Design - plastics Flashcards
1
Q
Design guidlines
A
- Consider proerties of the plastic: non-linear stress behaviour, and temp, time, moisture, UV, and chemical dependent
- Consider application of product - what environment will it be exposed to?
- Consider manufacturing method and economy
2
Q
What are mistakes often due to?
A
- Bad choice of material
- Insufficient knowledge about plastics
- Design without consideration of manufacturing
- Designed towards metals rather than plastics
3
Q
Injection molding - design guidlines
A
- Aim for even wall thickness - use as thin as possible to save material and cooling time
- Use ribs instead of large wall thickness
- Use correct radius (not too small nor large)
- Avoid large flat areas to prevent warp
- Integrate as many functions as possible (ex. hinges)
- Use cost-efficient joining methods
4
Q
Injection molding - wall thickness
A
- Even - max. 15% of variation
- Uneven leads to warpage due to uneven stresses
- Normal wall thickness = 1,5-4 mm
- Changing thickness affects: weight, process cycle time, bending stiffness, quality
- Cooling-time = (wall thickness)^2 x 2,5
5
Q
Bending stiffness - how to increase
A
Can be increased by:
- Larger wall thickness
- Adding ribs
- Use material with higher Youngs modulus
6
Q
Ribs
A
- Used to minimize risk of sink marks but too thick ribs cause sink marks as well
- Height of them < 3 times of wall thickness
- Thickness of them = 1/2-1/3 of wall thickness
- Avoid sharp angles - use release angles, depend on height of ribs, normally 0,5-3 degrees
7
Q
Radius
A
- Plastics are sensitive to sharp corners
- Radius = 1/2 of wall thickness
- Too small - stress concentration causing failure
8
Q
Weld lines
A
- Where materials meet
- Try to minimize number of weld lines since they are defects
- Don’t place weld lines in areas with high stresses
- Fibers = no strengthening in weld lines
9
Q
Joining of plastics - guidlines and types
A
- Try to avoid joining
- Big risk of failure
- Don’t join plastic + metal since joint will loosen due to relaxation of plastic
Types:
- Welding: friction, ultrasonic, hot plate, hot bar
- Adhesives
- Mechanical fasteners: rivets, snap-fits
10
Q
Welding - properties
A
- Cost-efficient for permanent joining
- Choice of welding depend on: geometry and type of plastic, mehanical loading, possibility to integrate welding into manufacturing
- Cannot weld amorphous and semi-crystalline thermoplastics due to poor bonding
- Moisture absorbing plastics must dry before
11
Q
Friction welding
A
- Friction heat is used to melt plastic at welding point and is generated by weld time, force, velocity of motion
- Linear = vibration welding: thermoplastics rubbed under force: quick, strong bonding, recycleble
- Rotational = spin welding: circular motion of one part against another under force
12
Q
Ultrasonic welding
A
- High-frequency mechanical motion generating heat and “reform” thermoplastics
+Quick
+Clean
+Efficient
+Strong bonds with small energy consumption
13
Q
Hot plate welding
A
- Heated tool is placed onto or near welding surface
- Can join thermoplastics with different Tm - use different plates to melt at different temp.
14
Q
Hot bar welding
A
- Simpliest welding for polymers
- Overlapping thermoplastic films are pinched between electrically heated bars
+Quick
+Cheap
+Clean
15
Q
Adhesives
A
- Almost all materials can be adhesively bonded
+Great design freedom
+Resist shear force
+Light-weight joints
-Hard to recycle
-Bad working environment
-Used for thermosets mostly
