230502 - Polymer Technology

Question 1

How do I calculate the residence time if I have the screw size?

Answer 1

Question 2

What is the melt cushion?

Answer 2

An amount of plastic that will serve as extra material to be injected in the holding phase.
5mm
x=0 in the nozzle:
* Melt cushion’s volume gets SMALLER - sometimes the screw can touch the nozzle (not enough material)
* Melt cushion’s volume gets BIGGER- the plastic compresses and the pressure does not get to the cavity

Question 3

Ratio dosing-length vs screw diameter

Answer 3

1 < Ratio dosing-length < 3

Question 4

Average residence time

Answer 4

3min< residence time < 6min
Otherwise, you may have an inhomogeneous plastification of the material.

Question 5

4 parameters that define the cooling time

Answer 5

• Thermal conductivity
• Dominant wall thickness.
• Melt temperature.
• Ejection temperature (experimentally determined, measured with a thermocouple).
  ///
  cooling time:
• 60%: holding time
• 40%: rest cooling time

Question 6

How do you determine the holding pressure time?

Answer 6

Experimentally determined: several tests and weigh them

Question 7

Ejection temperature

Answer 7

Every point in the part should be below the ejection temperature, so that the part does not get pierced.

Question 8

Dominant wall thickness

Answer 8

Cooling: THICK walls take more time to cool down.
Clamping force: Thin walls thickness require more pressure.

Question 9

Crystallization influence on cooling down process***

Answer 9

• The crystallization process at cooling starts at a given temperature (crystallization temperature), and it keeps constant during this process.
• Intrinsic heat source from the crystallization.
• Heat up : heat sink, an ice cube has the same temperature until it’s completely molten

Question 10

What is thermal diffusivity? Relation to cooling process?

Answer 10

It characterizes the speed at which temperature changes can propagate through a material, how quickly heat spreads within a material.
* HIGH thermal diffusivity, distributes heat more readily, distribute temperature changes more quickly
* LOW thermal diffusivity, slower in distributing heat, retain heat for longer periods.

α = κ / (ρ * c)

Question 11

cooling time influencing factors

Answer 11

Wall thickness - Increase (squared)
Bigger thermal diffusivity - decrease
cooling time:
* 60%: holding time
* 40%: rest cooling time

Question 12

What are the calculations we must do for a given process?

Answer 12

Machine
cooling time
screw size