injection molded magnets - pros and cons · geometry large variety determined by injection molding...

Injection Molded Magnets - Pros and Cons

Thomas Schliesch

Max Baermann GmbH, Bergisch Gladbach

Germany

1 Injection Molded Magnets – Pros and Cons

Magnetics 2017, Orlando, FL - Thomas Schliesch, Head of Research and Development

Email: [email protected], Phone: +49(0)22048309141

• Injection molded magnets: Overview

• Aspects of magnetic strength in different sorts of applications

• Examples for motor and sensor applications

• Design and quality control

• Problems and future perspectives

Contents of Presentation




Injection Molded Magnets: Applications/Overview

Advantages •Complicated shapes (pro)

•High variety of field distributions (pro)

•High density of functions (pro)

•Large output rates (pro )

Disadvantages •Reduced fields and forces (con)

•Limited temperature range (con)

•Limited mechanical stability (con)

Industry •Automotive

•Household

•Computers and electronics

•Medicine

Injection Molded Magnets – Pros and Cons Magnetics 2017, Orlando, FL - Thomas Schliesch, Head of Research and Development


3

Applications: •Field sources for sensors

•PM for electrical machines

•Magnetic couplings

•Magnetic bearings

•Gaskets

Injection Molded Magnets: Manufacturing




Injection Molded Magnets: Manufacturing by In-Mold-Magnetizing




pro

Mainly used

for Ferrites

pro

Injection Molded Magnets: Magnetization by Pulse Discharge Process



6

Mainly used

for Rare Earth magnets

Features Values Remarks

Remanent polarization

Br

0.1T-0.8T Decreased by polymer delusion (con)

Intrinsic coercivity Comparable with bulk

materials

Application temperature -60°C - +200°C Mainly limited by polymer behavior (con)

Tensile strength 20MPa - 100MPa

(room temp.)

Large variations with temperature due to

polymer behavior (con)

Magnetization patterns

and fields

Large variety, any

locations

Tailoring of internal polarization patterns

and anisotropies possible (pro)

Combinability with

other parts

Easy By overmolding processes,

cost effective (pro)

Geometry Large variety Determined by injection molding (pro)

Injection Molded Magnets: Table of Characteristics




Injection Molded Magnets: Strength of Magnetization

=> Volume fraction of magnetic material determines performance !

E.g. a typical volume fraction is 60% (which means ~ 90% mass percent of

magnetic material for many sorts of compounds) => Mc ~ 0.6M0 (con)

C

i

0

C

i0

C

i

CV

V

MV

VM

V

m

M

iii

or



8

C

M0C

V

VMM con

Magnetic particles with raw material magnetization M0 diluted in a polymer matrix.

Vc compound volume, VM total magnetic volume, Vi volume of single particle,

Mc: Resulting magnetization of compound

Permanent Magnets – Strength of M Determines Strength of H and B

From Maxwell equations follows for the field H of permanent magnetic materials:

Example: Axial field above an uniaxially magnetized disc (center position):

22

a

22

a dr

d

)hd(r

hd

2

MH(d)

A V

'

'

''

'

'

dVrr

M

4π

1dA

rr

nM

4π

1)r(H



9

a0 MHμB )'r(fM)'r(M a

(Ma average strength of magnetization)

outside magnet

Magnetic Sensors and Injection Molded Magnets

Injection molded magnets are very popular for magnetic sensors due to:

- High variety of field shapes

- Geometry

- Combinability

- Fast production of large numbers




The lack of field strength of injection molded magnets often plays a minor role for magnetic

sensors due to:

- The development of more sensitive sensors

- Position sensors that detect or calculate field directions are independent

of field strengths over wide ranges of operation (pro)

- Field strengths can often be increased by slight dimensional changes (pole face) or by use of back iron

assemblies

0

rad

tan2 )

B

B(atan

p

1

pro

(measured angular position φ as a function of two field components,

p-> number of pole pairs)

Injection Molded Magnets for Magnetic Clamping and Coupling



11

dAnˆμμ

1F A

0r

)B2

1(BBBBB

BB)B2

1(BBB

BBBB)B2

1(B

ˆ

22

zyzxz

zy

22

yxy

zxyx

22

x

Maxwell Stress Tensor Θ:

2

aMF

0F 0.36 F

Magnetic Coupling, A-> air gap area, torque T:

if filling grade is 60% volume

e.g. for a clamping system

tanrad BBAT 2

aMT

Due to the dependence of the square of magnetization injection molded magnets

are used only for low strength couplings or clamping systems.

Magnetic Clamping, force F:

con

con

Injection Molded Magnets for Rotating PM Machines

MATH Maximum torque TH is proportional to strength of

magnetization M (and air gap area A) (con)

2

H

F )T

T(1η => Maximum efficiency decreases with decreasing strength of

magnetization (con), TF -> frictional torque

Even though the strength of injection molded magnets leads to inferior motor

performance, machines of lower power consumption are often built by using such

magnets. This is mainly done when:

- Complicated geometries of rotors are needed (pro)

- Special field shapes are demanded (pro)

- Magnet performance can be balanced by volume enlargement or increase of numbers

of windings



12

Examples: Sensor Magnets for Rotary Position Detection (On-Axis)

Off-axis position sensors: Sensor is located at or near the

center of rotation. 2D-Hall sensors or MR field direction

sensors are mostly used, two poles in the majority of cases

0

rad

tan2 )

B

B(atan

p

1

)sin(BB 0rad

)(cosBB 0tan



13

1p

Examples: Sensor Magnets for Rotary Position Detection (Off-Axis)

Off-axis position sensors: Sensor is located radially

outside external circumference of cylindrical

specimen. 2D-Hall sensors or MR field direction

sensors in most cases. Amplitudes are usually not the

same for both field components, field shape is not

always sinusoidal. Number of poles starts with 2 but

can also be quite large.

0

rad

tan2 )

B

B(atan

p

1

)f(BB 1rad

)g(BB 2tan



14

1p

24 poles

4 poles

Examples: Rotor Magnets for Electrical Machines

Rotor magnets with different polarity

and field shapes and different inserts

(metal shafts or plastic bushes)

Different traces for torque and

signal generation are used fairly often



15

FEM-Design Method for Field Oriented Magnets

Magnetizing Field

H’

Calculation of

Br and µr

mold magnet



16

pro and con Optimization of magnet performance by tailoring field shapes raises the variety of technical solutions (pro)

but is fairly challenging (con)

FEM-Design Method for Pulse Magnetized Magnets

Magnetizing Field

H’

Calculation of

Br and µr

magnet

magnetizing coil




pro and con

Optimization of magnet performance by tailoring field shapes raises the variety of technical solutions (pro)

but is fairly challenging (con)

Magnetic Design by Analytical Software

A

dA''r-r

'nM

4π

1dV'

'r-r

M'

4π

1)r(

rr

V

rM

E.g. by solution of

Poisson equation: ΦH

Covers approx. 70%

of our daily design

work

Problem input and

solution are done

within a few seconds

only !

Treatment of many sorts

of different magnets

with up to 256 poles



18

Covers molded

and sintered magnets

Magnetic Quality Control

Automated sensor device with 4 different types of sensors for quality control in production sites



19

pro

Magnetic Quality Control

Check of each sort of magnet with own measuring routines, statistical data evaluation,

results accessible via intranet



20

pro

Mechanical Stability Issues and Their Solution

Different coefficients of thermal expansion

can lead to large internal stresses and

cracks. Solution is often an intermediate

layer made of low filled polymer for

damping



21

con

pro



22


System design with the help of mechanical stability simulations

(pro and con). Attempts to develop materials with higher

elasticity/plasticity and comparable magnetic performance

to standard compounds (pro).

Maximum stress variation inside a magnet at

temperature cycle between -40°C and 125°C

Mechanical parameters of standard and improved

Ferrite/PA materials


- Rotors for water pumps made of PPS/Ferrite: Special PPS compositions can solve problems

to withstand temperature shock tests (pro).

- Mechanical stability control of magnetic compounds has to be applied continuously

for water pump magnets (con).



23

Attempts to Solve the Strength Issue: Ferrites

Raise of remanence by increase of filling grade up to

70% of volume for a Ferrite-polyamide composition.

Filling grade enhancements are mainly limited by

mechanical strength issues.

Hybrid compounds: Compositions can be tailored to fill the gap between Ferrites

and Rare Earth materials by addition of e.g. NdFeB, but materials become expensive !



24

pro

pro and con

Strength Issue: Anisotropic Rare Earth Compounds

-4000

-3000

-2000

-1000

0

1000

2000

3000

4000

0 40 80 120 160 200 240 280 320 360

Flu

x d

en

sit

y [

G]

Angle [°]Ferrite

NdFeB-anisotropic

Comparison of a multipolar rotor magnet made of

Ferrite and anisotropic NdFeB-HDDR powder

(PA-binder)

Injection molded anisotropic NdFeB-HDDR materials can originate large

remanences between 700mT (PPS based) and 800mT (PA based), but thermal stability

often does not meet automotive specifications. Similar results are valid for

SmFeN-materials

Demagnetizing curve of anisotropic NdFeB

compound with PPS binder



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pro and con

Injection Molded Magnets - Pros and Cons



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Thank you for your attention !

injection molded magnets - pros and cons · geometry large variety determined by injection molding...

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