ITEC 142 Injection Molding

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ITEC 142Injection Molding
Professor Joe Greene CSU, CHICO
Itec 142
February 23, 1999
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Chapter 11 Injection Molding

• Overview
• Equipment
• Material and product considerations
• Operation and control of the process
• Specialized injection molding processes

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Introduction

• Background
• Concept is simple
• Melt plastic, flow into mold and take part shape,
  cool, demold
• Injection molding makes parts in discrete
  (discontinuous) process
• More injection molding machines used for plastic
  processing than any other equipment
• Almost all thermoplastic and some thermosets
  materials can be injection molded
• Process is automated and highly repeatable parts
• Injection molding parts are finished with little
  post molding operations
• Very complex parts can be made
• Machines are expensive
• Molds are expensive, usually P-20 steel

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Injection Molding Equipment

• Function
• Injection
• Molding
• Clamping

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Injection Unit

• Purpose
• Melt solid pellets to liquid form and then inject
  into mold
• Steps
• Hopper- manual or pneumatic loaded. Can have a
  mixer, volumetric or gravimetric units to meter
  material.
• Screw
• Reciprocating screw
• most common
• similar to general purpose extrusion screw
• much shorter than extrusion screws, L/D of 121
  to 201 (E 201 to 301)
• compression ratios (diameter of feed to diameter
  of metering) are often 21 to 5 1 which is lower
  than for extrusion.
• lower compression ratio means less mechanical
  action and heating
• Step 1 turns of the screw melts resin and
  collects it at end of screw
• Step 2 the screw moves forward via a hydraulic
  mechanism
• Step 3 retraction of screw
• Step 4 part cooling and removal

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Injection Molding Steps
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Injection Molding Ram Injection

• Ram injection
• plunger type machine
• used prior to the invention of the reciprocating
  screw
• Step 1 resin melts via thermal heaters and
  collects in a pool called injection chamber
• Step 2 resin pushed forward by action of plunger
  (ram or piston) driven by hydraulic system at the
  head of the machine. A torpedo or spreader is
  used in barrel to improve melting and mixing.
• Step 3 resin flows into mold
• Step 4 part cools and is ejected
• Ram injection advantages
• less expensive
• better for marbling of plastics
• Reciprocating screw advantages
• more uniform melting
• more uniform mixing
• lower injection pressures
• larger permissible part area
• fewer stresses in part
• faster total cycle

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Injection Molding Ram Injection
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Injection Molding Terms

• Shot size- maximum weight of injection molding
  machine that can be injected.
• Typical shot sized for injection molding machines
• 0.7 ounces (20 g) to 700 ounces (20 kg)
• Rating system for injection molding machines is
  shot size
• PS is the standard material since thermoplastics
  have varying densities
• Screw machines have a wider range of shot sizes
  than ram injection machines
• Rule of thumb
• reciprocating screw has a range of 1/200 of
  total size to max shot size
• ram injection has a range of 1/5 of the total
  size to max shot size

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Injection Molding Terms

• Shot size- maximum weight of injection molding
  machine that can be injected.
• Typical shot sized for injection molding machines
• 0.7 ounces (20 g) to 700 ounces (20 kg)
• Rating system for injection molding machines is
  shot size
• PS is the standard material since thermoplastics
  have varying densities
• Screw machines have a wider range of shot sizes
  than ram injection machines
• Rule of thumb
• reciprocating screw has a range of 1/200 of
  total size to max shot size
• ram injection has a range of 1/5 of the total
  size to max shot size

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Injection Molding Molds

• The mold includes the shape of the part and is
  located between the stationary and movable
  platens of the injection molding machine
• Key terms
• sprue bushing- part of mold (cooled)
• nozzle- end of injection (heated)
• sprue channel- from bushing to runner
• runners- feeds material from sprue to part
• gate- mold area between runner and part
• mold cavity- concave part of mold
• mold core- convex part of mold
• multi-cavity- more than one part in a cavity
• ejectors- knock out pins
• mold inserts- multiple cavities for same base
• mold base- inserts used in same base
• MUD base- Master Unit Die
• draft angle- minimum angle from bottom to top of
  part
• parting line- the split between core and cavity
  molds

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Runner System

• Several types of runners
• single part runner
• multiple part runner
• symmetrical runner
• non-symetrical runner
• runner-less designs with hot manifolds

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Runner System

• Runner size considerations
• Although properly sizing a runner to a given part
  and mold design has a tremendous pay-off, it is
  often overlooked since the basic principles are
  not widely understood.
• Pros and cons of large runners
• While large runners facilitate the flow of
  material at relatively low pressure requirements,
  they
• require a longer cooling time, more material
  consumption and scrap, and more clamping force.
• Pros and cons of small runners
• Designing the smallest adequate runner system
  will maximize efficiency in both raw material use
  and energy consumption in molding. At the same
  time, however, runner size reduction is
  constrained by the molding machine's injection
  pressure capability.

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Runner System

• Runner Balancing is an essential for a balanced
  filling pattern with a reasonable pressure drop.
• Payoffs of good runner design
• A runner system that has been designed correctly
  will
• Achieve the optimal number of cavities
• Deliver melt to the cavities
• Balance filling of multiple cavities
• Balance filling of multi-gate cavities
• Minimize scrap
• Eject easily
• Maximize efficiency in energy consumption
• Control the filling/packing/cycle time.

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Hot Runner System

• The ideal injection molding system delivers
  molded parts of uniform density, and free from
  all runners, flash, and gate stubs.
• To achieve this, a hot runner system, in contrast
  to a cold runner system, is employed. The
  material in the hot runners is maintained in a
  molten state and is not ejected with the molded
  part. Hot runner systems are also referred to as
  hot-manifold systems, or runnerless molding.

FIGURE 1. Hot runner system types (a) the
insulated hot runner, (b) the internally heated
hot-runner system, and (c) the externally heated
hot-runner system
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Gate System

• Several types of gates
• rectangular simple gate
• fan gate

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Clamping System

• Several types of clamping systems
• rectangular simple gate
• fan gate

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Clamping Unit

• Clamping Force
• Clamping unit holds the molds together while the
  resin is injected, packed, and cooled, and
  ejected.
• Clamping force is the rating of the injection
  molder, e.g., 150 tons clamping force.
• Clamping force Injection Pressure x Total
  Cavity Projected Area
• Projected area is the area projected into a
  single plane, that is, the widest area of the
  part.
• Examples
• The force necessary to mold a part that has 100
  in2 projected area and has 3,000 psi is 3,000
  100 300,000 lbs force 150 tons (note 1 ton
  2000 lbs)
• The maximum projected surface area of a part on a
  200 ton machine with a maximum injection pressure
  of 2,000 psi is 400,000 lbs force / 2,000 psi
  200 in2

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Ejector System

• Several types of ejector systems
• ejector plate
• ejector pins
• mechanical plate
• hydraulic pins

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Plastics Design for Injection Molding

• Part Design
• The underlying principles behind part design,
  other than part functionality are
• cooling of plastic from melt to glassy state
• heat transfer from various sections
• thermal shrinkage of the plastic parts
• Heat transfer is best when the parts have the
  same thickness.
• Inside portions of parts cool more slowly than
  the part surfaces
• Center portion will shrink more than the surface

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Injection Molding Process
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Injection Molding Materials

• Thermoplastic Materials
• Most thermoplastic materials are injection molded
• A few thermoset materials are injection molded,
  silicone rubbers

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Injection Molding Operations

• Cycle Time Injection Pressure

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Injection Pressure Equations

• Equations
• Based on a simplification of classic fluid
  mechanics theory
• P is the injection pressure and n is a material
  constant (the power-law coefficient), which
  typically ranges from 0.15 to 0.36 (with 0.3
  being a good approximation) for a variety of
  polymer melts.
• Circular channel flow
• The melt flow in the sprue, runner, and
  cylindrical gates
• Strip channel flow
• Such as melt flow in a thin cavity

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Injection Pressure Graphs
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Injection Molding Thermal Process

• Temperature History in part

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Injection Molding Operations

• Fountain Effect Flow
• Hot resin flow from the middle of the flow
  channel to the walls and cools

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Injection Molding Process

• Fill time
• How long it takes to fill part. Faster filling
  rate shorter fill time
• Volume of part divided by volumetric flow rate
• Note Pressure is a function of the flow rate.
  Faster flow rate higher pressures, except at
  very slow fill which causes larger core and
  smaller flow channel and then higher pressures.

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Viscosity and Temperature and Shear Rate

• Effects of temperature and pressure
• Since the mobility of polymer molecular chains
  decreases with decreasing temperature, the flow
  resistance of polymer melt also greatly depends
  on the temperature. The melt viscosity decreases
  with increasing shear rate and temperature due to
  the disentanglement and alignment of the
  molecules and enhanced mobility of polymer
  molecules, respectively. In addition, the melt
  viscosity also depends on the pressure. The
  higher the pressure, the more viscous the melt
  becomes.
• Shear rate velocity divided by distance.
• Higher shear rate lower viscosity

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Cavities

• The number of cavities depends on the available
  production time, product quantity required,
  machine shot size and plasticizing capacities,
  shape and size of the moldings, and mold costs.
• Number of cavities
• Product Quantity If the dimensional tolerance of
  the part is not very critical and a large number
  of moldings is required.
• Machine shot capacity Number of cavities S / W

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Sprue Guidelines

• The sprue must not freeze before any other cross
  section. This is necessary to permit sufficient
  transmission of holding pressure.
• The sprue must de-mold easily and reliably.

Dco ? tmax 1.5 mm Ds ? Dn
1.0 mm ? ? 1º - 2º tan ? Dco -
Ds / 2L
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Runner Guidelines

• Common runners
• Full-round runner
• Trapezoidal runner
• Modified trapezoidal runner (a combination of
  round and trapezoidal runner)
• Half-round runner
• Rectangular runner