The process engineer seeks to reduce downtime and defective product. In sheet extrusion there are some defects: stains, contamination, orange peel, thickness variation and bubbles or fisheye. These have in common the cause of accumulated dirt inherent in the process.
- Surface staining can be caused by grime accumulated on the lip of the die.
- The internal contamination comes from the extruder or die: carbon and contaminants that passed the filters. Some holes are produced by thermoforming due to these contaminants. Capped screens also cause thickness variations as pressure changes.
- Vacuum pits and orange peel caused by lack of vacuum in the extruder are due to volatiles within the dough; When the gas is formed, it is released, bursting into bubbles.
The following steps can help the process engineer to keep the three mentioned areas (die, meshes, and vacuum) clean as part of best sheet extrusion practices.
- Mark the foil so the packer knows you will start cleaning and it is clear how far to separate the good product.
- Clean the lip of the socket with the tool provided by the socket manufacturer or in some cases a copper or bronze bar of the correct thickness can be used to carve this area.
- Now do the filter change on the mesh carrier. Some processes are cleaner than others because of the type of raw material used. One way to know whether the screen is worth changing is by monitoring the pressure just before the screen. The higher it is, the more covered it is. Define an optimal pressure point to change it this way.
- Clean the vents area. This is that the orifice of the extruder is clean, also make sure that the pipe to the pump is not clogged and the volatiles (kind of oils) are flowing. Keep in mind that your vacuum pump must generate enough suction to remove the gases trapped in the dough. Important note the pump can mark enough vacuum, but if the pipe is plugged then the volatiles will not come out of the extruder. In the same way the pipe can be clean, but the pump does not suck enough, so the volatiles will not come out either.
- Allow reasonable time for the plastic flow to stabilize and check other machine control areas such as speeds, pressures, temperatures, gauges, etc.
- Once the controls are within range and you see good product, mark the inspection point so the packer knows whether to pack or wait for product release for other tests. And so, you know from which point you already have a product with the correct quality.
By applying cleaning to these areas at the same time you avoid generating waste and downtime that would occur if they were done separately. In an annual production we are talking about tons and hours that we can earn.
Other defects can be prevented with a good purge program and recommendations that we have for your successful production.