# Compression molding

Compression molding across thermosets, thermoplastics, and elastomers: charge preparation, flow and solidification, reinforcement, tooling, defects, and acceptance.

Reviewed: 2026-11-01

Tags: processes, shaping, compression-molding

A hypothetical **20 MPa hydraulic pressure** acting on a **20,000 mm² piston** gives **400 kN ideal press force**. Applied over **50,000 mm² of projected cavity area**, it averages **8 MPa**. Doubling that area would require **800 kN** to maintain the same average. Qualify mold reactions, press capability and the pressure distribution needed to consolidate the actual charge.

Set the thermal route by charge. Thermosets and crosslinking rubber require a cure. Thermoplastics consolidate and solidify through cooling.

Specify the material grade, charge preparation, mold, press and acceptance together. Retain reinforcement and charge layout when comparing results.

## Material families and charge forms

Molded Fiber Glass's *Technical Design Guide for FRP Composite Products and Parts*, carrying **2018** on its cover, distinguishes thermosets that cannot be remelted and reshaped from thermoplastics that can be melted and solidified by cooling. Its matched-die discussion describes charge placement in an open mold, closure, and pressure, and distinguishes compression from transfer processes that force material from a separate chamber. This is process taxonomy, not an interchangeable machine cycle. [MFG technical design guide](https://moldedfiberglass.com/wp-content/uploads/2020/01/pdf_Technical_Design_Guide_for_FRP_Composite_Products_and_Parts_2018.pdf)

| Material/charge | Main forming and setting requirements | Important boundary |
| --- | --- | --- |
| Thermoset SMC/BMC | Flow and consolidate before the specified cure prevents further flow | A filled-looking part may still be undercured |
| Thermoplastic powder or preheated charge | Heat/consolidate, then cool through the qualified solidification route | Cooling history can change structure and properties |
| Crosslinking elastomer preform | Fill the cavity and achieve compound-specific vulcanization/cure | Demolding and post-cure requirements depend on formulation |
| Reinforced sheet, bulk compound, or preform | Manage flow, reinforcement distribution, and consolidation | Properties need not be uniform or isotropic |

IDI describes **SMC** as resin/filler paste combined with chopped glass between carrier films and matured to the required viscosity. Its **BMC** is a batch-mixed, dough-like compound with short reinforcement. Both are supplied uncured, but their charge forms and process behavior differ. “SMC” or “BMC” is not a complete specification of resin, fiber content, electrical rating, strength, or shrinkage. IDI's product page has no verified technical issue date. [IDI SMC and BMC](https://www.idicomposites.com/product/smc-bmc/)

For elastomers, WACKER SILMIX identifies high-consistency silicone rubber cured by either peroxide crosslinking or platinum-catalyzed addition chemistry, and supplies preforms adapted to compression molding. This supports identifying the compound and cure system before defining the route; it does not make every silicone molding compound interchangeable. The North American product page is undated. [WACKER SILMIX silicone compounds](https://www.silmix.com/products/silicones/)

## Charge placement, flow, and reinforcement

Control charge identity, mass, shape, location, layer arrangement, handling, and time before closure. Charge weight is only one variable: two equal-weight charges placed differently can follow different paths around ribs, bosses, or inserts. Preserve the qualified pattern rather than adding arbitrary small pieces to correct mass.

MFG's SMC/BMC discussion explains that flow can alter fiber orientation and local properties. It distinguishes flowing compounds from preform reinforcement and describes reinforced-thermoplastic charge routes as a separate material/process choice. Initial random reinforcement does not establish isotropic properties in the molded component. Evaluate load direction, local flow, and geometry when applying coupon data. [MFG guide, material forms and comparisons](https://moldedfiberglass.com/wp-content/uploads/2020/01/pdf_Technical_Design_Guide_for_FRP_Composite_Products_and_Parts_2018.pdf)

Specify whether critical load paths may contain meeting flow fronts, resin-rich regions, or changing fiber orientation. For structural acceptance, use the actual material/process and representative locations or validation evidence. A formulation's tensile value is not automatically the allowable strength of every rib or corner.

## Bounded thermoplastic example: VICTREX PEEK

Victrex's *Compression Molding Processing Guide* prints **©September 2016** on its final page, despite “jan2022” in the filename. Its PAEK route includes drying/cleanliness, powder packing, heating to establish melt homogeneity, and cooling under pressure. **PEEK 450PF** is a named fine-powder example. These stages consolidate and solidify a thermoplastic; they are not a thermoset cure cycle.

The guide warns that substituting granules can leave boundaries and voids, with different mechanical behavior. It also explains that cooling history affects crystallinity and cautions that specimens machined from compression moldings may differ significantly from future injection-molded parts. Consequently, a machined compression-molded prototype cannot by itself qualify an injection-molded design. Obtain grade-specific current instructions rather than borrowing one plotted profile for every PAEK charge. [Victrex guide, pages 2–5 and final-page date](https://www.victrex.com/-/media/downloads/technical-guides/victrex_compression-molding-brochure_jan2022.pdf)

## Tooling, flash, and defects

A process specification should identify cavity/parting geometry, charge access, venting, flash-control features, ejectors, heating/cooling zones, inserts, and allowable finishing. Press capacity must be compatible with the projected loaded area, tooling support, stroke/daylight, load distribution, and required pressure sequence. Mold closure speed and force buildup are process variables, not simply the fastest available settings.

IDI's *Molding Guide for BMC & SMC* prints **June 2017** on its title page; its 2026 upload directory is not the issue date. Its troubleshooting sections associate blisters/porosity with multiple possible causes, including trapped air, contamination, charge pattern, pressure, and cure conditions. Non-fill can involve insufficient charge, excessive flow distance, delayed closure, or tool deflection. Knit-line defects involve meeting flow fronts and reinforcement patterns. These are diagnostic leads, not proof of one cause from one visual symptom. [IDI troubleshooting guide, pages 3, 21, 25, and 27](https://www.idicomposites.com/wp-content/uploads/2026/08/idi-molding-troubleshooting-guide.pdf)

Separate acceptable removable flash from damage to the functional edge or excessive material loss. Define the trim boundary and permitted witnesses. Control ejection and support during subsequent cooling or handling: a sound-looking surface does not establish complete cure, internal consolidation, or dimensional conformity. Investigate process records before changing pressure or temperature; one change can improve fill while creating another defect.

## Original example: press force versus cavity pressure

Use the stated hypothetical piston and cavity areas, ignoring friction, mechanical losses, pressure nonuniformity and other mold reactions.

| Quantity | Calculation | Result |
| --- | --- | --- |
| Ideal press force | 20 N/mm² × 20,000 mm² | 400,000 N = 400 kN |
| Average cavity pressure | 400,000 N ÷ 50,000 mm² | 8 N/mm² = **8 MPa** |
| Force for twice that cavity area at the same average pressure | 8 N/mm² × 100,000 mm² | **800 kN** |

A hydraulic gauge reading of 20 MPa is therefore not 20 MPa on the charge in this example. Doubling cavity count at unchanged force would halve the ideal average pressure if total projected area doubled. Actual sizing must include the mold's load path, tooling reactions, press capability, and the qualified process margin. Average pressure also does not demonstrate that remote ribs filled or all locations consolidated.

## Acceptance and production handoff

Inspect the part after the required cure/cooling/post-treatment and defined conditioning. Separate dimensions and warpage, surface appearance, flash, voids, reinforcement distribution, and functional properties. Relevant verification can include material identification, cure or thermal analysis, mechanical/electrical tests, sectioning or suitable nondestructive methods, and leak testing where required. Select methods and sampling for the actual failure risks and governing specification.

A useful handoff contains four groups of information:

1. **Material:** exact compound/grade, reinforcement, charge form, lot, storage/conditioning, and permitted age or rework.
2. **Charge and tool:** mass and layout, carrier-film removal where applicable, inserts, mold identity, vents, trim surfaces, and critical flow regions.
3. **Qualified cycle:** temperature evidence, loading delay, closure/pressure sequence, cure or cooling criteria, demolding, and any post-cure/anneal.
4. **Acceptance:** dimensions and conditioning, property limits, defect standards, test locations, sampling, and traceable cycle records.

## Related reference

[Manufacturing processes](../index.md)