1.4 PRACTICE PROBLEMS
1. TRUE / FALSE - Plastic is melted in the hopper before it is extruded.
2. TRUE / FALSE - A reciprocating injection moulding process has a screw that moves.
3. TRUE / FALSE - Steam is the most common method used to weld plastics.
4. The key tag shown below has a flat profile. There is a hole on the right hand side for the keyring, and a large hole in the main body of the ring.
a) Design and sketch a complete injection mold for the part. Indicate parting lines, ejector pins, runners, gates, and all other important features.
b) What important design features must be considered for a part like this?
5. List 5 tradeoffs injection molding and reaction injection molding?
6. Describe the operation of an injection molding machine using figures and notes. This should describe the entire cycle from when plastic enters the hopper to when it is injected into the part.
7. Name the three zones on a plastic compression screw and briefly describe the function of each zone.
8. List the general advantages of thermo plastics over metals. List the disadvantages.
9. A six cavity mold has been designed for a 200 ton injection molding press. The press is no longer available, and the mold has to be put in a 160 ton press. How many cavities should be blocked off to ensure the mold will fill properly?
10. a) Describe the operation of the screw in an injection molding machine.
b) How would the operation of the screw change for a continuous extrusion machine?
c) What does the tonnage of an injection molding press refer to?
d) What types of cooling channels can be used inside an injection mold. Describe how they operate.
11. Why do powder metal parts change shape after processing? What can be done about it?
12. What are the advantages of injection molding over other processes.
13. What is the purpose of gates in an injection mold? What are the different types, and what are their advantages?
14. What are parting and flow lines?