Injection Molding Simulation

•How to interpret simulation results

Introduction of Filling

� Filling Pattern� During the filling process, polymer

melt is propelled into the mold cavity by pressure.

� The pressure source comes from the sprue where the pressure is the

Melt Front

sprue where the pressure is the highest.

Tube Flow

Flow between Plates

Melt Front

Radial Flow

Introduction of Filling

� Filling Pattern� The highest pressure is

from gate� Pressure reduces due to

flow friction� The melt front propagates

Hig

h

pre

ssure

� The melt front propagates due to pressure gradient

� The coarse melt front time isocurves imply smooth filling

� The dense melt front time isocurves imply high flow resistance

Thin area is

difficult to flow

Thick area is easy to

flow

Introduction of Filling

� Filling Pattern� Normally, the flow towards

areas of the cavity with minimum resistance.

� If the flow is faster, it indicates the area has a indicates the area has a lower resistive force against the flow.

� On the other hand, a very slow advancement of melt front represents an area having a large resistive force.

Introduction of Filling

� Effects caused by part thickness� Since the thermal conductivity of

thermoplastics is low, the thick portions cannot release its heat easily.

� Thicker portion are usually the � Thicker portion are usually the hotter areas in the cavity at the same time.

� Thinner portions of the plastic parts have a large flow resistance and the flow is therefore much more difficult.

� High shear rates due to low thickness will result in significant viscous heating.

Introduction of Filling

� High-Speed filling: Flow control� High speeding filling results in a

high shear rate.� Viscosity of thermoplastics may

decrease due to shear heating and thus reduces its flow resistance.thus reduces its flow resistance.

� Viscous heating effect can also reduce the thickness of the solidification layer in the thin area.

Introduction of Filling

� Low-speed filling: Heat transfer control� Slow filling will result in low

viscosity shear thinning effect, which implies more energy is required to force the melt to fill required to force the melt to fill the cavity.

� Cooling effects are especially critical at regions with low thickness.

� Thicker portions are easier to get filled with low-speed filling than with high-speed filling rate.

Introduction of Filling

� Fountain flow� The flow front is of a spreading

or smearing nature, which is generally referred as fountain flow due to the no-slip flow characteristics on the mold characteristics on the mold surface.

Introduction of Filling

� Fountain flow – weld line� The picture shows most of the

macromolecular chains at the melt front are parallel to fountain flow.

� There exist some clear lines on the surface of the part, which are the surface of the part, which are the so-called weld lines.

� Weld lines are not only negative effect but also generating stress problems due to its microscopically subcompact structure.

Weld lines

Introduction of Filling

� Hot welding� Weld lines are formed by inserts

splitting the flow of the polymer at high temperature areas.

� It often occurs during the filling process. The melt temperature process. The melt temperature is sufficiently high to prevent poor weld line strength.

Insert Weld line due to hot welding

Introduction of Filling

� Cold welding� It occurs under low temperature. � It is commonly seen at the end

of filling and at the intersection of different melt front.

� The melt temperature is usually

Gate

� The melt temperature is usually low at the end of filling, so the weld line strength is poor.

Weld lines due to cold welding

Gate

Gate

Introduction of Filling

� Displacement of weld line� The thrust action would

displace the internal weld surface.

� It is caused by internal melt flow and is regarded as an and is regarded as an underflow effect.

� It would further weaken the strength of weld line.

Introduction of Filling

� Molecular orientation� The behavior of molecular

orientation is caused by the difference in the external forces on the macromolecule. i.e., velocity difference.velocity difference.

Introduction of Filling

� The trend of molecular orientation� Molecular chains are generally

aligned along the main flow direction.

� The shear rate at the middle � The shear rate at the middle layer is the lowest, and molecular chains are randomly oriented.

� Molecules near the walls are oriented along the flow direction.

Introduction of Filling

� Hesitation� The area having a

hesitation is prone to freeze the plastic flow faster than expected, which generally leads to which generally leads to incomplete fill.

� To avoid hesitation, the gate should be located far from the thin areas as much as possible.

Hesitation

Introduction of Filling

� Racetrack� The race track phenomenon

is due to different flow resistance within filling process.

� Racetrack tends to form � Racetrack tends to form defects like internal welding line, burnt mark and incomplete filling.

Possible internal

weld lineRacetrack due to

different wall thickness

Introduction of Filling

� Air trap� Inspect the melt front

advancement for any air-trap problem.

� Moldex3D can identify air traps automatically.traps automatically.

� Air traps usually result in burn marks

Air trap

Introduction of Filling

� Flow balance (single cavity)� Pay attention to flow

balance while using multiple gates.

� Always try to balance the flow so each gate provides

Flow mark

flow so each gate provides a similar flow contribution.

� You need eDesign to evaluate runner effects

Unbalanced runner system

Dominant gate

Introduction of Filling

� Flow balance (multiple cavities)� According to the analysis of melt

front advancement, molds with multiple cavities can be checked if each cavity is filled evenly and if the flow balance is good.

Unbalanced runner system

the flow balance is good.� You need Moldex3D/eDesign to

perform flow balance of multi-cavity mold.

Cavities that are

overpacked

Cavities that may be

incompletely filled

Introduction of Filling

� Shear rate distribution� If the shear rate is too high,

it could stretch the molecular chains too much and lead to chain breakage.

� Normally, high shear rate

Material Max. allowable shear rate (1/s)

ABS 50,000

PP 100,000

PS 40,000

HIPS 40,000

PA6 60,000

PA66 60,000� Normally, high shear rate occurs at gates and thin cavities.

PA66 60,000

PBT 50,000

PET 50,000

PC 40,000

PC/ABS 40,000

PMMA 40,000

POM 40,000

PPS 50,000

LCP 60,000

Introduction of Filling

� Shear stress distribution� If the shear stress is too

high, it could lead to residual stress, excessive warpage, painting problem…etc

Material Max. allowable shear stress(Mpa)

ABS 0.30

PP 0.25

PS 0.25

HIPS 0.30

PA6 0.50

PA66 0.50problem…etc� Normally, high shear stress

occurs at gates and thin cavities.

PA66 0.50

PBT 0.40

PET 0.50

PC 0.50

PC/ABS 0.40

PMMA 0.40

POM 0.45

PPS 0.50

LCP 0.50

Introduction of Filling

� Pressure history� Pm: Pressure profile in the

metering zone of the injection screw.

� Pn: Pressure profile at the injection nozzle.injection nozzle.

� Pg: Gate pressure profile at the end of runner, that is the pressure profile at the inlet of the cavity.

� Pc: Pressure profile at cavity end. Internal cavity pressure is lower than gate pressure due to the loss of pressure inside the cavity.

Introduction of Filling

� Filling stage

� tf-tf1: Flow control stage. Polymer starts filling the empty cavity; maintaining a steady flow and the cavity pressure increases graduallypressure increases gradually

� tf1-tp: Pressure control stage. During the solidification of molten polymers, cavity pressure increase quickly and plastic flow begin to reduce its volume. Mold filling pressure is transferred to the end of the cavity.

Introduction of Filling

� Pressure distribution (single cavity)� Evaluate pressure drop condition

while designing the multiple gating system.

� Find out the dominant gate location (lower pressure drop one)

Flow mark

location (lower pressure drop one) and eliminate/modify unnecessary gates (higher pressure drop, lower flow).

Lower effective pressure

due to higher pressure

drop in runner

Dominant gate

higher effective

pressure due to

lower pressure

drop runner

Introduction of Filling

� Pressure distribution (multiple cavities)

Unbalanced runner system

� While evaluating molds with multiple cavities, find out if there exist uniform pressure distribution at each gate of mold cavity and interior of cavity in

Cavities that are

overpacked

Cavities that may be

incompletely filled

P1 P2 P3

P1>P2>P3

cavity and interior of cavity in order to achieve the flow balance.

Need more from simulation?

� Moldex3D/eDesign� A complete suite of analysis tools for tool designer, it provides

• Quick and direct CAD/CAE integration with automesh• Runner analysis• Cooling analysis• Warpage analysis• Warpage analysis• Fiber orientation analysis• Multiple-component molding• Extensive parallel computing

Moldex3D products?

www.Moldex3D.comwww.Moldex3D.com