1. Optimizing the Molding Parameters Chapter 4
Professor Joe Greene
CSU, CHICO
MFGT 144
September 20, 1999

2. Chapter 4 Topics Need for Control Optimizing the Parameters
Part Quality
Part Cost
Optimizing the Parameters
Temperature
Pressure
Time
Distance

3. Need for Control
Important to produce parts to meet product performance and cost requirements.
Consistency is achieved through tightly controlling process parameters that affect how the plastic fills the mold.
Part Quality
Developed as a series of performance specs through meetings with customer, e.g., bumper 5 MPH test
Example, part dimension weight, surface finish, gloss level
Toothbrush- shape and clear plastic with functionality.
Fin- shape, rigid, parts hold together during surfing
Straw- outside and inside dimensions are critical
Worms- flexible, shape, color, sparkles

4. Need for Control Part Cost Part Design Manufacturing process
Amount and type of material
Amount of complexity in design affects scrap rate
Assembled parts or secondary operations required
Manufacturing process
Cycle time
Scrap rate due to process inefficiencies
Secondary operations efficiency
Reduce cost through expertise in part production and cycle time reduction.
Changes in plant environment and personnel affects cycle time and quality

5. Need for Control
Parameter Effects
Table 4-1. Molding Parameters

6. Proper Parameter Values
The Setup Sheet (Chart 4-1)
Needed to monitor the events and process conditions at each machine.
Usually done electronically today through data acquisition.
Needed to meet quality rating systems
ISO9000
Q/S System

7. Proper Parameter Values
Installing and Setting Up the Mold
Sizing and Inspection of machine
Proper hydraulic oil level
Heater bands in place and operating
Mold temperature controllers operable
Injection cylinder empty and screw forward
Hopper shutoff closed and hopper wiped clean
Proper material available and dried
Granulator clean and available
Safety gates and mechanisms operating
Vent Hoods clean and operating
Machine lubricated
Alarms and lights operable

8. Proper Parameter Values
Installation Procedure
1. Make sure mold has connecting strap for A and B sides during transportation. Transport mold to machine using mold eyebolt.
2. Start machine and move injection sled to back position.
3. Open clamp wide enough to accept mold.
4. Lower mold from the top of machine
5. Position the mold such that the locator ring will slip into locating hole.
6. Level and square mold with platen.
7. Locate clamps and bolt the A side to the stationary platen.
8. Place ejector rods in mold and slowly bring platten forward to clamp the B side until pressure is reached desired level.
9. Shut off the machine. Locate clamps and bolt B side to the moving platen.

9. Proper Parameter Values
Installation Procedure
10. Remove chain fall hook, eyebolt, and connecting strap from mold
11. Recheck clamps and insure all are tight.
12. Start machine and open mold slowly.
13. Rest from photocopy

10. Optimizing Temperature
Four basic categories
Temperature
Pressure
Time
Distance
Temperature
Pressure
Time
Distance

11. Injection Molding Parameters
Temperature
Injection Cylinder Feed Throat
Biggest Concern is Bridging, where polymer material heats up, melts, and then solidifies across the feed throught.
Must have adequate cooling in throat area. (Temp between 80F and 120F)
Injection Cylinder Nozzle Zone
Heated with a heater band (Fig 4-4) usually one band.
Temperature should be same as (or 10F higher) the melt temp
Example,
PC suggested melt temperature is 550F (288 C).
Checking temperature of melt via a surface pyrometer
Inject outside of mold with a purge shot and stick probe in hot plastic

12. Injection Molding Parameters
Melt Temperature (Table III-1. Suggested Melt Temp at nozzle)
Acetal (coploymer) 400 F
Acrylic F
ABS F
Liquid Crystal Polymer 500 F
Nylon F
Polyamide-imide 650 F
Polyarylate F
Polycarbonate F
Polyetheretherketone 720 F
Polyethylene LDPE 325 F
Polyethylene HDPE 350 F
Polypropylene F
Polystyrene F
Thermoplastic polyester (PBT) 425 F
Urethane elastomer 425 F

13. Injection Molding Parameters
Temperature
Injection Cylinder Front Zone
Controlled by a series of heater bands (usually 3 to 6 bands)
Located directly behind the nozzle and consists of the first third of the total length of the heating cylinder.
Proper temperature 10 to 20F less than nozzle temp.
Example, PC should be 530 to 540 F
Injection Cylinder Center Zone
Located directly behind the front and consists of the second third of the total length of the heating cylinder.
Proper temperature is the average of the front and the rear zone.
Example, PC should be 500F

14. Injection Molding Parameters
Temperature
Injection Cylinder Rear Zone
Controlled by a series of heater bands (usually 3 to 6 bands)
Located directly below the feed throat and consists of the last third of the total length of the heating cylinder.
Proper temperature 15% less than the front zone.
Example, PC should be 459F or 460F
Then the center zone should be the average of 538F and 460F or 500F
Insulation Jackets
Placed around injection barrel. Can save 25% electricity
Preheating Material
Good practice with the use of a dryer to minimize shock.

15. Mold Temperatures
Temperature Control provided with flow cooling or heating channels (Figure 4-9.
Cooling process
Coolant is passed through tubes in mold that removes heat from the hot plastic as it is flowing in mold.
Design of cooling channels,
diameter of tube,
number and location of channels, and
distance between the channels and the surface of the mold
Critical to the performance of the molded part
Use Mold Cool Analysis in Moldflow Dynamic or C-Mold Advanced Series

16. Injection Molding Parameters
Mold Temperature Control (A and B Sides within 10F)
Cascades (Bubblers) (Figure 4-10)
Used when have a deep metal core, molds for waste baskets
The cooling medium (water) comes from the main cooling channel, enters at the bottom of the bubbler, flows up through an inner tube, cascades inside the unit, and flows down through an outer tube, exiting into the main cooling channel.
Cooling Pins (Figure 4-11)
Heat is transferred from the plastic to the highly conductive cooling pin (BeCu).
Insulation Sheets
Insulation sheets mounted on the outside surfaces of mold.
1/4in to 1/2in thick plates of fiberglass sheets.
Can save 25% if mounted on all 6 sides of mold

17. Injection Molding Parameters
Temperature (Table III-1. Suggested Melt Temperatures at nozzle)
Acetal (coploymer) 400 F
Acrylic F
ABS F
Cellulose acetate 385 F
Ethylene vinyl acetate 350 F
Liquid Crystal Polymer 500 F
Nylon F
Polyamide-imide 650 F
Polyarylate F
Polycarbonate F
Polyetheretherketone 720 F
Polyethylene LDPE 325 F
Polyethylene HDPE 350 F
Polypropylene F
Polystyrene F

18. Injection Molding Parameters
Cooling Related to Cycle times
In general, Temp should be set to guide in Table III-2
Guidelines for any given wall thickness
If wall thickness doubles, the cooling time increases by 4 times
Example, If thickness is 0.040in requires 3 sec to cool, then if the wall is increase to 0.080in, the cooling time would require 12 sec.
Example, likewise if thickness is reduced, the square rule applies
Figure 4-12 Average Cooling time versus thickness
Cooling related to standard Runners
Runners can be a controlling factor in cooling times.
Need to be rigid enough to demold
Usually Diameter is 1/16” to 1/4” diameter in thickness.
Plastic cools from outside surface to inside

19. Injection Molding Parameters
Cooling related to standard Runners
Runners can be a controlling factor in cooling times.
Cooling time for runners is usually longer than part. (Fig 4-12)
Ways to improve performance
Have cavity images placed close as possible to the sprue in the mold.
This reduces length of runner and allows for smaller diameter
Make sure the hole diameter at the large end of the sprue bushing is no larger that required
Rule of thumb (diameter of sprue)
Diameter of runner should provide same cross sectional area as the total of all runners coming to it.
Example, (Figure 4-13) radius = in each runner
Area of runners = r2 = 3.14 (0.06) (0.06)2 = in2
Diameter of sprue = Area = r2 = Diameter =0.085 in
This ensures that there is always enough material being fed to the runners by the sprue to keep equal pressure on those runners

20. Injection Molding Parameters
Cooling related to Hot runners
One that is kept molten during the molding process and thus does not require cooling time allowance. (Fig 4-14)
Molten plastic enters the mold through a special bushing , similar to the standard runner (with special heaters to maintain the molten state of the plastic all of the way to the cavity image.
Just before it gets tot the cavity image, the plastic flows through a special nozzle that allows material to flow until the cavity is filled, then shuts off and keeps the plastic molten ready for next cycle.
Costs are typically 40% to the cost of the mold
Results in shorter cycle time and less material usage.

21. Injection Molding Parameters
Machine and Oil Temperatures
Machine Temperature of the hydraulic oil
Desired range is 80F and 120F
Too cold, the oil is sluggish due to higher viscosity
Too hot, the oil has thermal breakdown
Heat Exchanger
Provide enough cooling to provide proper temperature
Can build up with rust, scale, minerals.
Scale buildup of 1/64in can result in 40% loss of efficiency
Should be cleaned monthly with acid-flush unit (portable)
Ambient Temperatures
Can affect mold temp, hydraulic temp, cycle time, quality, cost
Ideal settings are: 68 to 79F and 30 to 50% Relative Humidity

22. Optimizing Pressure Injection Unit Injection Pressure (Figure 4-15)
Applied to the molten plastic and resulting from the main hydraulic pressure pushing against the back end of the injection screw (or plunger).
Usually 1,000 psi to 5,000 psi
Lower than hold and pack pressure which be between 10,000psi and 20,000 psi
Calculated from line or hydraulic pressure
Example,
2000 psi on a
8 in diam hydraulic
with a 2.5in screw
What is the injection pressure?

23. Pressure Control

24. Pressure Control Injection unit (continued)
Hold pressure (pack pressure)
Used to finish the filling of the mold and pack the part.
Rule of thumb: Hold pressure = 50% of injection pressure.
Hold pressure applied against a pad or cushion of material.
Applied at the end of the initial injection stroke, (Figure 3-5), and is intended to complete the final filling of the mold and hold pressure to solidify while staying dense or packed.

25. Pressure Control Injection unit (continued) Back pressure
Applied after the injection phases are complete.
When holding pressure is complete the screw begins to turn in order to bring new material to the front of the barrel in preparation for next shot.
As material fills the cavity, the screw is pushed back (Fig 3-6).
Back pressure is small compared to injection pressure (between 50 psi and 500 psi (screw may not turn if exceeded).
Procedure is to start with small amount of back pressure and steadily increase in increments of 10 psi.
Ensures consistency in part weight, density, and material appearance.
Squeezes out any trapped air or moisture.
Minimizes voids in molded parts.

26. Pressure Control Clamp Unit
The purpose of developing clamp pressure is to keep the mold clamped shut against the forces developed when the injection pressure pushes plastic into the closed mold.
Clamp pressure is applied mechanically or hydraulically

27. Pressure Control Pressure Required
Total clamp force is determined by projected area.
Total force = projected area times injection pressure
Rule of thumb 4 to 5 tons/in2 can be used for most plastics.
Example, (Figure 4-18 and 4-19)
Part is 10 in by 10 in by 0.1 in
Projected area is all of the sides of the cube.
Neglect 0.1 in thickness.
Surface area = 10in x 10 in = 100 in2
Pressure = 15,000 psi for PC
Tonnage required to keep mold closed is
100 in2 x 15,000 psi= 1,500,000 lbs = 750 tons (note : 2000 lbs = 1 ton)
10 in
10 in

28. Shrinkage Controlling Shrinkage is the best way to control warpage
All plastic materials have shrinkage value
Shrinkage (or shrink rate) is a value to predict how much the plastic part will reduce in size after it cools down.
Shrinkage is measured by inches (or cm) per inches (or cm) of linear length in all 3 dimensions.
Plastics shrink ranges from in/in to in/in
Shrinkage IS the same in all 3 directions (x,y,z) for most polymers (isotropic = same property in all 3 directions)
Shrinkage is NOT the same (anisotropic) in 3 directions (x,y,z) for polymers (thermoplastics and themosets) with glass fibers where shrinkage is less in direction of flow than transverse
Crystalline materials have high shrinkage rates than amorphous
Crystalline materials have greater shrinkage in flow direction than in across the direction of flow (Figure 4-22)

29. Shrinkage Molds have to be built larger to account for reduced size.
Shrinkage is like a % reduction, (Figure 4-20)
e.g., 0.02 = 2% shrink, = 0.5% shrink
Example,
Shrinkage of in/in, what is effect on part that is 6 in long?
Multiply shrink value by each part dimension (L W H), or for 6 in part, shrink will be in/in x 6 in = inches.
Thus, mold would have to be inches bigger, for part to cool down and shrink to 6.0 inches.
(Figure 4-21) For mold of dimensions, in by in, the final part dimension will be 6.0 in by 0.5 in, since it will shrink in (6in x in/in = 0.060in) in one direction and in (.5in x 0.010in/in = in) in other dimension.

30. Process Effects on Shrinkage
Temperature
Higher the plastic temperature, the greater the amount of shrinkage because the activity of the plastic molecules is temp dependent.
Increasing Barrel temperature 10% can increase shrinkage 10%
Example, If shrink rate (or shrinkage) is in/in at a barrel temp of 500F, then decreasing barrel temp to 450F, can decrease shrinkage to
Example, If shrinkage is in/in and parts are too big with a barrel temp of 400 F, then increasing barrel temp to 440F, can increase shrinkage 10% to in/in and help make the part smaller and fit better.
Mold temperature can affect shrinkage. Hot mold will create less shrinkage because a co mold solidifies the plastic skin sooner than a hot mold resulting in a shrinking of a plastic before full injection pressure is applied.
Hot mold allows molecules to continue to move and be compressed by injection pressures before solidifying resulting in less shrinkage.
Rule of thumb: 10% mold change = 5% shrinkage change

31. Process Effects on Shrinkage
Pressure
Injection Pressure has direct effect on shrinkage
Higher injection pressure, the lower the shrinkage due to packing.
Rule of thumb: 10% change in pressure can cause 10% change in shrinkage
Pressure has to applied until part cools down to solidification
Postmold Shrinkage
Lower the desired amount of shrinkage requires longer cycle times
95% of cooling and shrinkage takes place within 5 min of molding.
Some parts can shrink and warp up to one month after molding.
Post molded shrinkage is controlled by quenching parts or placing in full fixtures.
Figure 4-23

32. Minimizing Stresses Defining stress (Fig. 4-24)
Biggest molding problem besides contamination.
Heat cycle: Plastics go from melt to rubbery to glassy states
Differential shrinkage due to different temperatures in part causes buildup of stresses and results in warpage.

33. Minimizing Stresses Product design (Fig. 4-25) Draft angles
Amount of taper required to allow the proper ejection of a molded part
Typical draft = 2º per side.
Minimum draft = 1º
Smaller the draft the more difficulty in demolding part.
Draft angles alter dimensions of part (Fig 4-26)
For every 1º of draft, the dimension increases by in per side for a part up to 1 in deep. Additional in per side for additional inch.
No draft consequences (Fig 4-27)
Vacuum due to molding causes part to stick in mold.
Uniform Walls (figure 4-28
Non-uniform wall thickness creates molded in stresses
Non-uniform walls results in temp differences causing stresses
Rounded corners (Figure 4-29 and 4-30) (Fig 4-31, Fig 4-32)
Square corners cause material to be stretched and stresses

34. Drying Material Importance
Moisture can cause defects in molded plastic parts.
Water turns into steam during molding
Hygroscopic materials
Materials absorb moisture from surrounding atmosphere
Nylon, ABS, and PC
Dried at 275F for 2 to 3 hours. Some Nylons require 24 hours
Most engineering resin should be dried at 275 for 4 hours
Nylon, PBT, PET, PC, ABS, PEEK
Hydrophyllic materials
Materials do not absorb moisture and generally do require drying
LDPE, HDPE, PP, PS ---- Refer to Mfg recommendations