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Injection Molding

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Injection molding is the most common method for producing a plastic part ... Cold Well Slug 'Hot Runners' Gates. Cavities. Single Cavity. Multicavity. Hot Runners ... – PowerPoint PPT presentation

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Title: Injection Molding


1
Injection Molding
Presented byDave SterlingProduct Development
EngineerFebruary 11, 2004
2
Injection Molding Outline
  • Introduction
  • Materials
  • Equipment/Tooling
  • Process
  • Process Variables
  • Applications
  • Pros/Cons

3
Basic Facts
  • Injection molding is the most common method for
    producing a plastic part
  • Over 10 billion lbs. annual usage in the US
  • Over 30 billion in North American sales annually
  • Automotive is the 1 consumer (9.5 billion)
  • The top three markets are electronics, consumer
    products, and automotive
  • Approximately 1355 molding plants in North
    America
  • Average price for an injection molding press
    205,000

4
Materials
  • Thermoplastic materials
  • PP, Nylon, ABS, PC, PEEK, PBT, Acetal, etc.
  • Note Some thermosets are RIM molded (e.g.
    silicone)
  • Unfilled or highly filled materials
  • Fill level and type affects processing and part
    performance
  • Glass, Carbon Fiber, Carbon Black, PTFE, Aramid
    Fiber, Silicone, Talc, Mica, Wollastonite,
    Kaolin, Calcium Carbonate, Colorants, etc.
  • Materials in the form of pellets typically 1/8
    in diameter and 1/8 long (1/2 for long-fiber)

5
Equipment
  • Injection Molding Press consists of three main
    units
  • Clamping Unit, Mold, Injection Unit

6
Injection Unit
  • Consists of
  • Hopper
  • Gravity vs. Pneumatic
  • Hopper Magnets
  • Barrel
  • Hardened Steel
  • Screw
  • General Purpose with feed, compression, and
    metering sections
  • L/D typically 181 to 251
  • Compounding (gt301)
  • Nozzle/Tip
  • Check Ring/Non-Return Valve
  • Drive Unit
  • Hydraulic vs. Electric
  • Barrel Heaters

7
Injection Unit
  • Converts pellets into a thermoplastic melt
  • Electric Band Heaters
  • Mechanical Shear
  • Performs some mixing
  • Color Concentrates
  • Conveys material, at high speed and pressure,
    into the mold via the nozzle
  • Note Size your machine to use a minimum of 25
    of the max. shot size (40 to 70 is preferred).

8
Mold/Tooling
  • Most common mold material is P20 or P21 tool
    steel
  • SS 420 is also used
  • Corrosion resistance
  • H13 is used for high temperature materials and
    ejector pins
  • Provides longer mold life
  • A2 and A6 steels are used when a Class A surface
    is needed
  • O1 and O2 are used when heat treating is needed
  • Aluminum 6061-T6 can also be used for prototypes
    or low-volume production

9
Mold/Tooling
  • Sprue
  • Standard
  • Hot Sprue
  • Runners
  • Main
  • Secondary
  • Cold Well Slug
  • Hot Runners
  • Gates
  • Cavities
  • Single Cavity
  • Multicavity

10
Hot Runners
  • Hot runner systems virtually eliminate runner
    scrap
  • Used in high-volume production environments
  • Utilize a shut-off at the tip/gate

11
Gate Design
  • Submarine or Tunnel Gate
  • Pinpoint or Restricted Gate
  • Fan or Edge Gate

12
Gate Design
  • Tab Gate
  • Sprue Gate

13
Gate Design
  • Flash Gate
  • External Ring Gate

14
Gate Design
  • Internal Ring Gate

15
Gate Design
  • When placing gates, try to gate thick to thin.

16
Clamping Unit
  • Injection presses are rated by their clamping
    force (i.e. 75 ton, 300 ton, etc.)
  • Responsible for keeping the two mold halves
    together during the injection molding cycle
  • Can be hydraulic, electric, or a hybrid
  • Toggle vs. Straight Hydraulic
  • Note A good guideline for sizing machines is
    that 350 lbs. of force is needed for each in2 of
    cross-sectional area parallel to the mold face.

17
Clamping Unit
18
Clamping Unit
  • Tandem systems allow for stack molding operations
  • e.g. top/bottom half, family of parts, etc.

19
Auxiliary Equipment
  • Dryers
  • Dehumidifier (Best)
  • Tray w/ desiccant bed

20
Auxiliary Equipment
  • Mold Heating
  • Water PP, ABS, HDPE, TPU, SEBS, TPV, RTPU,
    PC/ABS
  • Oil Electric Nylon, PC, PEEK, PPS, POM, LCP,
    PPA
  • Material Conveying Systems
  • Pneumatic, Gravity Fed
  • Part-Pickers
  • On-Line QA Equipment
  • Grinders
  • 20 re-grind usually acceptable

21
Process Cycle
  • Three Parts
  • Screw Forward Time
  • Injection
  • Injection Hold
  • Packing
  • Hold/Cooling Time
  • Holding
  • Screw Rotation and Retraction
  • Cooling
  • Mold Open Time
  • Ejection/Part Removal

22
Screw Forward Time
  • Injection
  • Speed and pressure control the fill time
  • Injection Speed
  • Fast injection offers good surface appearance,
    weld-line strength, and minimizes orientation
  • Can cause fiber breakage
  • Injection Pressure
  • Start low and increase until the part is filled
    just short of flashing
  • Packing
  • After initial injection
  • Packs more material into cavity to make up for
    the material shrinkage

23
Hold/Cooling Time
  • Gives polymer melt time to set up
  • Holding
  • Keeps pressure on the molten material so it does
    not flow back into the barrel
  • Screw Rotation/Retraction
  • Process of building a shot
  • Cooling
  • Cooling times vary based on the resin and fillers
  • Amorphous materials typically have shorter
    cooling times than crystalline materials
  • Time can be minimized via proper cooling circuit
    design

24
Mold Open Time
  • Mold opening
  • Ejection of the part
  • Ejector Pins
  • Conveyors
  • Part Pickers
  • QA Systems
  • Optical Measurement
  • Weight
  • Possibilities are Endless!
  • Limit mold open time
  • Heat Loss
  • Cycle Time

25
Process Variables
  • Barrel Temperature
  • Hot enough to melt material
  • Too hot leads to material degradation
  • Mold Temperature
  • Hotter Better Surface, Flow, Properties
  • Screw Speed
  • Slow as possible with fiber filled
  • Can be higher for other fillers to promote mixing
  • Screw retraction should stop 1-2 seconds before
    mold opens for ejection
  • Injection Speed/Injection Time
  • Injection speed should be as fast as possible to
    minimize knit-lines
  • High speed also gives good surface appearance
  • High speed can also generate a large amount of
    shear heat

26
Process Variables
  • Injection Pressure
  • Highest pressure just short of flashing provides
    the best physical properties
  • Backpressure
  • Low for filled compounds (50-100 psi)
  • Very low for long-fiber
  • Higher if mixing is needed (i.e. color
    concentrates)
  • Always need some backpressure so no air is
    introduced into the melt
  • Hold Time
  • Can be low for materials that set-up quickly
  • Increases with part thickness
  • Cooling Time
  • Varies with material, part thickness, mold
    cooling circuit, and mold temperature
  • Moisture Content
  • As low as possible
  • Suggested level is 0.01-0.2 depending on the
    material

27
Troubleshooting
  • Troubleshooting Guide from RTP
  • !!!Make sure the material is dry!!!
  • Low residence time is best

28
Applications
  • Automotive Components
  • HVAC Components
  • Medical Devices

29
Applications
  • Buttons Knobs
  • Electronics
  • Sports Recreation
  • Power Tools
  • Appliances
  • Toys

30
Advantages
  • Can be low-costfor high volume
  • Complex shapes are easily molded
  • Cycle times are usually short
  • Materials are reusable/recyclable
  • Wide variety of materials available including
    custom materials
  • Precise dimensional tolerances
  • Repeatability

31
Disadvantages
  • Large initial capital investment for a molding
    press and auxiliary equipment
  • Large initial capital investment for a tool
  • Does not afford easy design modifications
  • Tools are cut for a specific material
  • Complex parts require complex tools
  • Cores, slide-action, etc.

32
Questions?
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