Mold Release Agents for Composites (Fiberglass, Epoxy, SMC/BMC): 3 Release Systems and Selection

Mold release agents for composites are the tool coatings and compounded additives that stop a curing fiberglass, epoxy or SMC part from bonding to its mould, and they come in 3 systems: semi-permanent coatings bonded to the tool, sacrificial waxes and film formers renewed before every cycle, and internal metal soaps such as zinc stearate compounded into sheet and bulk moulding compound. Only the third system is a plastics additive in the strict sense, so which one a composite part needs depends first on whether the release travels with the compound or sits on the tool.

Release agents are one of the processing-modifier families among the plastic additives covered on this site, and composites are the part of that family where the tool, not the formulation, usually carries the release. Unsaturated polyester, vinyl ester and epoxy are thermosets moulded as plastics, which is why their release belongs in this reference.

This page explains why composite parts stick, compares the 3 systems, matches them to fiberglass, epoxy, carbon-fibre and SMC/BMC parts, shows how stearates migrate in SMC, and covers failures, checks, the 3 sets of rules and the producers. The parent page on mold release agents for plastics and composites compares all 6 release-agent types across every moulding process.

In brief:

• 3 release systems in composite moulding: semi-permanent coatings, sacrificial waxes and film formers, internal metal soaps
• About 120 °C vs about 160 °C (248 °F vs 320 °F): zinc vs calcium stearate melting point (Baerlocher), the reason zinc stearate is preferred in SMC and BMC
• 1.23 wt%: the stearate level of Peter Greven's Brookfield paste-viscosity comparison, a test condition, not a recommended dosage
• 0.1 %: the maximum for each of D4, D5 and D6 in silicone mixtures placed on the EU market since 6 June 2026 (Regulation (EU) 2024/1328)
• 5 mg/kg: the EU specific migration limit for zinc in food-contact plastics, which reaches zinc stearate (Regulation (EU) 2020/1245)

What Is a Mold Release Agent for Composites?#

A mold release agent for composites is a release agent, a chemical that prevents one material from bonding to another surface, used between a curing thermoset resin (unsaturated polyester, vinyl ester or epoxy) and the tool it is moulded on. Fiberglass, the North American trade word for glass-reinforced plastic (GRP, FRP or GFRP), epoxy laminates and the compression-moulded compounds SMC and BMC are the 3 part families this page covers.

Across all plastics, the 4 industrial classes of release agent are waxes, fatty esters, silicones and metallic soaps, the list given in Ullmann's article on release agents (Lammerting, 2000); the general family, with its 6 types, is set out on mold release agents. Composite shops say release agent, mould release, parting agent or demoulding agent for the same product, and data sheets add form release and anti-adherent.

Why do fiberglass, epoxy and polyester parts stick to the mould?#

Fiberglass, epoxy and polyester parts stick because the resin goes onto the tool as a liquid and cures there into a solid, so any surface it can wet and bond to becomes part of the moulding unless a release layer sits in between. The bond forms during cure, not after it: a thermoset resin reacts while it touches the tool, so every pore and scratch it wets on the tool face is locked into the cured laminate.

Where can that release layer come from? It comes from the tool, as a coating applied before moulding, or from the compound, as an additive that migrates to the part surface during the press cycle, and that split decides who chooses the release.

Is a composite release agent a plastics additive or a process chemical?#

A composite release agent is a plastics additive only when it is compounded into the material, as zinc stearate is in SMC and BMC; the coatings applied to fiberglass and epoxy tools are process chemicals that never enter the formulation. An internal release agent is mixed into the plastic and migrates or blooms to the part surface, while an external release agent is a coating or spray applied to the mould and belongs to the process. This reference covers external release agents only where they mould a plastic, which means 5 processes.

• Injection moulding
• Polyurethane and RIM
• Composites in epoxy, unsaturated polyester and fiberglass
• Rotational moulding
• Thermoforming

The consequence for a buyer is practical: an internal release appears on the compound's formulation sheet and its food-contact declaration, while an external one appears on the moulding shop's process sheet.

What Are the 3 Release Systems Used in Composites?#

The 3 release systems used in composites are semi-permanent coatings bonded to the tool, sacrificial waxes and film formers renewed before every cycle, and internal metal soaps compounded into SMC and BMC, and semi-permanent coatings dominate epoxy and fiberglass tooling. The first 2 systems are external products carried in water, solvent or a co-solvent blend; the third is part of the compound itself. The table sets the 3 systems side by side by where the release sits, how often it is renewed and which composite processes use it.

Chemistries are those recorded in our source library; products on the market use further chemistries that are not verified here.

1. Semi-permanent release coatings: bonded siloxane, silane and fluoropolymer films#

A semi-permanent release coating is a release agent that bonds to a clean tool surface and gives multiple releases per application, usually as a reactive siloxane or silane film, which is why it is not reapplied before every part. The bonded film is often silicone-based, and the chemistries recorded in our source library are reactive silane and siloxane coatings and fluoropolymer-based products; polydimethylsiloxane (CAS 63148-62-9) works by leaving a low-surface-energy film at the part-tool interface.

Why do composite shops favour it? An epoxy or polyester tool runs a series of parts between cleanings, and a coating that survives more than one cycle removes a release step from each of them. How many parts one application releases depends on the product and the process, and no verified figure is held in our source library. Temperature ratings, the subject of searches for a high temperature mold release agent, are product data, and none is verified here either.

Two rules follow the chemistry: silicone-based products sold in the EU have been held to 0.1 % each of D4, D5 and D6 since 6 June 2026 (Regulation (EU) 2024/1328), and fluoropolymer-based products fall inside the EU universal PFAS restriction proposal (Annex XV dossier of 7 February 2023, pending).

2. Sacrificial release agents: waxes and film formers renewed every cycle#

A sacrificial release agent is a release film applied to the tool before every moulding cycle and consumed by it, and in composite shops it is usually a wax or a film former rather than a bonded coating. The chemistries recorded in our source library for this route are wax-based products and silicone oils and emulsions, delivered in water, solvent or co-solvent carriers. The tool therefore starts every cycle with a fresh release layer.

The products sold for this job in composite shops are carnauba-based paste waxes, PVA (polyvinyl alcohol) film formers applied over wax, and release films, and all 3 appear in the search results for the queries this page answers, together with market names such as "honey wax" (Google results, 24 October 2026). No verified data on their composition, coat counts or drying times is held in our source library, so this page names no product and gives no application routine; those figures come from the technical data sheet of the exact product.

Tool waxes are a different product from the compounded waxes a formulator adds to a polymer. PE, oxidised PE, Fischer-Tropsch and montan grades act as lubricants inside the melt and are covered under polymer waxes.

3. Internal release agents: zinc and calcium stearate in SMC and BMC#

Internal release agents in composites are metal soaps, above all zinc stearate, compounded into the unsaturated-polyester paste of sheet and bulk moulding compound, where they migrate to the surface of the part during the press cycle and form the release layer themselves. Peter Greven's Plastics brochure (09/2025) states that metal soaps "migrate to the surface of the material", which is the whole mechanism: the soap is distributed through the paste at compounding, and press heat moves it to the interface.

Zinc stearate is preferred because it melts at about 120 °C (248 °F) according to Baerlocher, roughly 40 °C (72 °F) below calcium stearate at about 160 °C (320 °F), and Peter Greven names it the preferred SMC and BMC release agent for exactly that reason. Calcium stearate is also used, and stearic acid (CAS 57-11-4), the fatty acid both soaps are made from, is offered by Peter Greven for SMC and BMC as well. Grade fineness, including the precipitated grades that release best, is covered in the SMC and BMC section below.

Compounded release in thermoplastics follows the same migration logic with other chemistries, from PETS in polycarbonate to amides in polyolefins, and is set out on internal mold release agents. For epoxy laminates, pultrusion and prepregs no internal release data is held in our source library, so the internal route on this page is described for SMC and BMC only.

Semi-Permanent vs Sacrificial vs Internal Release: How Do They Compare?#

A semi-permanent coating lasts multiple cycles per application, a sacrificial release agent is renewed before every cycle, and an internal release agent travels inside the compound, so the choice turns on whether the release should belong to the tool or to the material. The semi-permanent and sacrificial definitions apply to every moulding process; what composites add is the third column, because SMC and BMC are the composite materials that carry their own release. The table compares the 3 systems on the 8 points a composite shop or compounder weighs.

Transfer, build-up and cleaning behaviour are product-specific and not quantified in our source library.

Which of the three, then, belongs to which composite process? Fiberglass and epoxy tools run mostly on semi-permanent coatings, sacrificial waxes and film formers remain the alternative in fiberglass shops, and SMC and BMC carry internal zinc or calcium stearate.

Which Mold Release Agent Suits Each Composite Process and Resin?#

The release follows the process: fiberglass and epoxy tools are mostly run on semi-permanent coatings, with sacrificial waxes and film formers as the alternative in fiberglass shops, carbon-fibre parts follow the epoxy route of their resin, and SMC and BMC carry their own release as zinc or calcium stearate in the compound. The table matches each composite process to the release system used and the number this site can source for it.

Choosing a composite release system takes 4 decisions, in this order.

1. Decide whether the release must travel with the compound (SMC and BMC) or sit on the tool (open moulding, infusion, prepreg).
2. Choose between a coating that lasts multiple cycles and one renewed every cycle, by how many parts the tool runs between cleanings.
3. Check what happens to the part afterwards: painting, bonding and printing are where a release film or an over-dosed internal release causes failures.
4. Check the chemistry against the rules: silicone-based products and the D4/D5/D6 limit, fluoropolymer-based products and the PFAS proposal, and food contact for stearates.

Fiberglass hand lay-up and spray-up (unsaturated polyester and vinyl ester)#

The usual mold release agent for fiberglass is a semi-permanent coating bonded to the tool, with a sacrificial wax, often topped by a PVA film former, as the alternative where a shop renews the release before every part. Unsaturated polyester and vinyl ester are the resins of glass-reinforced plastic, and in hand lay-up and spray-up the gelcoat, the first resin layer against the tool, is the surface the release protects, on a fibreglass mould as on any GRP tool.

Which of the 2 is "best" depends on how many parts a tool runs between cleanings and on what happens to the part afterwards. No comparative release, build-up or cost data is held in our source library, so this page does not rank products and gives no coat count.

The glass in the laminate is a reinforcement chosen for strength, not for release, and its sizing chemistry is covered under glass fiber reinforced plastics.

Epoxy laminates, infusion and prepreg tooling#

Epoxy laminates, infusion parts and prepregs are released mainly with semi-permanent coatings bonded to the tool, because an epoxy tool runs a series of parts between cleanings and a coating that survives multiple cycles removes a step from each one. An epoxy mold release agent for composite work, or a mold release agent for epoxy resin laminates, is therefore chosen by the same logic as one for polyester: parts between cleanings and what happens to the part afterwards.

Does the epoxy itself carry any release? No internal release data for epoxy laminates or prepregs is held in our source library, so the tool coating is the only route this page can describe. Epoxy poured into a mould as a casting is a separate case, covered below.

Everything else that goes into the resin, from tougheners and fillers to UV stabilizers and flame retardants, is set out under additives for epoxy resins and composites.

Carbon fibre and aramid parts#

The best release agent for carbon fiber is the one that suits its resin, because the release layer sits between the resin and the tool and never touches the fibre: an epoxy carbon-fibre part follows the epoxy route of semi-permanent tool coatings. A carbon fiber mold release agent sold under that name is therefore a resin-side product, and a polyester or vinyl ester carbon part follows the fiberglass route instead.

Carbon fibre is chosen for stiffness at low weight, and its role in plastics is covered under carbon fiber reinforced plastics.

Aramid fibres follow the same rule, and their use in plastics is described under aramid fibres in plastics.

SMC and BMC compression moulding#

Sheet and bulk moulding compound are released from the inside: zinc stearate, or calcium stearate, is mixed into the unsaturated-polyester paste and migrates to the part surface in the heated press, so the tool needs no separate coating for every shot. SMC and BMC, unsaturated-polyester compounds also made with vinyl ester, are the one composite route where the release is a plastics additive chosen by the compounder. Why zinc is preferred, how fineness changes release and paste viscosity, and which grades are sold for SMC are covered in the next section.

How Do Metal Stearates Migrate and Release SMC and BMC Parts?#

Metal stearates release SMC and BMC parts in 3 steps: the soap melts in the heated press, migrates through the curing paste to the part surface, and spreads there into a thin layer that keeps the moulding from bonding to the steel. Baerlocher attributes this to zinc stearate's low melting point, which lets it spread evenly when heated.

Metal soaps are made by 4 production routes: precipitation (the indirect route), direct conversion, the melt process and the continuous COAD process. Precipitated grades are very fine with a high surface area and release best, while melt and COAD products are the dust-free forms. The same soaps also lubricate melts, scavenge acid and stabilise PVC, the functions set out under metal stearates.

Ullmann's encyclopaedia, as cited in Wikipedia's article on the compound, calls zinc stearate "the most powerful mould release agent among metal soaps", a ranking this page reports as a quotation, not as its own finding. Rabinovitch, Lacatus and Summers (1984) add that the labels "internal" and "external" describe where an additive tends to act and explain performance only in part.

Why an additive moves to a surface at all, and how fast, is modelled under additive migration in plastics.

Zinc stearate vs calcium stearate in SMC and BMC#

Zinc stearate melts at about 120 °C and calcium stearate at about 160 °C according to Baerlocher, and that 40 °C gap is why Peter Greven names zinc stearate the preferred internal release agent for SMC and BMC, with calcium stearate also in use. The lower melting point means the zinc soap is already molten at a press temperature where the calcium soap is still solid. Baerlocher sells a dedicated SMC grade of each: ZINCUM SW 1626 (zinc stearate) and CEASIT SW 1725 (calcium stearate).

Zinc stearate, zinc distearate in chemical naming (CAS 557-05-1), is the release agent of choice in this application. Production routes, the five plastics functions and the full regulatory status are on zinc stearate.

The table sets out the identity and release-relevant properties of the 2 soaps used in SMC and BMC.

21 CFR 184.1229 prints the CAS number of calcium stearate as 1529-23-0, a typographical error for 1592-23-0.

Melting points differ by source because supplier figures describe commercial grades while PubChem describes the reference compound, so the table gives each source's value rather than one number. The calcium soap's acid-scavenger role in polyolefins and its PVC uses are covered under calcium stearate.

Particle size, paste viscosity and dosage#

Fineness decides the trade-off in SMC and BMC: a finer stearate reaches the same release at a lower dosage, while a coarser one raises the viscosity of the paste less at the same dosage, which is why precipitated grades release best but are not always the right choice. The compounder therefore weighs release against paste handling rather than maximising one of them. Three variables are balanced against each other in an SMC paste.

• Particle size: a fine precipitated grade or a coarser melt or COAD grade
• Dosage: the level of the soap in the paste
• Paste viscosity: measured by Brookfield viscometer in Peter Greven's comparison

The only number in our source library is the 1.23 wt% at which Peter Greven ran its Brookfield comparison, and it is a test condition rather than a recommended level; the working dosage is set by the compounder's own trials against paste viscosity and release. Cost-in-use of a dosage change can be compared with the additive dosage and cost-in-use calculator.

What Goes Wrong with Release on Composite Parts?#

Release on composite parts goes wrong in 5 recognisable ways: parts that still stick, paint and adhesive that will not hold, a hazy or greasy surface, an SMC paste that is too thick, and deposits that build up on the tool. The table links each symptom to its likely cause and to the change that addresses it.

Painting, bonding and secondary operations after release#

Paint and adhesive fail on a released composite part when release agent is left on its surface, because the same low-energy film that stops the part bonding to the tool also stops a coating or adhesive bonding to the part. A part that releases easily also refuses paint, and Baerlocher's lubricant performance matrix records the same effect for externally acting additives: worse printability, weldability and adhesion.

"Paintable" release products are sold for this reason, and the claim is the supplier's to prove. No verified data on paintability is held in our source library, so a paint-adhesion test on a released part is the check that decides. Silicone transfer, the most discussed case, is covered in the rules section below and on the silicone page.

Over-dosed internal release: exudation and surface haze#

An internal release agent that is over-dosed shows on the part as exudation, a greasy or hazy surface film, because more soap reaches the surface than the release layer needs. Internal release migrates to the surface by design, so the line between enough and too much is the line between a clean release and a visible film.

The physics of an additive coming out of a polymer is covered under blooming and exudation in plastics.

Baerlocher's matrix records haze and exudation at high dosage for lubricants in PVC; SMC follows the same mechanism, but no SMC threshold is held here.

Defect-by-defect diagnosis across all additive families is in troubleshooting additive-related defects.

How Is Release on a Composite Tool Tested and Checked?#

There is no standard test for composite mould release: release is confirmed on the tool itself, and the laboratory work around it checks the paste, the additive and the part surface instead. Our source library holds no ejection-force, demoulding-force or release-force standard, so a moulding trial on the real tool remains the decisive check for every release system. The table lists the 4 checks and what each one tells.

Every method used on this site is indexed under testing plastic additives.

The trial proves release, the Brookfield reading proves the paste can be processed, and the surface checks prove the part can be finished.

Identifying a release agent in a finished part is a deformulation job, covered under additive analysis and deformulation.

Which Rules Apply to Composite Release Agents?#

Three sets of rules reach composite release agents: the EU limit of 0.1 % for D4, D5 and D6 in silicone-based products, in force since 6 June 2026; the EU universal PFAS restriction proposal for fluoropolymer-based products, still pending; and the food-contact listings of zinc and calcium stearate where an SMC or BMC part touches food. External release coatings are process chemicals, so they need no additive listing under Regulation (EU) No 10/2011; only the internal stearates enter the food-contact route. The table lists each rule, what it covers and which composite release system it reaches.

REACH Annex XVII entry 70: D4, D5 and D6 in silicone-based release products#

REACH Annex XVII entry 70 has limited D4, D5 and D6, the cyclic siloxanes that remain as residues in silicone products, to below 0.1 % each in mixtures placed on the EU market since 6 June 2026, and silicone-based release coatings and sprays are mixtures. The limit comes from Regulation (EU) 2024/1328, whose later dates for leave-on cosmetics (6 June 2027) and medical devices and medicinal products (6 June 2031) do not concern composite tooling. The entry and its derogations are explained under REACH Annex XVII restrictions.

Derogations exist, including residues in silicone polymer mixtures for specific uses, and whether a given professional release product falls under one is for its supplier's declaration; this page states no product's status. The 3 cyclosiloxanes, D4 (octamethylcyclotetrasiloxane, CAS 556-67-2), D5 and D6, have been on the SVHC Candidate List since 27 June 2018 as PBT and vPvB substances.

Silicone release in every moulding process, with the full derogation list, is covered under silicone mold release agents.

Fluoropolymer-based release products and the EU PFAS restriction proposal#

Fluoropolymer-based release products fall within the scope of the EU universal PFAS restriction proposal, which ECHA published as an Annex XV dossier on 7 February 2023 and which is still pending, so they are under review rather than banned. PTFE (CAS 9002-84-0) is a fluoropolymer inside the PFAS group the proposal covers, and fluoropolymer and PTFE films appear in Google's AI Overview for epoxy release as a product type (checked 24 October 2026). No restriction is in force on 24 October 2026, and this page claims no PFAS content for any named product.

Where the proposal stands, and which plastics uses of PFAS it names, is tracked under PFAS restrictions.

Food-contact composites: stearate listings under EU 10/2011 and the FDA#

Where an SMC or BMC part touches food, its internal release is covered as a salt of stearic acid (FCM No 106) under Regulation (EU) No 10/2011, with a specific migration limit of 5 mg/kg for zinc and none for calcium. The stearates have no FCM number of their own and are authorised through Article 6(3)(a); the zinc limit has applied since Regulation (EU) 2020/1245, and the overall migration limit of 10 mg/dm² applies to the whole part.

In the US, zinc stearate is GRAS under 21 CFR 182.8994 and calcium stearate under 21 CFR 184.1229; calcium stearate is also a prior-sanctioned stabilizer under 21 CFR 181.29. Cross-linked polyester articles for repeated food contact are governed by 21 CFR 177.2420, and our source library records no release-agent clause in that section, so this page states none.

External release coatings stay outside this route as process chemicals. The FCM numbers and the overall migration limit are explained under EU 10/2011.

Who Supplies Release Agents for Composites?#

Internal release grades for SMC and BMC come from metal-soap producers such as Baerlocher, with ZINCUM SW 1626 and CEASIT SW 1725, Peter Greven, with its LIGASTAR zinc and calcium stearates, and Struktol. The table lists the producers of internal release grades named in our source library.

Semi-permanent coatings, waxes and film formers for composite tools come from specialist release-agent makers, none of which is verified in our source library, so this page names none. Buyers should compare SMC grades by CAS number, particle form (precipitated or melt) and the paste viscosity they give at the intended level, not by brand name.

Plants and product lines of the stearate producers are listed in the directory of calcium and zinc stearate manufacturers and suppliers.

─── The sections below cover resin casting, neighbouring moulding processes and uses outside plastics, which sit next to composite release but outside its core. ───

Where Does Composite Release End? Resin Casting, Other Moulding Processes and Non-Plastics Uses#

Composite release ends where the part stops being a moulded fiber-reinforced thermoset: resin castings, polyurethane and thermoplastic mouldings use release agents too, while rubber, concrete and plaster moulds fall outside a reference on plastics additives. Cast epoxy and polyester parts are still thermoset plastics, so their release question is answered here in brief, with the depth on sibling pages.

Mold release for epoxy and polyester resin casting#

A mold release agent for resin casting does the same job as on a composite tool, keeping the cured epoxy or polyester from bonding to the mould, and the same rule applies afterwards: release left on the casting stops paint and adhesive from holding. The search results for a mold release agent for resin are dominated by craft resin casting into silicone moulds, which uses the same resins as composites without the fibre, and the 3 questions below are the ones casters ask most.

Can I use silicone spray as a mold release?#

Yes, silicone sprays release cured resin, but the silicone film transfers to the part and blocks later painting and bonding, and silicone products sold in the EU have had to stay below 0.1 % of D4, D5 and D6 since 6 June 2026. Where silicone release fits and where it does not is covered under silicone release agents.

Do you need a release agent for silicone moulds?#

Usually not: a silicone mould is already a low-surface-energy surface, which is the property a release agent is there to provide, so the question only arises when silicone is cast against silicone or when a cast resin bonds chemically to it. In those 2 cases the release choice follows the resin, not the mould.

What can I use instead of resin mould release spray?#

Our source library holds no data on petroleum jelly, aerosol lubricants or cooking sprays as release agents, so this page gives no verdict on them; an industrial release is chosen against a declared composition, a food-contact position and a cyclosiloxane declaration, and a household product supplies none of the three. The household question is answered on the parent page for mold release agents for plastics.

Release in polyurethane, RIM and thermoplastic injection moulding#

Polyurethane and RIM moulding release mainly from the tool, with wax and silicone emulsions, while thermoplastic injection moulding uses both an internal additive and an external spray. PU foam and RIM release is covered under mold release agents for polyurethane.

The thermoplastic route, from PETS in polycarbonate to stearates in polystyrene, is covered under mold release agents for injection molding.

Release agents outside plastics: rubber, concrete and plaster moulds#

Release agents for rubber, concrete, plaster, die casting and food moulds share the name but not the chemistry problem, and this reference, which covers additives for plastics only, does not cover them. The 5 excluded uses are listed below.

• Rubber moulding
• Concrete formwork
• Plaster moulds
• Die casting
• Bakery and food moulds