Injection moulding questions

Hello everyone,
I have some questions regarding injection molding, If anyone has any information regarding these questions kindly let me know I would appreciate your help.
thank you in advance

1. What is a one dvantage and one disadvantage of using collapsible cores to eject mouldings
with undercuts?

2. Why excessive cooling can cause defects in mouldings? you can explain it, with the help of diagrams,

3. What type of cooling method would you recommend to cool the core of the impression of a
pen barrel?