PTE Mold Design

Names for A side

A-half

Stationary half

Cavity half

Injection half

Names for B side

B-half

Moveable half

Core half

Ejection half

Name the mold plates from top to bottom

Locating ring

Top clamping plate

A-plate (cavity retainer)

B-plate (core retainer)

Support plate

Parallels (rails)

Ejector retainer plate

Ejector plate (Knock out rod bumps this)

Bottom clamping plate

Support pillar

Ejector guide pin and ejector guide pin bushing

Return pin

Name the additional plates in a 3-plate mold

Runner stripper (floating plate)

Runner plate

Puller pin/sucker pin

What does the runner stripper plate do in a 3-plate mold?

Comes forward after the runner was pulled out to eject the runner off the puller pin so it can be ejected out of the mold

What does the puller pin/sucker pin do in a 3-plate mold?

Undercut to hold the runner and pull it out of the part and away from the steel

Calculate cavity size required to mold a finished part with a length of 5.125”. Shrink factor of 0.017 in/in

Mold = Part / (1-shrink)

5.2136”

Impact of three draft neutral plane locations on a boss.

Top - Thick wall at bottom = strong and less pressure to fill but main wall may freeze off before can pack out (sink)

Middle - Good balance

Bottom - Thin wall at top = easiest to pack out and least amount of sink but weakest and may have hard time filling thin tip.

Reasons why sub-inserts may be used

1. High wear area

2. Easier to make design changes

3. Improved cooling opportunities (sub-insert material)

4. Improved venting opportunities

Review mold drawings from Review 1

A-side vs B-side

Plug

Ejector return pins

Ejector pins

Leader pins

Leader pin bushing

Support Pillars

Locating ring

O-ring grooves

Dog eared pockets

Design objectives of the ejection system

1. Actuate quickly and reliably

2. Minimize part distortion

3. Mold must be open and all undercuts cleared

4. A force must be applied to the molding

5. Minimize cooling interferences

6. Minimize impact on part aesthetic and function

7. Minimize complexity and cost

List components of a slide

1. Angle, horn, or cam pin

2. Slide

3. Wear plate

4. Heel or wedge block

5. Retainer

List 3 reasons why ejection blade may be used rather than pins

1. Thin wall or rib where a round pin may not fit on the part

2. Edge of a part

3. The blades larger area would help minimize the force applied to the part and reduce buckling of ejection component

What are the advantages of a stripper plate in a 2-plate mold base

Improved cosmetics

Uniform ejection force

Increased ejection surface contact area

NOT maximize runner design options

How might a mold designer reduce the risks associated with a slide that moves out of position prior to the mold closing?

1. Add hydraulic or pneumatic core pull in case the spring fails

2. Add proximity sensors to confirm the location

What considerations should be made by the Mold Designer to determine the required displacement of a constant angle lifter?

Undercut size

Clearance

Part shrinkage

Warp

Modification to part design to reduce shear stresses throughout the entire part forming cavity during filling?

Increase wall thickness - reduce pressure, easier to fill, less drag on the melt

If increase melt temperature by 20°F, which material’s viscosity will decrease the least?

HDPE (semi-crystalline). Nylon next

Flat rectangle part with fan gate and constant wall thickness. At 1 second fill time, highest volumetric shrinkage in this part should normally be expected near or opposite the gate?

Opposite/away from gate

What impacts volumetric shrinkage?

Temperature and Pressure

Do narrow or broad molecular weight distributions require higher pressure?

Narrow - Lacks smaller chains that act as lubricants

Name a process change to eliminate residual stresses in the molded part.

Hotter mold - may increase cycle time but will keep the chains in a relaxed state. Cool longer to keep them in this relaxed rubber band like formation.

Polycarbonate, 8 cavity mold, filling in 0.5 seconds, pressure to fill the mold is 20,000psi. What process chain reduces this filling pressure?

Higher melt temperature to lower viscosity

What affects shear stress?

Velocity and Viscosity

Flat part with two “A” paths going to one wider “B” path.

If shear stresses were the same at A & B and you increase injection rate, the shear stresses will be higher at point ___ than ___

A than B

Flat part with two “A” paths going to one wider “B” path.

If you increase the mold temperature from 100 to 250F, how much reduction of the residual stresses in region A of the final part are expected?

Significant

Flat part with two “A” paths going to one wider “B” path.

If you increase the mold temperature from 100 to 250F, how much reduction of the shear stresses in region A of the final part are expected?

Minor

What are residual stresses?

Stresses in the final part after it has been molded and frozen. The orientations, variations, and shrinkage has been locked in.

Flat part with two “A” paths going to one wider “B” (2”) path.

If decrease width of A from 1” to 0.5”, molecular orientation at A would ___

Increase - velocity and shear stresses are increased so more molecular orientation

Review amorphous vs. semi-crystalline materials

Polycarbonate - Am

Nylon - SC

PBT - SC

POM (Acetal) - SC

HDPE - SC

Polypropylene - SC

PMMA (Acrylic) - Am

ABS - Am

Flat rectangle part. Gate A on thin wall vs. Gate B middle of long wall.

Advantage of A over B?

Simple flow pattern, minimizing conflicts and shrinkage that would cause potential for residual stresses and warpage

Flat rectangle part. Gate A on thin wall vs. Gate B middle of long wall.

Advantage of B over A?

Less pressure to fill and pack due to shorter flow length. Lower clamp tonnage due to lower pressure.

What increases the magnitude of orientation in a molded plastic part?

Increasing shear stress

Increasing shear rate

Increasing pack pressure

Decreasing mold temperature

NOT increasing melt temperature

Bowl or saddle?

Neat extensional orientation

Bowl

Bowl or saddle?

Neat radial orientation

Saddle (surface)

Bowl or saddle?

Fiber extensional orientation

Saddle

Bowl or saddle?

Fiber radial orientation

Bowl (surface)

Why don’t tunnel gates freeze even though they are thin wall areas?

High velocity leads to high shear.

During packing, the flow is going through the small pinpoint area which keeps the velocity during pack relatively high and prevent freezing.

Constant or variable runner?

Highest pressure drop in the runner

Constant

Constant or variable runner?

Constant shear rate in the runner

Variable

Constant or variable runner?

Fastest cooling time in the runner

Constant

Constant or variable runner?

Lower pressure drop across the part

Constant

Constant or variable runner?

Highest shear rates in the runner

Constant

Constant or variable runner?

More material volume

Constant

Given a geometrically balanced runner layout, list 2 reasons why there may be filling and packing variations?

1. Flow lengths are the same so geometrically balanced, but flow rates are imbalanced since one branch splits to feed 2 vs 4 cavities

   1. Flow group 1 will have lower viscosity due to shear which will cause the first cavities to fill differently (rheologically imbalanced)

Packing is not a limiting factor, what is the main disadvantage of using a runner sizing method that makes all runner diameters 1.5*wall thickness at the gate vs. other conventional runner sizing methods?

Small runners may be hard to fill and pack

3 advantages or disadvantages of hot runner valve gates vs thermal gates

1. Higher cost

2. Stack height

3. More maintenance

   1. Dead flow regions (hard for color changes)

Key words for cold runner

High precision

Minimize mold cost

Minimize maintenance

Key words for hot runner

Frequent color changes

Avoid regrind

High production volume

Key words for insulated

Lower cost

Low tolerance resin

What type of mold?

8 cavity, high precision small round disk parts. Nylon and concerned with concentricity. Concerned with low level of training operators will have

Three plate cold runner

What type of mold?

Low tolerance thin wall part, polypropylene, frequent color changes. 16 cavity mold, minimize mold cost, avoid regrinding of runner. Highly skilled and experienced tooling and manufacturing group familiar with all types of melt delivery systems

Insulated hot runner

What type of mold?

.040” thick cup, polypropylene. High production volume, gated in the center of its base. 16 cavities. Concerned with hot runner leakage issues due to inexperience with hot runners and low skilled work force.

Internally heated manifold

Internally heated drop

What type of mold?

8 cavity, automatic degating. No special gating position limitations. Minimize mold cost, skill, and maintenance required to operator mold.

2-plate cold runner

Label components of hot runner

Cylinder

Piston

Mainfold Bushing

Valve Stem

Nozzle Housing

Nozzle Heater

Nozzle Tip

Describe start up procedure for hot runner

1. Turn on mold cooling before hot runner heaters

2. Soft start (90°C for 15 minutes) to drive out moisture from heaters

   1. Heat soak (Melt operating temp for 15-30 minutes) to allow thermal expansion

Advantages of internally vs externally heated manifolds (hot runners)

Less leakage problems

Better isolation of heat from cavity

NOT lower pressure drop during mold filling

NOT better for color changes

List combinations of hot manifolds and hot drops

Externally heated manifold w/ externally and internally heated drop

Internally heated manifold with internally heated drop

Insulated heated manifold with insulated and internally heated drop

Recommended land length for vents

.040” - .060”

Recommended vent depth for amorphous materials

.001” - .0025”

Recommended vent depth for semi-crystalline materials

.0003” - .001”

Tensile stress-strain curve

Up until point A the graph is linear. Name of this location?

Proportional limit

Tensile stress-strain curve

What is location B called? 1st peak

Yield point (where it’s failing)

Tensile stress-strain curve

What is location C called? final peak/point

Ultimate yield, strain, or strength

Tensile stress-strain curve

Where is the elastic limit?

Between A and B (proportional limit and yield point)

List 2 ways to get direct cavity/core cooling

1. Threaded pipe extensions directly into cavity/core blocks

2. Have water enter blocks from plate below then insert then add o-ring groove for water to cross the boundary

3. Jacket cooling (still need o-rings)

Why is temperature rise in a cooling circuit important to understand in relation to the heat transfer equation for conduction?

Q= k A deltaT / delta X

delta T is the difference between plastic temp and coolant temp?

As coolant flows through it gets warmer. If coolant warms by too much, delta T decreases so Q (heat transfer rate) also decreases. Excessive temperature rise across the circuit indicates we are cooling one region of the part at a slower rate than another part which can lead to dimensional instability.