A Note About Drying

I often get calls asking about material drying and why they have problems.

Engineering polymers need drying before they can be processed. These include PET, PC, ABS, Nylon and Acetyl. These are the popular ones but there are others, many of them blends.

A standard hot air dryer can handle many applications. When problems arise it is normally due to some easily fixable fault with the dryer.

  1. Make sure that the fan is turning in the correct direction. This is normally counter-clockwise. It might sound stupid, but dryers are sometimes moved from one machine to another and the phases might not be the same when they are reconnected. Check the direction if the dryer has been moved or is newly installed.
  2. Most dryers have a rotating shut off flap mounted on the front of the fan. The flap needs to be partially to fully open. The dryer cannot work properly if this flap is closed and it is being starved of air.
  3. There are two sieves in a vertical hot air dryer. Sometimes the inverted sieve with the cross shaped steel rods in not reinstalled after the dryer has been cleaned after a material change. It is very important that this sieve is in place in the dryer. The small side of the cone must face upwards. This sieve stops the machine from drawing a column of cold wet material from the top of the dryer. It forces the material to travel slowly down to the machine throat and be dry before it arrives for melting and moulding.
  4. Faulty heater rods or bridges. If you remove the fan from the vertical heating tube in the dryer you will find a series of elements. These are normally bridged in three places by a bridge. These bridges are sometimes made of a soft pewter type material which cracks and stops the heater tubes from working properly. Make sure these are all intact especially if the dryer is not warming up sufficiently. 
  5. If you suspect that the dryer is not reaching temperature although it is reading on the round analog dial mounted at the bottom of the neck of the heating tube, then remove the dial and test it. The easiest way is to boil a kettle and put it into the boiling water. At sea level it should be really close to 100 degrees. You will have to adjust accordingly if at altitude. If it is reading 100 degrees when the dryer is set to 100 degrees then the dryer is working properly. This removes any doubt that the dryer is not reaching temperature.
  6. Follow the manufacturers material drying guidelines. There are minimum and maximum drying temps for all types of materials and the best place to start is to check the processing guidelines for the exact grade that you’re using. You might be using a temperature that is too low for that grade.
  7. Make sure that the dryer is big enough for the throughput of material. It’s easy to check the throughput of the machine and cross check against the processing guidelines for drying time to make sure that the machine is not using more material per hour than the dryer can dry.
  8. Sometimes the evidence of splash or splay marks on components can be solved by processing conditions. A small amount of back pressure will often solve these marks. Make sure to use as little suck back (screw decompression) as possible. Also set the speed of the suck back as low as possible. Mould vents slowly become gummed up with resin from the gasses exhausted through them when a mould has been running for a while. I have found this to be a major factor when processing ABS and PS in particular. The solution is to bring the ejector pins forward and wipe everything including both faces of the mould with a solvent. This removes the gum and allows the vapour to escape again. 
  9. If you’re using master batch through a dosing unit make sure that the carrier is the same as the material you are adding it to. If the percentage of dosage is quite high (around 4% and up) then water marks could well be from wet master batch. You need to dry it first. 

I hope this will help one day when you have problems with drying or splay marks. If not then feel free to give me a call.

Hopper Dryers
Mould Temperature Controllers & Water Type Chillers