230516 - Polymer technology Flashcards
influence of sprue shape
linear sprue promotes a better flow of the material, point sprue will not promote a uniform
Insert
metal inserts:
security against twisting and tearing out
heat insert to expand, if inserted cold, the polymer is hot and the insert will expand and crack the plastic
holes
reduce notch effect by adding a pestañita around the hole
keep the thickness constant
threads
external: best. The thread would be unmolded with core and cavity.
internal: unfavorable. favorable if a socket is inserted outside of the boss to prevent crack formation or creep of the plastic. the mold would need to “screw out” of the internal thread.
design variables composite materials
- fiber volume. high fiber content increases risk fiber to fiber contact and not bonded fibers.
- length
- orientation of layers
- shift sequence
- different fibers, carbon and glass fiber together for example (cost-effective). aramyd or PE and carbon (impact strength improves).
design rules composite
- first and last layer 45deg
- neighbouring layers should not differ more than 60deg: tranverse ffibers high shrinkage, paralel less shrinkage. high shear, delamination or warping. (mechanical pproperties of the composite are smaller in transverse direction to the fiber)
*Avoid sharp corners: not an appropriate positioning of the laminates. (tool made or hand made).
crack stoppers
bad: thick laminates
good: thin laminates with a variation in the fiber orientation stacked, will stop the propagation of the cracks. variations in fiber: improves resistance to shear stresses.
sttiffeners
- ribs
- double shell structures.
- sandwiches panels
What is cold pressing? VIDEO
- Cold tool (60deg).
- preimpregnated thermoplastic material GMT (glass mat reinforced thermoplastic), LFT (long fiber reinforced thermoplastics). preheating.
- close tool.
- pressure.
Long fiber. injection molded: short fiber.
comparison between long fiber and short fiber
higher mechanical properties compared to short fiber.
more expensive.
cycle time
most of it in heating
forming a very short
cooling: long, not that long
effects on corners
it may be necessary to clamp the organo sheet on the corners to avoid distortion of the fibers.
a further injection molding process can be included to mold snap fits or other features.
differences between metal and silicone stamps
special silicone, not a high complexity.
What is thermoplastic winding?
more fiber volume content
the fiber is winded around the core, which is itself heated by radiation.
What is thermoplastic winding? VIDEO
more fiber volume content
the fiber is winded around the core, which is itself heated by radiation. pullable cores, dismountable, melting and dissolving cores, lost.
