Exam 2 Flashcards
What are the three causes of excessive wear and tear of the screw?
a. Abrasive wear caused by fillers
b. Adhesive wear caused by metal to metal contact
c. Corrosive wear caused by chemical attack
Why the exact shape of the extrusion die and the extruded part are different from each
other? Use a diagram to explain your points
Because of die swell (due to viscoelastic behavior of polymers), the shape and dimensions of
the die are different than the extrudate
Name two advantages of short L/D and two advantages of long L/D extrusion screws. 5
points
Short: • Less residence time • Less space • Requires less torque • Requires less energy • Less initial cost
Long:
• Larger output
• Better design adoptability
• Can be designed for less shear and more conduction
What is the function of the screen pack in an extruder?
Filter impurities out of the final part. It is located in the head section and gives a clean and clear extrusion for the plastic
A new compounder is planning on purchasing an extruder.- Do you recommend if
he/she purchases a:
a) Twin-screw or a single screw extruder? Why?
b) Co-rotating or counter-rotating extruder? Why?
a) Twin-screw because of better mixing capability
b)Co-rotating because of better mixing capability. Counter-rotating is better for
larger throughput.
Name three possible part defects that may be present in an extruded part.
a. melt fracture
b. sharkskin or alligator hide
c. uneven flow and surging
d.
-degradation
-poor mixing (color streaks)
-contamination
-bubbles and void space in the extrudate
List 5 advantages of the injection molding process over other polymer processes. Be
clear, concise, and specific (avoid subjective terms as much as possible).
a. Suitable for high volume parts (fast cycle time)
b. Part consolidation
c. Part size variation (from fraction of a gram and up)
d. Tight tolerance parts
e. Complex geometry
- Various surface textures
- Highly repeatable process
- Low cost in high volume production
- Automated process, low labor cost
- Net shape
- Parts consolidation
- Name five rules of thumb when designing an injection molded part.
a. Constant wall thickness
b. Minimize wall thickness (up to a point. Why?)
c. Ample draft where possible
d. Use gussets to reinforce the wall.
e. Use ribs to increase rigidity
6
Avoid undercuts if possible
Reduce weld lines
Avoid sharp corners
What is the purpose of cold-well extension in the runner system?
Cold Slug Wells are highly desirable in an Injection Mold. The Cold Slug Well provides
a small reservoir (well) to trap air, and impurities before they enter the Runner, Gate
and Cavity.
A Cold Slug Well is located above the Sprue Puller Pin. Typically, as the runner
changes from a primary to secondary, and, secondary to tertiary there is also a cold
slug well at each intersection.
From a molder’s point of view, why is it important to have all the sections of an
injection molded part as uniform as possible?
More uniform flow in the mold
More even cooling
Less chance of warpage (this happens because of other reasons too such as fiber
orientation)
What is a hot runner system? What are the advantages and disadvantage of this
over a cold runner system?
It is a heating system built into the mold that keeps the materials and the sprue
molten while waiting for the injected part to cool. By contrast, a cold runner is
simply a channel formed between the two halves of the mold, for the purpose
of carrying plastic from the nozzle to the cavities. These channels freeze during
each cycle of part production.
Hot runner eliminates material waste, shortens the cycle time, easier to start
the process, but it is more expensive system.
What is the meaning of balancing the runners? Why is this important? Use
schematics to make two recommendations in order to have a better chance of
having a balanced runner system.
What?
It means that the flow of the material into the cavities through the runner
system is even so that the cavities fill at the same time and packing pressure in
various cavities is about the same.
Why?
As stated above if the runner system is not balanced, the cavities will not feel at
the same time. Also the parts in various cavities could have different properties.
recommendations:
- avoid sharp corners
- use equal runner length
- adjust runner cross section
- use circular runner system
- use CAE analysis
