Plet 222 exam 1

List 4 main types of hazards on an injection molding press and example where they can be found

• Pinch point - Mold halves/ejectors/screw

• Burn/High Temp - Barrel

• High Pressure - oil lines/nozzle tip

• Electric Shock - electrical box

5 steps of design hierarchy for safety

1. Eliminate the hazard

2. Guard

3. Warning signs

4. Training

5. PPE

Main purpose of lockout tagout system

Control Hazardous Energy

Thoroughly explain Hazardous Communication (MSDS)

All chemicals used in an area need to have a data sheet which describes appropriate safety gear and what to do in event of spill, fire, etc.

Why is it extremely important to keep a clean and organized work enviornment

Eliminates slip hazards/ inspectors look closer at cluttered areas/ disorganized

Most important rule of ladder safety

3 points of contact

List and explain steps of injection molding cycle

1. Mold closes - Mold closes

2. Fill - Inject part to 95%-99% full

3. Pack - Finish filling + pressurize cavity

4. Cooling time - material in mold cools

5. screw rotate - screw rotates back to shot size

6. Mold open - Mold opens

7. Eject - Ejectors move forward + backwards

List 4 main plastic variables and why they are important

• Temperature - affects viscosity

• Flow rate- affects viscosity

• pressure gradient - affects shrinkage

• Cooling - affects shrinkage

List 7 steps of optimizing a process in order

1. Optimize Velocity

2. Optimize Hold Pressure

3. Optimize Hold Time

4. Cooling + Recovery Time

5. Minimize clamp pressure

6. other variables

7. set safeties

Where do you set injection fill time at the start and why we set it there

15-25 seconds

to set scale on graph

where do we set injection fill time at end of the process and why

10% higher then actual

accommodate material variation / safety so we don’t flash mold

Where do we set injection fill pressure at the start of process set up and why

Max

don’t pressure limit velocity

Where do we set injection fill pressure at the end of process set up and why

200 hyd psi / 2000 psi plastic above actual

accommodate material variation / safety so we don’t flash mold

Where do we set screw rotate speed at the start of process set up and why

90% max

don’t want recovery time to limit cycle

Where do we set screw rotate speed at the end of process set up and why

so it returns 1-2sec before end of cooling time

want screw to rotate as slow as possible w/o affecting cycle time

Where do we set hold pressure at the start of process set up and why

0 psi

trying to get 95% full part on fill only

Where do we set hold pressure at the end of process set up and why

avg of just full and flashy pressures

packout cavity without flashing

Where do we set transfer position at the start of process set up and why

0.1 inch

don’t want to screw overtravel to affect velocity data

Where do we set transfer position at the end of process set up and why

Where necessary to provide a cushion of about 10% of shot size while maintaining 95% full part

Need to maintain psi in cavity w/o degradation + 95% full part at transfer

Why do we need a cushion during the injection molding cycle?

Maintain pressure in cavity

where should cushion be set in an optimized process?

10% shot size

Why do we set the cushion at 10% shot size in optimized process?

want to maintain pressure without degradation

if it is not the right size how do we adjust it? (cushion)

adjust shot size and transfer position by same amount

If BY turns off the hold pressure at the end of your practical the parts must be 95% full. Why wouldn’t the part be 95% full at this point?

The mold has heated up and now there is a thinner frozen layer which allows the material to flow easier

What are the 3 main functions of the clamping unit.

1. Move the mold

2. hold the mold closed

3. provide a means of ejection

Hydraulic clamping advantages

1. Pressure directly behind the mold

2. more consistent clamp pressure

Hydraulic clamping disadvantage

low spring rate / less energy efficient

Mechanical clamping advantages

1. High spring rate

2. more energy efficient

mechanical clamping disadvantage

clamp force outside of platens / harder to maintain

Hydro-mechanical clamp advantages

1. Pressure directly behind mold

2. faster operation

Hydro-mechanical clamp disadvantage

Expensive / hard to maintain

What is #1 and its function?

injection cylinder - provides linear forward motion to push plastic into the mold

What is #2 and its function?

Screw - rotates to mix and melt plastic material, moves back while rotating

What is #3 and its function?

Non-return valve - prevents plastic from flowing back over the screw during injection

What is #4 and its function?

Pull-in cylinder - moves injection unit forward to contact the mold

What is #5 and its function?

mold - provides geometric shape for plastic to melt

What is #6 and its function?

clamp cylinder - moves mold and holds it closed during injection

What is #7 and its function?

machine base - support structure

What is #8 and its function?

tie bar - stretches to build clamp tonnage, main tension element in load path

What is #9 and its function?

movable platen - supports movable half of mold

What is #10 and its function?

stationary platen - supports stationary half of the mold, provides space for nozzle tip to contact mold

What is #11 and its function?

nozzle tip - adapter between injection unit and mold

What is #12 and its function?

Barrel - heated, houses the screw

What is #13 and its function?

feedthroat - where material enters the barrel and falls on the screw

What is #14 and its function?

Hopper - material storage during processing

What is #15 and its function?

Screw motor - rotates the screw causing it to move back to shot size

What is #16 and its function?

Rear platen - supports clamping cylinder, floats

What is #17 and its function?

Ejection cylinder - provides means to move ejection plate through holes in movable platen

Tie bar stretch equations

• Stress = Force / Area

• Modulus (E) = stress / strain

• Strain = ΔL / L

• Area = [(πd²)/4] × 4

modulus of steel

3 × 10⁷ psi

Intensification ratio equations

1. Hydraulic area / screw area

2. plastic pressure / hydraulic pressure

why does loading factor affect adjusted fatigue strength

the type of load will affect how fast a component will fatigue

why does failure rate factor affect the adjusted fatigue strength

takes into account scatter testing data for initial fatigue data

why does surface treatment factor affect the adjusted fatigue strength

the surface treatment can put the surface component in a state of compression

Four main functions of the screw and which section performs each

• feed - moves material

• compression - melts material

• metering - mix material

• non-return valve - builds pressure for injection

Most common non-return valve

Check ring

Other 2 non-return valves and why would you use them

ball check, poppet

low viscosity materials

which barrel zone is most likely to over-ride and why

compression zone - that’s where the shear heat is generated and there is no where for the heat to go.

why is it important that all of the tie bars stretch the same amount

puts uneven stress on the mold

why do we only weigh parts when we perform a gate freeze study

we are only concerned where the gate freezes and not the runners

What nozzle is this?

reverse taper

What nozzle is this?

General purpose

What nozzle is this?

nylon

3 types of injection unit

• ram extruder (plunger)

• ram-assisted screw extruder

• reciprocating screw

Explain why temperature is one of the main things that affect a materials viscosity?

At high temps molecules are further apart making them flow past each other easily

Explain why strain rate is one of the main things that affect materials viscosity?

It isn’t

Explain where the screw is most likely to wear and why it wears there?

At the back of the flights in the feed zone, the polymer is not melted and fillers rub against flights

How can you tell if either the screw or barrel is leaking?

You cant maintain a cushion. If you move your shot size and transfer position and still cant maintain a cushion then it is probably the check ring. Barrel usually only worn in the front of metering zone.