Drying the Plastic

PET plastic must be dried before extrusion. Any residual water can lead to hydrolysis during extrusion. Hydrolysis in turn causes the melt viscosity to drop. Low viscosity PET is difficult to make into quality filament. To dry the flake, it should be heated to between 160 and 175 deg C for 4 hours.

A drying hopper has been incorporated into the design. This allows for continuous drying of the flake before and during extrusion. If you are not using a drying hopper, the plastic can be dried in a bench top oven before extrusion. Either method should be done in a well ventilated area as fumes can be released from the plastic. Make sure you have appropriate safety equipment such as a fire extinguisher.

Using the Drying Hopper:

Step 1: Make sure the plastic flakes are clean and free of fines. Load the plastic flakes into the drying hopper.

Step 2: Place the lid on the drying hopper and make sure that the ventilation system is turned on.

Step 3: Turn on the drying hopper and set the temperature to 180 °C. Note: The set temperature on the drying hopper is 180 °C because the temperature sensor is upstream of the plastic. The actual plastic temperature will be closer to 170 °C.

Step 4: Allow the plastic to dry for four hours before starting the extruder.

Thoughts - Frequently in industry, moisture is removed by blowing cool air through desiccant. The air is then heated before going through the flakes. After that, it is cooled and recirculated. Use of pre-dried air helps to achieve even drier flake, especially in humid environments. We have not tested how much improvement could be gained in filament quality using such a system.

Using a Bench Top Oven

Step 1: Make sure the plastic flake is clean and free of fines. Spread it out on a tray no more than 2.5 cm thick.

Step 2: Turn on the oven and set it to 165 °C. Use a temperature prove to verify the temperature if the oven does not display it.

Step 3: When the temperature is stable, place the plastic into the oven.

Step 4: Dry the plastic for 4 hours, periodically checking the temperature.

Step 5: Remove the plastic from the drying oven and put in the hopper of the extruder. Warning: The plastic and tray will be very hot! Use gloves for protection.

NOTE: Air circulation will probably not be as good when using a bench top oven because air is not being forced through the plastic. If your oven has a convection bake setting, use it.