In the dry pressing of isotropic soft ferrite cores—MnZn and NiZn grades for power conversion and EMI suppression—polyvinyl alcohol functions as a press binder and granulation aid. The powder agglomeration step typically begins with high-intensity mixing of spray-dried or pre-calcined ferrite powder with an aqueous PVA solution at a concentration of
8 wt% to 12 wt%, corresponding to a dry binder addition of
0.8 wt% to 1.5 wt% on the powder mass. Zinc stearate at
0.2 wt% to 0.5 wt% is often co-sprayed as an internal lubricant to reduce die-wall friction during punch ejection. The slurry is then atomized in a co-current spray dryer with inlet air at
180°C to 220°C, producing free-flowing granules with a size distribution of
80 µm to 250 µm and a residual moisture of
0.3% to 0.8%. Compaction is performed on mechanical or hydraulic uniaxial presses at
100 MPa to 130 MPa forming toroidal, E-I, or pot core geometries. Green density after pressing ranges from
2.8 g/cm³ to 3.0 g/cm³ for MnZn, with a green flexural strength exceeding
2.2 MPa measured per ASTM C1161-18, which ensures intact handling prior to sintering in a controlled oxygen partial pressure kiln at
1250°C to 1350°C. The binder burnout profile requires a ramp not exceeding
1°C/min between
200°C and 450°C to avoid carbon entrapment and bloating. Finished cores must satisfy IEC 62317-13:2015 dimensional tolerances and exhibit initial permeability tested at
10 kHz, 0.25 mT per IEC 62044-2. PVA grade selection demands ash content below
0.5% per ASTM D5630-13 to prevent magnetic performance drift due to residual sodium or calcium.
What Premature Low-Temperature Debinding Does to Carbon Residue in NdFeB Green Compacts
Pressed anisotropic NdFeB magnets fabricated from melt-spun or HDDR powders incorporate PVA as a forming binder to impart crush strength prior to sintering. The binder is introduced as a
10 wt% aqueous solution blended with the magnetic powder at a dosage of
1.0 wt% to 2.0 wt% dry basis, often combined with
0.5 wt% ethylene bis-stearamide wax as a complementary binder/pore former. Compacts are pressed in a transverse magnetic field at
1.5 t/cm² to 2.5 t/cm² to achieve a green density of
4.0 g/cm³ to 4.4 g/cm³. The critical process window is the thermal debinding and sintering sequence performed in vacuum or flowing argon. If the heating rate exceeds
1.5°C/min in the
250°C to 400°C range, incomplete decomposition of PVA’s acetate groups leaves carbon residue above
500 ppm, which diffuses into grain boundaries and reduces the intrinsic coercivity H
cJ by
10% to 15%. Mass spectrometry of outgassing confirms acetaldehyde and acetic acid evolution peaking at
340°C. The optimal thermal profile holds at
380°C for
2 h under partial hydrogen pressure of
50 kPa to catalytic hydrogenate carbon remnants, then ramps to
1060°C for liquid-phase sintering. Final magnets are characterized by remanence B
r ≥
1.20 T and H
cJ ≥
1200 kA/m as per IEC 60404-8-1:2023, with strict reject limits on carbon not exceeding
300 ppm verified by combustion infrared detection per ASTM E1019-18. Failure to control the binder burn-off often manifests as a “fisheye” fracture surface pattern identifiable in SEM fractography of overfired parts.
Tape Cast Slurry Rheology and PVA-Plasticizer Interaction in NiCuZn Ferrite Processing
Multilayer chip ferrite components—beads, filters, and common-mode chokes—begin as flexible green tapes formed by doctor blade casting of a non-aqueous or aqueous slurry. PVA of intermediate hydrolysis degree (
87 mol% to 89 mol%) and weight-average molecular weight near
30,000 g/mol to 50,000 g/mol serves as the primary binder, plasticized with glycerin or polyethylene glycol (
10 wt% to 20 wt% relative to PVA dry mass) to impart elongation at break exceeding
5%. The slurry formulation contains NiCuZn ferrite powder at a volume loading of
45 vol% to 55 vol%, a phosphate ester dispersant at
0.6 wt%, and the PVA binder dissolved in deionized water at
12 wt% concentration. Viscosity is adjusted to
1500 mPa·s to 3000 mPa·s at
10 s⁻¹ shear rate, measured by rotational rheometry, ensuring uniform tape thickness of
25 µm to 125 µm after drying at
60°C in a conveyor oven with six-zone temperature profiling. The dried green tape must withstand a handling tensile stress of at least
3 N/mm² as per ISO 1184-1983, while maintaining sufficient ductility to survive screen-printing of silver paste and lamination press cycles at
70°C, 20 MPa. During co-firing at
900°C, the binder decomposes below
500°C, leaving porosity that collapses during liquid phase sintering. The final ceramic body exhibits an initial permeability of
100 to 800 at
1 MHz depending on formulation, and must pass 1000-hour load life tests at
85°C/85% RH per AEC-Q200 Rev D. PVA purity in tape-cast applications is monitored via ICP-OES for residual catalysts, with sodium and calcium content individually restricted to
less than 80 ppm to avoid dielectric breakdown in co-fired modules.
If Water-Soluble Debinding Is Retained in Iron-Silicon MIM up to 600°C
Soft magnetic components for solenoid plungers and sensor cores are net-shape fabricated by metal injection molding (MIM) using a binder system composed of PVA and low-molecular-weight PEG as a water-leachable backbone. Spherical Fe-
6.5%Si gas-atomized powder with a D
50 of
12 µm is kneaded with the binder in a sigma-blade mixer at
160°C to a solids loading of
60 vol% ±
2%, then granulated and injection molded on a screw-type machine with a
22:1 L/D ratio and shut-off nozzle. The molded parts are first immersed in stirred deionized water at
40°C for
8 h, achieving a PEG extraction above
90% and opening interconnected porosity. This water debinding step is rate-limited: if parts are not uniformly pre-dried, rapid water absorption exceeding
0.5 wt%/h can cause blister formation in walls thicker than
4 mm. Subsequent thermal debinding under flowing nitrogen at
3°C/min to
480°C removes the residual PVA; spectroscopic tracking of carbonyl absorbance at
1730 cm⁻¹ confirms complete removal before advancing to sintering at
1300°C in cracked ammonia or pure hydrogen. Sintered density reaches
7.45 g/cm³ to 7.55 g/cm³, enabling a saturation flux density B
s of
1.85 T and maximum permeability above
8000, verified by ASTM A773/A773M-21 for DC hysteresis. The PVA/PEG system is deliberately chosen where chlorine-free chemistry is mandated for resistance to intergranular corrosion, referencing MPIF Standard 35 SR-grade material designation. Dimensional reproducibility across production lots is within an isotropic shrinkage factor of
1.165±0.003, an order of magnitude improvement over press-and-sinter routes for complex geometries.
| Application Sector | Binder Specification or Process Parameter | Reference Standard / Test Method | Critical Acceptance Criterion |
|---|
| Dry-pressed MnZn ferrite cores | Ash content of PVA | ASTM D5630-13 | <0.5 wt% |
| NdFeB green compacts | Residual carbon after debinding | ASTM E1019-18 (combustion IR) | <300 ppm |
| Tape cast NiCuZn ferrite | Tensile strength of green tape | ISO 1184-1983 | >3 N/mm² |
| Fe-Si MIM components | Magnetic saturation polarization | ASTM A773/A773M-21 | Bs ≥1.85 T |
| Anisotropic Sr-ferrite arcs | Remanence orientation ratio | IEC 60404-8-1:2023 | Br/Bs >0.88 |
| Extruded flexible ferrite | Magnetic remanence ratio | EN 60404-8-1 | 0.65–0.72 |
| Soft magnetic composites | Total loss at 1.0 T, 500 Hz | IEC 60404-12:2021 | <90 W/kg |
Press-ready granules of anisotropic Sr-ferrite intended for wet magnetic compaction originate from a spray-dried slurry in which PVA serves as a granule binder. The feedstock begins as a water-based slip containing
60 wt% to 65 wt% solids—predominantly hexagonal SrFe
12O
19 with a median particle size of
0.8 µm—and
0.8 wt% of PVA solution (hydrolysis
88 mol%) added prior to centrifugal atomization. Dried spherical granules with a target moisture of
0.5 wt% and bulk density of
1.8 g/cm³ are fed into a cavity equipped with a pulsed magnetic alignment field of
1.2 T. The PVA does not plasticize the compact; instead, it disintegrates at punch impact, releasing individual crystallites that rotate into alignment before the punch applies
30 MPa to 50 MPa of compaction pressure. Green strength after demolding remains at
1.5 MPa to 2.0 MPa (three-point bending) which must satisfy the vibration resistance of robotic pick-and-place handling lines operating at
1.2 s cycle time. Sintered magnet segments achieve B
r of
0.40 T to 0.42 T and H
cJ of
270 kA/m to 300 kA/m per IEC 60404-8-1, with periodic inspection of orientation ratio (B
r/B
s) kept above
0.88. PVA’s role here is indirect: without adequate granule fracture during filling, alignment suffers and leads to a coefficient of magnetic remanence variance exceeding
3% across a production batch.
Thermal Oxidative Decomposition of Residual Acetyl Groups in PVA and Its Effect on Oxygen Content in Sintered MnZn Ferrite
High-permeability MnZn ferrite cores destined for xDSL signal transformers or common-mode chokes demand an initial permeability µ
i above
10,000 and coercivity below
2 A/m, parameters exquisitely sensitive to oxygen stoichiometry and grain-boundary chemistry. Polyvinyl alcohol used for dry pressing these cores must be characterized not merely by ash content but by the fraction of residual acetyl groups after pyrolysis. PVA grades with a degree of hydrolysis below
88 mol% retain more acetate structure, which during sintering in a closed tunnel kiln with oxygen partial pressure of
0.05% to 0.5% generates unsaturated aldehydes and carbon monoxide below
300°C. Evolved gas analysis coupled with TGA-FTIR shows that the degradation onset shifts from
220°C for fully hydrolyzed PVA to
180°C for
87 mol% hydrolysis. If carbon burn-out is incomplete, the resulting over-reduced ferrite surface layer causes a
15% to 20% drop in permeability at
10 kHz while increasing power loss (P
v) at
100 kHz, 200 mT by up to
30%, as measured per IEC 62044-2. To mitigate this, a specific grade of PVA with ash
0.3% max, chloride
50 ppm max, and hydrolysis
98.5 mol% to 99.0 mol% is prescribed. Binder addition is limited to
0.6 wt% to 0.8 wt% along with a polycarboxylate dispersant to achieve granule density of
2.65 g/cm³ for toroids weighing up to
3 kg. The sintering cycle in a nitrogen/oxygen bleeder atmosphere initiates binder burn-out at a ramp rate of
0.5°C/min between
180°C and 450°C with an intermediate soak at
260°C to volatilize acetyl fragments before the main pore-closure regime above
800°C. Finished cores are batch-tested for oxygen content by inert gas fusion per ASTM E1019, targeting a stoichiometric index Fe
2O
3 deviation within
±0.01 wt%. This degree of control prevents the skin-core permeability gradient that would otherwise exceed
5% and result in differential impedance at multi-megahertz switching.
Does the Plasticizing Method Influence Magnetic Alignment in Extruded Ferrite-Bonded Profiles?
Flexible bonded ferrite sheets and profiles for rotary encoder rings and refrigerator gaskets are manufactured by compounding hard ferrite powder—typically barium ferrite—with a thermoplastic elastomer such as chlorinated polyethylene (CPE) or ethylene-vinyl acetate (EVA). Polyvinyl alcohol is often integrated prior to final compounding as a surface-compatibilizing intermediate to render the magnetic powder dispersible in the rubber matrix during twin-screw extrusion. In this pre-treatment, the ferrite powder is wetted with an
8 wt% aqueous PVA solution at a ratio of
4 wt% PVA to powder dry weight, kneaded, dried, and then pulverized to a coated masterbatch with a controlled agglomerate size of
100 µm to 400 µm. The resulting PVA encapsulation survives the early mixing stage at
110°C in a corotating twin-screw extruder and does not thermally degrade until final profile die temperatures approach
140°C; above that threshold, the elimination of water and acetate vapor creates microporosity of
5 vol% to 8 vol% in the extrudate, which reduces the effective remanence B
r by
3% to 5% compared to a PVA-free but poorly dispersed compound. Magnetic alignment is induced by a
0.8 T magnetic field at the die land via permanent magnet poles installed in the extrusion head. The finished strip with dimensions up to
120 mm × 6 mm exhibits a magnetic remanence ratio (B
r/B
s) of
0.65 to 0.72 depending on the PVA masterbatch loading. Product compliance falls under EN 60404-8-1 for magnetically hard materials, and abrasion resistance is verified by weight loss
<3% after
1000 cycles Taber abrasion according to ASTM D4060-19. The critical process incompatibility emerges when PVA-coated powder is stored at relative humidity above
60%: moisture uptake softens the coating, turning the free-flowing masterbatch into a caked mass with a flow function coefficient below
3 as per ASTM D6128-22, and feed interruption in the extruder results in periodic fluctuations of B
r by
±2% across the extruded length.Soft magnetic composites for axial flux motor stators incorporate PVA as a fugitive pore former within a silicone-phosphate insulation system deposited on iron powder particles. Atomised pure iron powder with a particle size distribution of
45 µm to 180 µm is first phosphatized with a diluted phosphoric acid solution to create a
50 nm to 120 nm thick inorganic insulation layer, after which an aqueous solution of PVA (hydrolysis
98 mol%, molecular weight
~75,000 g/mol) is blended at a binder-to-powder weight ratio of
0.4 wt% to 0.7 wt%. The coated powder is dried at
150°C and compacted in a warm die at
600 MPa and
150°C to form complex geometries such as claw-pole stator segments. During subsequent curing at
500°C to 550°C in a nitrogen atmosphere, the PVA undergoes thermal decomposition, leaving behind dispersed micropores of
0.1 µm to 0.3 µm diameter (confirmed by Hg porosimetry) that act as distributed air gaps, increasing the effective electrical resistivity to
800 µΩ·m to 1200 µΩ·m measured by four-point probe per ASTM B193-20. This nanostructured discontinuity reduces interparticle eddy-current loss at
400 Hz by
40% to 50% compared to PVA-free insulated composites, without sacrificing saturation flux density above
1.55 T. The trade-off is a drop in maximum permeability from
400 to
320, which is often acceptable for high-frequency traction machine designs. Finished SMC components are magnetically characterized according to IEC 60404-12:2021, with specifications covering maximum total loss at
1.0 T, 500 Hz not exceeding
90 W/kg. PVA grade purity is again paramount: sodium residues greater than
150 ppm catalyze localized oxidation of the iron insulation interface, leading to a time-dependent increase in eddy-current loss of
5% per 1000 hours at
180°C aging, a metric monitored closely for automotive qualification under AEC-Q200 thermal stressing.
Polyvinyl alcohol (PVA) functions as a sacrificial organic binder in the production of ferrite magnets, rare-earth bonded magnets, and metal injection molding (MIM) feedstocks for soft magnetic composites. Grades supplied for magnetic applications are distinguished by degree of hydrolysis (
87–89 mol% partially hydrolyzed,
98–99 mol% fully hydrolyzed), 4 % aqueous solution viscosity at
20 °C ranging from
5.0 mPa·s to
60.0 mPa·s, and residual ash content specified below
0.5 wt% for standard types or below
0.05 wt% for low-sodium variants critical in high-frequency MnZn ferrites where ionic contamination shifts permeability spectra. Powder loading, green machining requirements, and debinding cycle tolerance determine the appropriate molecular weight and hydrolysis combination; fully hydrolyzed grades with a degree of polymerization near
1,700–2,400 deliver elevated tensile green strength in dry-pressed anisotropic Sr-ferrite tiles, while partially hydrolyzed types reduce solution viscosity at equivalent solids, improving wetting of platelet-shaped powders during tape casting without compromising interparticle adhesion after solvent evaporation.
How Hydrolysis Degree Governs Binder Burn-Out and Magnetic Property Recovery
Thermal decomposition behavior dictates binder selection when magnetic phase purity is sensitive to residual carbon. Fully hydrolyzed PVA homopolymer (vinyl acetate content
<1 mol%) exhibits a sharp weight-loss onset at
230–240 °C under air, measured by thermogravimetric analysis per
ASTM E1131, with complete burnout achieved below
500 °C at a ramp rate of
2 °C/min. In production-scale continuous debinding furnaces operating with a
4-zone profile, partial pressure of oxygen must be maintained above
10 vol% in the critical
280–380 °C window; otherwise, carbonaceous residues exceeding
300 ppm are trapped within the sintered microstructure, degrading the maximum energy product (BH)
max by
3–5 % as confirmed by Helmholtz coil measurements on ring samples. Partially hydrolyzed copolymers leave marginally higher residue levels because acetate side groups undergo slower oxidative scission, and in atmospheres containing less than
5 % O
2, generation of acetic acid vapor can accelerate corrosion of molybdenum heating elements. Consequently, for sintered NdFeB magnets processed under argon partial pressure, PVA grades with sodium content below
50 ppm and a degree of hydrolysis exceeding
99.0 mol% are specified to minimize both carbon and alkaline earth contamination that depresses intrinsic coercivity.
When the binder must serve as both pressing lubricant and burnout vehicle in axial die compaction of wet anisotropic ferrite powders, viscosity stability under alkaline slurry conditions becomes the controlling parameter. Ferrite slip typically carries
30–35 vol% solids with pH adjusted to
10.0–11.5 using ammonium hydroxide; fully hydrolyzed PVA in this environment undergoes negligible acetal formation over
72 h holding times, whereas partially hydrolyzed grades can develop gel bodies through intermolecular hydrogen bonding catalyzed by residual acetate at temperatures above
40 °C. Observed in twin-screw compounding trials with
L/D 36:1 extruders, a viscosity shift beyond
15 % of initial value within a single shift correlates with inconsistent fill density in the die cavity, directly translating to weight variation outside the
±0.5 % tolerance window required for
ISO 9001 motor magnet production.
Binder Migration and Its Effect on Anisotropy in High-Pressure Wet Forming
During dewatering of a
15 mm thick ferrite cake at pressures exceeding
40 MPa, PVA macromolecules fractionate according to hydrodynamic volume: low-molecular-weight chains (
<50 kDa) preferentially migrate with the expelled water, enriching the surface layers and depleting the core. The consequence is a gradient in organic content from
0.8 wt% near the pressing punch to
1.4 wt% at the mid-plane, detected by
FTIR mapping of carbonyl absorbance at
1,730 cm⁻¹ after drying. This non-uniform distribution acts as a die-wall friction modifier during subsequent dry-back operations, inducing differential springback that compromises the alignment of particle easy axes. Magnetic remanence ratio (B
r/B
s) measured on discs cut from the compact’s center falls by
0.04–0.06 compared to edge specimens, a discrepancy eliminated only when the PVA grade’s polydispersity index (PDI) is constrained below
2.5 via fractionation. A dual-binder approach—combining a high-fraction (
70 wt%) PVA of
98.5 mol% hydrolysis and
1,700–1,800 degree of polymerization with a low-MW poly(acrylic acid) dispersant—has been verified on
500-ton hydraulic presses to confine organic gradient to
0.15 wt% across the green compact, restoring remanence uniformity.
Direct application without an overlying header often suits operational boundary conditions where a single property cliff-edge defines process viability. Pre-drying of PVA powder is mandatory in environments where equilibrium moisture content exceeds
10 wt%; a
48-hour residency in a desiccant dryer at
40 °C reduces agglomeration during solution make-up. Failure to control inlet moisture results in poorly dispersed gels that clog
50-µm filter screens downstream of the dissolver, a failure mode documented on continuous coating lines running slurry at
12 m/min. Furthermore, when comparative selection between PVA and polyvinyl butyral (PVB) is framed around debinding schedule flexibility, the critical differentiator becomes the exothermic peak separation in differential scanning calorimetry. PVB decomposes with a sharper enthalpy release between
350–400 °C, generating a temperature overshoot of
8–12 °C inside
5 mm cross-section parts, necessitating a plateau of at least
4 hours to prevent microcracking. PVA’s broader burnout profile, with an oxidation exotherm spanning
230–500 °C and a maximum heat flow below
15 W/g, permits a continuous ramp of
1.5 °C/min without thermal runaway, shortening total cycle time by
18–22 % in production tunnel kilns.
Comparative property matrix for organic binder systems used in magnetic powder processing
| Property / Test Method | PVA (fully hydrolyzed, low-ash) | PVB (typical plasticized grade) | Acrylic emulsion (water-based) |
|---|
| Ash residue after 600 °C, air, ASTM E1131 | 0.05–0.15 wt% | 0.01–0.05 wt% | <0.02 wt% |
| Sodium content, ICP-OES | <40 ppm (ultra-low grades) | <10 ppm | <5 ppm |
| Green flexural strength, ISO 178, 3 mm bar | 8–15 MPa | 12–20 MPa | 2–5 MPa (without co-binder) |
| Decomposition onset, air, TGA | 230 °C | 170 °C (plasticizer loss), 300 °C (main chain) | 280 °C (depolymerization) |
| Solution viscosity at 10 wt% solids, 25 °C | 300–5,000 mPa·s | Not water-soluble; ethanol/toluene solvent | 50–500 mPa·s (emulsion) |
| Compatibility with alkaline ferrite slurry (pH 11) | Stable >72 h | Phase separation in aqueous slurry | Coagulation above pH 9.5 |
Why Acrylic Emulsion Binders Fail to Deliver Adequate Interlaminar Strength in Multilayer Tape Casting
Acrylic latex binders, while offering near-zero ash and rapid drying, produce a film formation mechanism that is inherently discontinuous around magnetic platelet fillers at volume concentrations exceeding
45 vol%. During the low-shear alignment stage in a
0.8 T DC magnetic field, acrylic particles coalesce into bridges that resist particle reorientation; the resulting green tape exhibits a Lotgering factor
0.2–0.3 lower than that obtained with a PVA solution where homogeneous polymer wrapping preserves particle rotational freedom. PVA-based formulations yield sintered tape with a crystallographic texture index of
0.85 (Lotgering f-factor,
XRD pole figure analysis on
(006) peak), sufficient for
>90 % of theoretical remanence in textured Sr-hexaferrite substrates used for self-biased circulators. Moreover, PVA’s solubility in cold water permits precise viscosity adjustment immediately before casting, whereas acrylic emulsions undergo irreversible viscosity drift due to evaporation-driven skin formation at the doctor blade tip when gap height is maintained below
150 µm.
Operational limitation: PVA solutions stored beyond
7 days at temperatures above
30 °C exhibit microbial growth that reduces molecular weight and produces acidic by-products, lowering pH from
6.5 to
4.0 and attacking the magnetic powder surface. Biocide addition must not introduce chloride ions above
10 ppm, verified by ion chromatography per
DIN EN ISO 10304-1, to avoid pitting corrosion in sintered NdFeB magnets.
In feedstocks for metal injection molding of soft magnetic Fe-Si alloys, PVA functions as a backbone polymer in a multi-component binder system. A typical formulation uses
65 vol% gas-atomized Fe-
6.5%Si powder with
D50 12 µm, a primary wax component, and PVA of hydrolysis degree
99.0+ mol% added at
5–8 wt% of the organic fraction. During compounding on a co-rotating twin-screw extruder (
L/D 40:1, screw speed
150 rpm), PVA imparts sufficient melt strength to produce pelletized feedstock with elongation at break measured at
4.5 % per
ISO 527-2 on pressed films, preventing crumbling in the hopper of
120-ton injection molding machines. Molded toroidal cores undergo a solvent debinding step in water at
60 °C for
8 hours that extracts PVA prior to thermal removal of the wax component, avoiding the carbon residue spike that occurs when all organics are burned simultaneously. Cores sintered to
7.45 g/cm³ density achieve DC coercivity of
25 A/m, meeting
IEC 60404-8.6 performance requirements for power inductor applications.
PVA grade selection guide for magnetic binder systems (representative viscosity grades, 4 % aqueous)
| Grade designation (typical) | Hydrolysis (mol%) | Viscosity (mPa·s at 20 °C) | Ash (wt%) | Primary application |
|---|
| Low-MW, fully hydrolyzed | 98.5–99.2 | 5.0–6.5 | <0.5 | Ferrite tape casting (high solids loading) |
| Medium-MW, fully hydrolyzed | 98.5–99.0 | 20–30 | <0.5 | Wet pressing, ferrite injection molding |
| High-MW, fully hydrolyzed | 99.0–99.8 | 50–65 | <0.3 (low-ash) | Isostatic pressing, anisotropic magnets |
| Partially hydrolyzed, low-MW | 87–89 | 4.5–6.0 | <0.2 | Rapid wet-out, low-viscosity spray dry binder |
When Solvent-Borne PVB Becomes the Preferred Backbone Despite Higher Processing Cost
In applications requiring zero moisture introduction—particularly binder-jet additive manufacturing of rare-earth magnet green preforms where water triggers oxidation of Nd-rich grain boundary phases—a PVB dissolved in a
60:40 ethanol:toluene mixture avoids this degradation path. However, the explosive vapor classification of the solvent system mandates explosion-proof coating equipment meeting
ATEX Zone 1 standards, elevating capital expenditure by a factor of
2–3 relative to aqueous PVA-based lines. PVA thus remains the default choice when factory humidity is controlled below
50 % RH and powder surface passivation with
0.1 wt% phosphate ester is applied.
Binder jetting with PVA aqueous binder (
10 wt% concentration,
0.3 wt% surfactant) on MIM-grade stainless steel powders produces green parts with compressive strength of
4.5 MPa, sufficient for automated depowdering stations. The soluble nature of PVA enables complete extraction during the initial water debinding stage, reducing total debinding time by
30 % compared to systems relying solely on thermal decomposition of waxy components. Published data for this specific configuration in anisotropic bonded magnets is limited; however, transfer of technology from oxide ceramic additive manufacturing predicts achievable surface roughness Ra below
8 µm without post-sintering finishing, provided layer thickness is kept at
50 µm and droplet volume does not exceed
80 pL.
