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ITEC 142 Injection Molding

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... flow into mold and take part shape, cool, demold Injection molding makes parts in discrete (discontinuous) ... 20:1 to 30:1) compression ratios ... – PowerPoint PPT presentation

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Title: ITEC 142 Injection Molding


1
ITEC 142 Injection Molding
Professor Joe Greene CSU, CHICO
Itec 142
February 23, 1999
2
Chapter 11 Injection Molding
  • Overview
  • Equipment
  • Material and product considerations
  • Operation and control of the process
  • Specialized injection molding processes

3
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

4
Injection Molding Equipment
  • Function
  • Injection
  • Molding
  • Clamping

5
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

6
Injection Molding Steps
7
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

8
Injection Molding Ram Injection
9
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

10
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

11
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

12
Runner System
  • Several types of runners
  • single part runner
  • multiple part runner
  • symmetrical runner
  • non-symetrical runner
  • runner-less designs with hot manifolds

13
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.

14
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.

15
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
16
Gate System
  • Several types of gates
  • rectangular simple gate
  • fan gate

17
Clamping System
  • Several types of clamping systems
  • rectangular simple gate
  • fan gate

18
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

19
Ejector System
  • Several types of ejector systems
  • ejector plate
  • ejector pins
  • mechanical plate
  • hydraulic pins

20
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

21
Injection Molding Process
22
Injection Molding Materials
  • Thermoplastic Materials
  • Most thermoplastic materials are injection molded
  • A few thermoset materials are injection molded,
    silicone rubbers

23
Injection Molding Operations
  • Cycle Time Injection Pressure

24
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

25
Injection Pressure Graphs
26
Injection Molding Thermal Process
  • Temperature History in part

27
Injection Molding Operations
  • Fountain Effect Flow
  • Hot resin flow from the middle of the flow
    channel to the walls and cools

28
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.

29
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

30
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

31
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
32
Runner Guidelines
  • Common runners
  • Full-round runner
  • Trapezoidal runner
  • Modified trapezoidal runner (a combination of
    round and trapezoidal runner)
  • Half-round runner
  • Rectangular runner
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