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ITEC 142Injection Molding

Professor Joe Greene

CSU, CHICO

February 23, 1999Itec 142

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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 12:1 to 20:1 (E: 20:1 to 30:1)– compression ratios (diameter of feed to diameter of metering) are often

2:1 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

APF

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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