Exam 2 Material Flashcards
(45 cards)
Process where you heat a sheet of plastic, then form to a mold using a vacuum
Thermoforming
Advantages of Thermoforming (7 things)
very low tooling cost
low mold cost
low pressure req.
large parts easily formed
fast mold cycles
extremely adaptive to customer design needs
paint-able and colored plastic available
Disadvantages of Thermoforming (6 things)
high cost of raw mats
high scrap
unable to mold parts with negative draft (concave)
limited part shapes
inherent wall thickness variation
internal stresses common
what materials can be used for thermoforming molds?
Cast or machined aluminum
wood
epoxy
steel
Areal Draw Ratio formula
Surface Area of part being formed
/
area of sheet needed to make part
Area Ratio formula
area of sheet used to make part before forming
/
area of sheet used to make part after forming
estimated initial sheet thickness (gauge) formula
(areal draw ratio) * (final thickness)
Pre-stretching tool used to improve dimensional consistency when the draw ratio is sufficiently large. It’s like a plunger that presses the material into the mold prior to vacuuming.
Plug Assists
in thermoforming, how do you determine the width or length of a part?
the larger dimension is considered the length, the smaller is considered the width
What are the major differences between skin packs and blister packs?
Skin pack has plastic vacuum formed to the product, custom
Blister pack plastic is pre-formed prior to use with product, generic
this process is used to shape thermoplastic sheets and films into discrete parts
Thermoforming
this process is different from extrusion and injection molding because the material is not melted and higher pressures are not required.
Thermoforming
What are the principal steps to Rotational Molding?
- loading/charging
- heating (under rotation)
- cooling
- unloading/demolding
What’s the typical rotation ratio between axes in Rotational Molding?
3.75:1
What constraints exist to get even coating in Rotational Molding?
Rotate about 2 axes simultaneously
different rotation speeds (major axis > minor axis)
Ratio of rotations not a whole number
Slow rotation (< 60 RPM)
Which process is capable of molding much larger parts than the other common thermoplastics processes?
Rotational Molding
For which Thermoplastic process does the cost-effectiveness of the part increase with the size of the part?
Rotational Molding
What are some common thermoset processes?
Reaction injection molding
extrusion
spin casting
foam molding
compression molding
transfer molding
What’s the primary/most-common process for thermosetting plastics?
Compression Molding
What process preheats a preformed “pill”, then uses a plunger to force the preform into sprue, runners, and mold cavity?
Transfer Molding
Which process mixes two liquids, as they are injected into a mold at high speed?
Reaction Injection Molding (RIM)
What’s a downside of Reaction Injection Molding?
Controlling metering, mixing, temperature, and pressure are generally complex and expensive
Which process rotates a disk-shaped mold about it’s centroidal axis, then pours in material, in order to force the material throughout the mold?
Spin Casting (Also called Centrifugal Rubber Mold Casting)
What are the variables to consider with compression molding?
Material/Resin amount
Energy needed to heat material
Mold/material heating time
Compression force for that material
Cooling time/technique
