Compression Molding
How Compression Molding Shapes RF Dielectrics and Radomes
The process begins with a measured charge of bulk molding compound (BMC), sheet molding compound (SMC), or a granular thermoset preform that is preheated and dropped directly into an open, heated cavity. The press closes, and the matched-metal halves squeeze the softened material outward to fill the cavity while heat drives the crosslinking reaction. Excess material escapes as flash through controlled relief lands, and once the part has gelled and cured it is ejected as a near net-shape component. Unlike transfer or injection molding, the material never travels through a narrow gate, so shear is low and reinforcing fibers do not fracture or align into weld lines. For RF parts this matters a great deal: fiber breakage and weld lines create local pockets of differing permittivity that distort an antenna pattern or detune a tuned radome wall.
Radome manufacturing is the marquee RF use case. A streamlined or planar radome must be electrically transparent at the operating band, which usually means a wall whose thickness is a half wavelength in the dielectric, or a thin skin well below a quarter wavelength. Compression molding lets a fabricator hold that wall thickness across a doubly curved surface while embedding 30 to 65 percent glass or quartz fiber for stiffness. The same approach forms thick dielectric blocks, lens cores, and standoff insulators. Cyanate ester and quartz-reinforced compounds are favored for high-performance radomes because their loss tangent sits near 0.003 to 0.008 even at Ka-band and above.
Encapsulation of RF and microwave subassemblies is a related application. A populated carrier can be over-molded with a filled thermoset to provide mechanical protection and a hermetic-like moisture barrier, though designers must account for the cure exotherm and the molding pressure so that fragile MMIC die and wire bonds survive. Filler loading is tuned to match the coefficient of thermal expansion of the substrate and to keep the encapsulant permittivity from loading nearby transmission lines.
Cure Kinetics and Shrinkage Control
Two effects govern dimensional accuracy. First, crosslinking itself shrinks the resin (cure shrinkage); second, the part contracts further as it cools from mold temperature to room temperature (thermal shrinkage). Cavities are machined oversize by a shrink factor, and high filler loadings are used specifically because mineral and glass fillers cut both the cure shrinkage and the coefficient of thermal expansion. The degree of cure follows an Arrhenius-driven rate law, so mold temperature and dwell time are traded against one another to reach full conversion without scorching.
Governing Relationships
dα/dt = A × e(−Ea / RT) × (1 − α)n
Net molding (clamp) force:
F = Pmold × Aproj (e.g. 20 MPa × 0.05 m² ≈ 1.0 MN)
Cavity shrink compensation:
Lcavity = Lpart / (1 − S) where S ≈ Scure + αCTE × ΔT
Radome electrical wall (half-wave):
t = λ0 / (2 × √εr) (at f0)
Where α = degree of cure, A = pre-exponential factor, Ea = activation energy, R = gas constant, T = absolute mold temperature, Pmold = molding pressure, Aproj = projected part area, S = total linear shrinkage, εr = relative permittivity, λ0 = free-space wavelength.
Compound and Process Comparison
| Compound / Method | Cure / Form Temp | Pressure | Typical εr | Loss Tangent | Best RF Use |
|---|---|---|---|---|---|
| Glass-filled epoxy BMC | 150 to 175 °C | 7 to 21 MPa | 4.0 to 4.8 | 0.015 to 0.025 | Structural radomes, housings |
| Phenolic BMC | 150 to 170 °C | 14 to 35 MPa | 4.5 to 6.0 | 0.03 to 0.06 | Low-cost enclosures, standoffs |
| Cyanate ester / quartz | 175 to 200 °C | 7 to 21 MPa | 3.0 to 3.4 | 0.003 to 0.008 | Ka-band and mmWave radomes |
| Glass-reinforced PTFE | Sinter > 360 °C | 35 to 70 MPa (cold) | 2.1 to 2.6 | 0.0010 to 0.0025 | Low-loss dielectric billets |
| Filled thermoset encapsulant | 150 to 175 °C | 3 to 10 MPa | 3.5 to 4.5 | 0.01 to 0.02 | MMIC / module over-molding |
Frequently Asked Questions
How does compression molding compare with injection molding for microwave dielectric parts?
Compression molding loads a measured charge into an open heated cavity and then closes the press, so the material sees low shear and short flow paths. That keeps long glass fibers intact and preserves uniform permittivity and low loss across the part. Injection molding forces material through a gate at high shear, breaking fibers and forming weld lines that can shift local permittivity 2 to 5 percent. For thick radome walls and large BMC blocks, compression molding gives lower internal stress and holds wall thickness near plus or minus 0.05 mm, while injection molding wins on cycle time (30 to 60 s versus 2 to 5 minutes) for high-volume connector insulators.
What cure temperature and pressure are used for glass-reinforced PTFE and thermoset radome parts?
Glass-filled epoxy or phenolic BMC cures at 150 to 175 °C under 7 to 35 MPa with about 1 to 3 minutes of dwell per millimeter of wall. Cyanate ester radome compounds cure near 175 to 200 °C and need a free-standing post-cure of 4 to 6 hours at 220 °C. Glass-reinforced PTFE is the exception: it does not crosslink, so it is cold-formed at roughly 35 to 70 MPa and then free-sintered above 360 °C in a separate oven cycle rather than cured in the mold.
How is mold shrinkage controlled to hold radome wall-thickness tolerance?
Total shrinkage is cure shrinkage plus thermal shrinkage on cool-down. Cavities are cut oversize by a shrink factor, typically 0.1 to 0.8 percent for glass-filled thermosets and 2 to 4 percent for unfilled PTFE. Loading 30 to 65 percent glass or mineral filler lowers both the coefficient of thermal expansion and the cure shrinkage, which is why radome compounds run high filler content. Uniform heating, controlled cool-down, and balanced charge placement keep shrinkage isotropic so the half-wave electrical wall stays within plus or minus 1 percent and the transmission penalty stays under 0.3 dB.