Optimizing Molding Parameters - Polymer Materials - Lecture Slides, Slides of Engineering Chemistry

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1

Optimizing the Molding Parameters

Chapter 4

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Chapter 4 Topics

• Need for Control

• Part Quality
• Part Cost

• Optimizing the Parameters

• Temperature
• Pressure
• Time
• Distance

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Need for Control

• Part Cost

• Part Design
  • 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 Docsity.com

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Need for Control

• Parameter Effects

• Table 4-1. Molding Parameters

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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 Docsity.com

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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. Docsity.com

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Optimizing Temperature

• Four basic categories

• Temperature
• Pressure
• Time
• Distance

Temperature Pressure

Time Distance

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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 Docsity.com

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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
  • Controlled by a series of heater bands (usually 3 to 6 bands)
  • 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 Docsity.com

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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.Docsity.com

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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 Docsity.com

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Injection Molding Parameters

• Temperature (Table III-1. Suggested Melt Temperatures at nozzle)
  • Acetal (coploymer) 400 F
  • Acrylic 425 F
  • ABS 400 F
  • Cellulose acetate 385 F
  • Ethylene vinyl acetate 350 F
  • Liquid Crystal Polymer 500 F
  • Nylon 6 500 F
  • Polyamide-imide 650 F
  • Polyarylate 700 F
  • Polycarbonate 550 F
  • Polyetheretherketone 720 F
  • Polyethylene LDPE 325 F
  • Polyethylene HDPE 350 F
  • Polypropylene 350 F
  • Polystyrene 350 F Docsity.com

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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 = 0.060 in each runner » Area of runners = πr 2 = 3.14 (0.06) 2 + 3.14(0.06) 2 = 0.00566 in 2 » Diameter of sprue = Area = πr 2 = 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 Docsity.com

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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.Docsity.com