Injection and tooling

Question 1

How is injection molding done?

Answer 1

~ injection chamber is heated so the mix is typically 125-160C

~ nozzle of the injection chamber is inserted into a clamped die, and the mix is injected (via plunger or screw) into the cold cavity under 30-100 MPa

~ pressure is maintained to compensate for feedstock contraction

Question 2

Why are tool steels required?

Answer 2

~ because ceramic particles are highly abrasive

~ nitrided surfaces or cemented carbides are also used

~ tool steels have around 100,000 cycles

Question 3

What is the gate?

Answer 3

~ the entry point into the die

~ made of purposefully constricted cross sectional area so the mix in it solidifies early, isolating the injection chamber and the die

~ die pressure decreases as the mix solidifies

~ the nozzle is then extracted. the mix in the sprue and the runner is still molten

Question 4

What can happen under high shear with filling the cavity quickly?

Answer 4

~ mixes can convert from shear thinning to shear thickening

~ jetting can occur, which would plug vents, causing trapped air and forming weld lines (newly-injected jet of mix not bonding well to an older jet of mix)

~ a side gate is often preferred in which mix contacts and flows from a cavity wall

Question 5

What is the result of slow filling?

Answer 5

~ the gate will solidify before filling is complete

Question 6

What are vents used for?

Answer 6

~ vents are cut into the mold out from the cavity to allow for air escape as the mix is injected

~ they must be too small in cross section to allow mix to flow in