230502 - Polymer Technology Flashcards
How do I calculate the residence time if I have the screw size?
tv=(Vscrew/Vshot@meltT)*tcycle
What is the melt cushion?
An amount of plastic that will serve as extra material to be injected in the holding phase.
5mm
x=0 in the nozzle:
* The volume gets bigger - the plastic compress and the pressure does not get to the cavity
* The volume gets smaller - sometimes the screw can touch the nozzle (not enough material)
Ratio dosing-length vs screw diameter
1<ratio<3
Average residence time
3min< residence time < 6min
Otherwise, you may have an inhomogeneous plastification of the material.
4 parameters that define the cooling time
- Thermal conductivity
- Dominant wall thickness.
- Melt temperature.
- Ejection temperature (experimentally, measure with a thermocouple).
Holding pressure time
Experimentally: several tests and weight them
Ejection temperature
Every point in the part should be the below the ejection temperature, so that the part does not get pierced.
Dominant wall thickness
Cooling: biggest wall thickness takes more time to cool down.
Clamping force: thinnest wall thickness requires more pressure.
crystallization influence on cooling down process***
The crystallization process at cooling starts at a given temperature, and it keeps constant during this process.
Intrinsecal heat source from the crystallization.
heat up : heat sink,
an ice cube has the same temperature until it’s completely molten
What is thermal diffusivity? Relation to cooling process?
cooling time influencing factors
Wall thickness - Increase (squared)
Bigger thermal diffusivity - decrease
cooling time:
* 60%: holding time
* 40%: rest cooling time
What are the calculations we must do for a given process?
Machine
cooling time
screw size
