230418 - Polymer Technology

Question 1

Pressure behavior during switchover points

Answer 1

Hydraulic pressure: Linear increase of the pressure then a steep rise until holding pressure.
Cavity pressure: Linear increase, then a non linear increment of the pressure and then decrease of it while the cooling down process.

Question 2

Switchover point determination

Answer 2

Practical design of experiments to determine the precise point.
Too late switchover: tool damaging, flash.
Too early switchover: sink marks, non complete filling.

Question 3

Total shrinkage time

Answer 3

16 - 24 hrs. After this time, the shrinkage and crystallization is over and the part can be gauged.
Shrinkage difference between reality and simulation can be very high (50%). Temperature distribution on surface of mold.

Question 4

Parameters machine choosing

Answer 4

Plastification size (5x cavity, enough depth of the flights in the dosing part of the screw)
-> Clamping force (***) -> size of plates -> opening stroke (3Xdepth)

Question 5

Incorrect machine selection

Answer 5

• Too big claping force: Expensive.
• Cycle time increases.
• Too high residence time in the plasticized damage, Too low, no homogenaization.

Question 6

Injection or Holding pressure driven part

Answer 6

High injection speed to reach the injection pressure fast enough. The material will get cold soon.
Lot of material must be injected before the gate is sealed.
ratio: flow path length divided by the thickness. If ratio > 100: injection speed inflluenced part.

Question 7

small flow path and big thickness

Answer 7

holding pressure influenced part

Question 8

Estimation of clamping force

Answer 8

Projected area * cavity pressure * security factor (asimmetry, sliders)
The projected area should be symmetric in direction of injection.

Question 9

Rheology

Answer 9

Science of deformation and flow of the body

Question 10

Shear stress

Answer 10

shear stress = viscosity * shear velocity
? = ? * ?
For each polymer, you can determine the viscosity in terms of the temperature, and factors A,B, C (carreau theory)

Question 11

Difference between dilatant, intrinsically viscous and newtonian fluid

Answer 11

Newtonian fluid: Linear relation between the velocity and the stress.
Dilatant: Small velocity increase requires a big stress increase.
Intrinsically viscous: Big velocity increase requires a small stress increase. Increase shear rate, decrease viscosity.

Question 12

Pressure drop

Answer 12

inverselly proportional to the wall thickness

Question 13

clamping pressure calculation

Answer 13

(pw): Reference values for flow path / wall thickness. Technical parts (1%), simple parts (2%) acceptable increase in weight
You can determine the clamping force in terms of the specified ratio.
If you have a bigger machine than the needed pressure, you can adjust the clamping force in hydraulical and electrical toggle machines

Question 14

too small screw

Answer 14

inhumogeneus melt, unfused material.
Viscotiy fluctuations

Question 15

too big screw

Answer 15

Hardly controllable injection process into the nozzle, material degradation,

Question 16

average residence time

Answer 16

average residence time = (density in melted * Volume inside of screw / mass of shot ) * cycle time
Should be between 3 and 6 minutes