230328 - Polymer Technology

Question 1

What’s the purpose of the holding stage?

Answer 1

First injection stage: 90 to 98 % of material.
Second injection stage: Packing OR Holding. Remaining plastic, surface finish, dimensions. Pressure is controlled.

Question 2

Differences between the unbalanced, natural balanced and artificial balanced runner

Answer 2

Naturally balanced: same flow pass lengths. Aritificial Balanced: change the thickness of the runner to have a uniform filling. Reduces overpacking and flash formation.

Question 3

3 plate tool

Answer 3

3 parting lines, separation of the runner with oone of the plates.

Question 4

Shrinkage in glasss fiber added plastic. Composite Materials.

Answer 4

In direction of the glass fiber: small shrinkage.
Thermal expansion coefficient:
* 8 microns per Kelvin glass fiber.
* 100 – 200 microns per Kelvin thermoplastic.

Question 5

Crystallization vs. Time

Answer 5

** Purely crystalline: light cannot pass through.
Fast cooling: low crystallinity (amorphous material). Flexible, impact resistant materials.
Semicrystalline: some areas have crystalline areas, no light pass through the folding molecules (milky material). Stiff, opaque materials.
Crystals take time to develop in the thermoplastic.
Crystallization favors shrinkage and higher tendency for warping.

Question 6

Characteristics of POM:

Answer 6

Polyoxymethylene
High stiffness, low friction, semmicrystalline material.
Gear wheels, eyeglass frames, ball bearings. Automotive industry, power windows, light shifting.

Question 7

Influence of mold temperature

Answer 7

• high cooling rate: the material do not fill the texture.
• Low cooling rate: the material is fluid enough to fill the texture.

Question 8

Material pre-drying

Answer 8

PA absorbs water. PET, PBT.
Amids are polar molecules and this attracts water (also a polar molecule): dipolar effect.
* Wet PA: low UTS, high ductility.
* Dry PA: high UTS, low ductility.

Question 9

Two types of resin drying

Answer 9

Hot air: pumping exterior humidity and this could increase humidity in the PA.
Dry air: Previous redrying.
Leave it open and drying for 3 days. But that depends on the humidity of the surrounding air.

Question 10

Hidrolytically degradation

Answer 10

Polymer bonds react with water molecules, break up, and produce new chain ends. The original chains break up into smaller segments, resulting in polymer degradation.

Question 11

What’s the difference between electrical injection machines and hidraulic injection machines?

Answer 11

• Electrical: faster, use less energy since they are only active when the motor acts, digital control. No oil to warm, faster start up times.
• Hidraulical: slower, more power,

Question 12

pressure

Answer 12

Thin part, big pressure.
Thick part: low pressure.

Question 13

Orientation of the fibers:

Answer 13

Boundary layer: oriented in the direction of the injection.
Core layer: not a big orientation.

Question 14

Increase of injection speed effects:

Answer 14

• Burn effects in the end of the injection phase.
• Weld lines: better weld lines behavior, the material is hot and it joins better. Also you can have local heating in the weld area.
• Jetting: increased jetting, bad mechanical properties, no orientation of the fibers. Increase the gate, or change the position.

Question 15

switchover point?

Answer 15

Process parameters: Change between the injection phase and the holding pressure (Nachdruckt). Velocity controlled to pressure controlled.
Monitoring parameters: injection pressure, measured at the tip of the screw pE (Einspritzzeit). Cavity pressure: measured in the interior of the cavity. PW (Werkzeug)

Question 16

Too early switchover point effects:

Answer 16

• Visible marks: increase.
• Incomplete mold filling: increase.
• Burrs and flash: decrease.
• Tool damage due to overload: decrease.

Question 17

Holding pressure

Answer 17

process parameters: Holding pressure Pn and Holding time tn
Monitoring parameters: cavity pressure Pw (this is the value that actually correlates with the quality of the part)

Question 18

Increase of holding pressure

Answer 18

The same cavity, but more and more material gets in with the increase in pressure 300-800 bar (machine capabilities are around 2000 bar, but makes no sense to increase it).
Internal stresses at the interior of the part, shearing stresses. Tolerances of the part get more accurate with the increasing of the material

Question 19

Low holding pressure effects:

Answer 19

• sink marks and voids: increase.
• Shrinkage: increase.
• Dimensional stability: decrease.
• Demolding problems: decrease.

Question 20

PVT diagram for semicrystalline thermoplastics

Answer 20

Low pressure, high temperature: high specific volume, low density.
High pressure, low temperature: low specific volume, high density.
It also can be used to understand the whole holding phase.

Question 21

Dosing melt temperature

Answer 21

Process parameters:
- Cilinder temperature Tz,
- Screw speed: ns (Vs=0.3 m/s),
- Back pressure: pstau = 50 bar compact the pellets to get a homogeneus melt in the dosing of the material.

Monitoring parameters: Melt temperature Tm.

Question 22

Increase in melt temperature:

Answer 22

• Shrinkage: increases
• Cycle time: increase
• Flow length: increase

Question 23

Tool wall temperature

Answer 23

Process parameters: flow temperature Tv (Vorlauf)

Question 24

How is the material molten in the barrel?

Answer 24

• Friction generated by the rotation of the screw, called drag flow.
• Heating bands outside of the barrel, to compensate for radiation losses.

Question 25

What are the three zones of the screw?

Answer 25

• Feed zone: Constant flight depth, half of the length, forces the material together.
• Melt zone: Decreasing flight depth, reduces the volume, platizicing the material.
• Metering zone: Constant flight depth, the nozzle has a one-way valve that lets the material flow only towards the mold. Check valve, check ring.

Question 26

Two different types of runners:

Answer 26

Cold runners: the runners remain attached as in little car models. Machine is cheaper, but the runners have to be removed as a further operation.
Hot runners: The runners are hot during the whole process and this makes the machine more expensive, but reduces scrap.
Sprue, primary runner system, secondary runner system, and gate.

Question 27

Phases of injection molding:

Answer 27

Filling, Packing, Cooling and Mold opening. Packing and Cooling times are the biggest portion.

Question 28

Different transition temperatures:

Answer 28

1. Glass transition temperature: Amorphous domains start to flow, the amorphous material can flow through the nozzle.
2. Melting temperature: Crystalline structures break, semicrystalline materials can start flowing through the nozzle.
3. Crystallization temperature: The point at which structures re arrange.

Question 29

Properties of amorphous and semicrystalline materials:

Answer 29

1) Amorphous materials: Dimensional stability. do not perform as well as gears or bearings, lower chemical resistance.
PMMA, PC, ABS.
2) Semicrystalline: Low coefficient of friction, bearings and gears, tough materials. More shrinkage in direction of flow, less in perpendicular direction.
PP, PE, PET.