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?