Plastics Flashcards
(31 cards)
What are a couple of advantages of using plastics vs. using metals/castings etc. (5)
- Virtually unlimited variety of part geometry
- Plastics usually is a net shape process, and does not require machining or anything after
- Less energy $$ vs. metals
- Product handling during production is easier b/c lower temperatures and lighter weight $$
- Painting/plating not required $$
What are advantages of using plastic materials? (9)
- Corrosion and chemical resistance
- thermal and electrical insulating properties
- isotropic or anisotropic
- has good strength-weight ratio
- light weight babaaayyyy
- Thermoplastics are easy to process/reprocess
- can be rigid or flexible (ex. PVC)
- can be crystalline or amorphous (transparent, translucent, opaque)
What are some disadvantages of plastics? (8)
- Dimensional Instability (coeff. of thermal expansion)
- limited useful thermal range
- fragile (scratch, crack, break easy)
- flammable (smoke, flame)
- some are Hygroscopic (absorb moisture)
- can ruin with some chemicals
- can cost a lot
- some of the components (stabilizers, additives) can have health risks
What are 4 types of Plastic Products (types)
- Molded Parts
- Extruded
- Coatings (like electrical wires)
- Fibers (textiles)
Name some types of plastics used very commonly in the world (13)
- PE - polyethylene
- HDPE - High density polyethylene
- LDPE - Low density polyethylene
- PP - Polypropylene
- PVC - polyvinyl chloride
- PS - Polystytrene
- PU - Polyurethane
- PET - Polyethylene terephthalate
- PA - polyamides (nylon)
- ABS - Acrylonitrile-butadiene-styrene
- PC - Polycarbonate
- PMMA - poly(methyl methacrylate) (acrylic)
- PLA - polylactic acid (a bioplastic)
What is a monomer vs. polymer?
- CH2=CH2 (ethylene)
2. CH2-CH2-CH2-CH2-CH2-CH2 (polyethylene)
What are the two categories of plastics? Describe them a lil with regards also to their polymer chains.
- thermoplastics - their molecular structure doesn’t change upon heating, and therefore can be heated and reheated
- Thermosets - “set” after heating, which causes a cross linking in the molecular structure, and becomes permanent. (Crosslinking)
- TIP: polymers can be either or.
What are the visual differences due to molecular structure?
Crystalline - not clear, which is due to close packed chains of polymers. (ex. milk jug)
Amorphous - the polymer chains are twisted and not tight tight tight, which allows for light to go through. (ex. PET or acrylic)
What are 2 forms of raw plastic used for manufacturing?
Pellets and powder (ground pellets or bead)
Are polymers high or low viscosity? Why?
- high (therefore slow flow rates)
- due to its high molecular weight
- This is why polymers must be subjected to 20-30ksi when forced into molds
How is viscosity of polymers measured? what are the units? what scale is used? (hint: machine in the back of the plastics classroom)
-they are measured in grams per 10 minutes. the scale used is considered the “Melt Index” (MI)
When it comes to Melt Index, how many MI (9g/10min) is needed in extrusion vs injection?
extrusion = 1-5 MI injection = 10-120 MI
How much color is added to a batch when wanting to change the color of the plastic?
1-5%, depending on the viscosity of the plastic
What are some other additives used for plastics? (to improve some qualities of the final product)
- antifogging
- foaming
- wood flour (plastic lumber)
- sort fibers (molded car parts like the intake manifold)
- impact modifiers
- UV inhibitors
- perfumes and animal repellent smells
- antistatic (blast hole liners)
What is the difference between Primary and Secondary Processes/Products?
Primary: products and processes that use powder or pellets, or molding compound (eg. the injection molder in lab)
Secondary: products made using pre-made products such as sheets and films (ex. thermoforming in lab using a pre-made sheet of high impact polystyrene)
What are some commonly extrusion made products?
- tubing, sheets and films, wire, tarps, paper.
- things like clamshells (those plastic packages for blueberries n shit) are extruded sheets, then thermoformed on a production line.
What kind of psi is used for extrusion processes?
2’500 - 6’500
-extrusion is a melting AND compression process
What is the process of Pumping?
-when a screw (ex. injection molding) forces forward, the plastic melts and sticks to the barrel and is pushed ahead.. The forcing forward generates heat, causing melting and mixing
Where are plasticating feedscrew’s used?
- Injection molding AND extrusion
- Melting and pumping used at the same time
- injection screws involve rotation, which helps maintain high pressure
What are the 3 sections in an injection molding machine?
Feed Section: where the feedstock is added to from a hopper
Compression: the pumping action turns the pellets (etc.) into a fluid and compresses it
Metering: the melt is mixed (homogenized) and pressure is developed which helps pump into the die.
How is the compression ratio measured in an injection molding machine?
Volume of flight in feed section divided by the volume in the metering section. (typical ratio= 2:4)
What are Twin Screw Extruders?
Two feedscrews co-rotation or counter-rotating (depending on if melting and pumping or shearing and mixing) are used.
-less common and more expensive
What is a Gate Clamp (extruders/injectors)
Dies are usually clamped onto a gate clamp.
allows for easy removal for cleaning
What is viscoelasticity die swell?
When a polymer is extruded through a die, it tends to want to return to its original shape, and will swell After being extruded. Therefore, when making a die this has to be included
