopper, placed over the material and then holding it down by some method of weight. The airflow through this restriction should equal no more than the outlet area. There is some experimentation to this, as we do not want a tight seal where the reduction of airflow is such that it results in a greatly reduce temperature on the return line. What we are looking for is to generate airflow uniformly over the pellets so as to take away moisture.
Issues:
1- The use of cardboard introduces a contaminate
2- The use of cardboard introduces moisture
3- Holding of this cardboard above or onto the material can be an issue
The end result is that our material is now dry in the standard time mentioned in the material supplier's data.
The use of either of these tips is up to the individual, but care should be taken so as to minimize any possible contamination, or airflow restriction to the drying process.
SL Silvey
silveysplastics@hotmail.com
