Foam extrusion was and is a relatively complicated process. There are many variables sometimes acting unpredictably, like 'magic'.
Our vision was to develop the foam extrusion technology which would eliminate the 'magic', and would be simple, user-friendly, quick responsive control of the system producing good quality products.
Basic functions in a foam extruder could be described as:
- Metering plastic and some additives
- Melting mixture
- Adding blowing gases
- Mixing gases in the melt
- Cooling melt to proper temperature
- Extruding the melt saturated with blowing gases through the die
The main challenges were:
- Getting a steady and high extruder output
- Developing steady and accurate metering of the blowing agents (gases)
- Developing an effective system to mix gas into a melted plastic
- Developing efficient cooling of the melt
Challenges to overcome:
There are many variables, some can have a positive effect on certain aspects of the process while simultaneously having a negative effect on another aspect of the process as well as the product. For example, high output and good mixing requires high screw RPM. Good cooling requires low screw RPM, because high RPM creates more heat. Good mixing requires high shear rate, but high shear rate creates more heat. Good cooling rule: if you do not create heat, you do not have to remove it.
Results of our R&D
At Alcot Plastics we managed to develop a single screw extrusion system which meets the challenges. We developed:
- Special screw which provides extremely intensive mixing even at low screw RPM without creating high heat, keeping in mind that if you do not create heat, you do not have to remove it.
- Highly efficient melt cooling even at high screw RPM enabling us to have such a low melt temperature that evaporation heat of the gas after the melt leaves the die could freeze the expanded melt and the product is ready just a few feet from the die.
- Modified gas metering pumps to pump the blowing gases steadily and more accurately.
The Advantage of Our System
- Small extruders at high RPM could produce the same or higher outputs as larger one's at low RPM
- Production lines are much shorter, saving space
- Energy consumption is less
- Capital expenses are lower
- Easy, quick responsive control of the system (time from resin entering the extruder to the time leaving the die as a product is on average 7 minutes)
- Effect of change of injected gas on density is a few minutes
- Change of amount of nucleation is seen in a few minutes
- Start up times are short, on average 10 minutes (i.e. 10 minutes after pressing start the product is ready to be packed)
- Possible to run quite efficiently small orders or run only 8 hour shifts without creating waste
- Such good mixing that we get extremely uniform cell structure and we do not have any problem with mixing even after injecting melted foam stabilizers
- On our smaller extruder, it is possible to make products which normally would require larger extruders running at low RPM
- Extrusion is very consistent, nevertheless we developed an automatic laser-like diameter (or height) control, sensitivity of which is about .001". This system is controlling the diameter and automatically correcting it if necessary.
We have developed an economical technology which is user-friendly, good and quick responsive to command changes in parameters such as density, colour or foam cell structure, outperforming larger extruders in output and quality of the products.