Injection Molding Flashcards
Examples of thermoplastics
Nylon
Polycarbonate
Polyethylene
Examples of thermosets
Silicon
Epoxy
Polyester
Polyeurathane
What’s the difference between thermoplastics and thermosets?
Heat can break secondary bonds for thermoplastics which allow them to reform.
Thermosets cannot be reshaped
Basic injection molding process
Close mold
Inject plastic from barrel into mold
Hold
Cool
Open mold
Eject the part
Things to consider during injection molding
Temperature of barrel and mold
Injection velocity
Cooling temperature
Injection pressure
Hold time
Screw speed
These factors influence dimensions and flatness and internal stresses
Define: Weld Lines and cause (4)
Defect on injection molded parts from two flow fronts meet during the process.
Part design - sharp corners or undercuts make it more likely
Mold design - complex geometries or thin walls
Material properties- more viscous material make this more likely
Injection parameters
What is a short shot
A scenario where region does not fill the full cavity
Define: Burning
Small brown/ black discoloration on the part.
Proper venting and injection speed can prevent
Define : Flash
Resin flowing outside of the cavity, usually around parting lines
Define: Sink
A shallow depression on the surface of the molded part caused by shrinkage
Define: Void
A hole occurring inside the part caused by shrinkage, usually in sections that are too thick
Design rules for injection molding
Use drafts on walls
Uniform wall thicknesses
Use rounds to help with transitions
Hollow out thick sections
Ribs can be used to increase stiffness in large flat sections
