Skip to content

Outgassing and Cleanliness Requirements on Invar Space Parts

By Invar Machining editorial team· Published September 26, 2026 · How we write these guides

A machined Invar bracket can be dimensionally perfect and still contaminate a vacuum optical instrument or an electronic assembly if it carries the wrong residue into service. Outgassing, the release of trapped or adsorbed material from a surface once it sits under vacuum, is a surface chemistry problem more than a bulk material one for a dense metal alloy like Invar, which means the machining, cleaning and handling steps after the part leaves the mill matter more than the alloy itself. This guide covers what ASTM E595 and similar outgassing tests actually measure, why the base metal is rarely the real risk, and how cleaning for a vacuum-destined part differs from the packaging and storage question already covered elsewhere on this site.

What outgassing testing actually measures

ASTM E595, the standard most commonly referenced by space programs for screening material outgassing, bakes a small sample of the material under vacuum at an elevated temperature, typically 125 °C, for a set duration, usually 24 hours, and measures two things. Total mass loss, TML, is the percentage of the sample’s original mass that evaporated under those conditions. Collected volatile condensable material, CVCM, is the percentage of that lost mass that condensed on a collector plate held at a cooler reference temperature, representing material that would deposit on a nearby sensitive surface rather than simply venting away. A related method, ASTM E1559, measures outgassing kinetics and condensable species in more detail for programs that need it.

A commonly cited acceptance threshold in the space industry is TML at or below 1.0 percent and CVCM at or below 0.10 percent, values that appear widely in materials outgassing databases used across the industry. This is a general reference point, not a standard we certify against, and it is not necessarily your program’s actual requirement. Confirm the acceptance criteria your specific program uses before assuming a general figure applies.

Term What it measures Commonly cited threshold
TML, total mass loss Percentage of sample mass lost during bakeout under vacuum At or below 1.0 %
CVCM, collected volatile condensable material Percentage of lost mass that condenses on a cooler collector surface At or below 0.10 %
Test temperature Elevated temperature the sample is baked at Typically 125 °C
Test duration Time the sample is held at temperature under vacuum Typically 24 hours

Values in the table are typical figures widely referenced in the industry, not a claim about any specific test result for Invar or for a matched shop’s process. Your program’s test report and acceptance criteria are the numbers that actually govern a given part.

Outgassing testing exists because a vacuum system is a closed, sensitive environment where a small amount of released material has nowhere to go except onto the nearest cold surface. In an optical instrument, that surface is often a mirror or a lens coating, and even a thin condensed film can degrade reflectivity or transmission enough to matter. In an electronic assembly, condensed outgassing products can affect connector contacts or optical sensor windows. The Wikipedia entry on outgassing covers the general phenomenon across materials, well beyond the space-specific test methods discussed here.

Where outgassing sensitivity matters most on Invar hardware

Not every Invar part in a spacecraft needs the same level of outgassing scrutiny. A structural bracket with no direct line of sight to a sensitive optical surface and enough physical separation from it usually carries a lower outgassing risk than a mount sitting inside a sealed optical cavity or directly adjacent to a mirror coating, the kind of application discussed in Invar for optical benches and telescope structures. Programs typically grade their outgassing requirements by proximity and exposure to sensitive surfaces rather than applying one blanket cleanliness standard to every part on the bus, which is worth understanding before over-specifying a costly cleaning and verification process on a part that does not need it.

Why the base metal is rarely the outgassing problem

Invar 36, Super Invar, Kovar and Alloy 42 are all dense, solid metal alloys without the porosity, plasticizers or volatile binders that drive outgassing in polymers, adhesives, potting compounds and some composite materials. A clean, bare metal Invar part has inherently low outgassing on its own. The practical risk on a machined part comes almost entirely from what ends up on its surface during and after machining: cutting fluid residue, handling oils from fixturing and inspection, airborne particulate picked up during storage, or a coating or surface treatment that was not itself qualified for vacuum service. Our guide to surface finish and coating options for machined Invar covers which finishes suit vacuum use and which do not, since a coating chosen only for corrosion resistance in an atmospheric environment can be a meaningful outgassing source in a vacuum system.

Cleaning and handling practice after machining

A part destined for a vacuum optical or electronic system typically goes through cleaning that a general commercial Invar part does not:

  • Solvent cleaning to remove cutting fluid, oils and handling residue, using a solvent and process compatible with the part’s finish and with your program’s approved materials list.
  • A bake-out step, holding the part at an elevated temperature under vacuum or dry nitrogen purge, to drive off residual volatiles before the part enters service, distinct from the stress relief bake covered in stress relief for Invar machined parts, which serves a mechanical purpose rather than a cleanliness one.
  • Controlled handling after cleaning, typically gloved handling and controlled storage, to avoid reintroducing the contamination the cleaning step just removed.
  • Witness samples, a coupon cleaned alongside the actual parts using the identical process, sent to a test lab for outgassing verification when the program requires documented data rather than a general cleaning claim.

None of this is something invarmachining.com performs directly. We match your RFQ with a shop, and the shop or a separate accredited test lab handles the actual cleaning, bake-out and outgassing verification according to your program’s specification.

How this differs from packaging and storage

Outgassing cleanliness and shipping packaging answer two different questions, and it is worth keeping them separate on an RFQ. Packaging and storage, covered in shipping and storing Invar parts without losing stability, protect a part from physical damage, corrosion and dimensional shift during transit and inventory. Outgassing cleaning is a surface chemistry requirement that has to be satisfied before the part is even packaged, since packaging materials themselves, foam, tape adhesive, some plastic bags, can recontaminate a cleaned part if they are not also selected with outgassing in mind. A part can be packaged perfectly for safe shipping and still fail an outgassing requirement if it was packaged in ordinary commercial materials after cleaning rather than low-outgassing packaging appropriate to its cleanliness class.

A hypothetical worked example

Take a hypothetical Invar 36 mounting bracket that sits inside a sealed optical instrument enclosure, close enough to a mirror surface that condensable outgassing products would be a direct contamination risk. The program specifies a cleanliness requirement referencing the commonly cited TML at or below 1.0 percent and CVCM at or below 0.10 percent thresholds, to be demonstrated on a witness sample cleaned with the same process as the flight bracket.

The shop machines the bracket, then cleans it with a program-approved solvent process and bakes both the bracket and a witness coupon at an elevated temperature under vacuum. The witness coupon goes to an accredited test lab for ASTM E595 testing, and the flight bracket is packaged in low-outgassing materials immediately after its own bake-out, rather than standard commercial packaging, to avoid recontamination before final assembly. This sequence, clean, bake, verify on a witness sample, package appropriately, is illustrative of how the pieces fit together, and a real program’s specific test and packaging requirements govern the actual process.

What to put on the RFQ

  • Whether the part requires documented outgassing data, and against which acceptance criteria, rather than a general cleaning claim
  • The cleanliness classification or handling protocol your program requires, if one is specified
  • Whether a witness sample cleaned alongside the parts is needed for test lab submission
  • Whether any coating or surface treatment on the part has itself been evaluated for outgassing, referenced against surface finish and coating options
  • Your program’s packaging requirement for cleaned, vacuum-destined parts, separate from general shipping protection

Getting matched

Send your drawing through our RFQ form with your outgassing and cleanliness requirements noted, and we match you with up to three shops that can clean, bake and coordinate witness sample testing to your specification. For the broader vacuum and thermal environment these parts often serve, see Invar for cryogenic and vacuum applications, and for material traceability and documentation expectations on a flight program more generally, see qualifying an Invar supplier for satellite structures. Back to the space and satellite hardware overview for the full picture.

Frequently asked questions

What does ASTM E595 actually test?

It bakes a small material sample under vacuum at an elevated temperature, typically 125 °C, for a set duration, and measures how much mass the sample loses (total mass loss, TML) and how much of that lost mass condenses on a cooler collector plate (collected volatile condensable material, CVCM). It screens materials and processes for how much they will contaminate a vacuum system in service, not the finished dimensional or mechanical performance of the part.

Does Invar itself have an outgassing problem?

Not inherently. Invar is a solid, dense metal alloy without the porosity or volatile additives that make some polymers, adhesives and composites outgas heavily. The outgassing risk on a machined Invar part almost always comes from surface contamination, cutting fluid residue, handling oils or an applied coating, rather than the base metal itself.

Does invarmachining.com or a matched shop perform ASTM E595 testing or certify outgassing compliance?

No. We are a matching service, not a test lab. ASTM E595 and similar outgassing tests are normally run by an accredited materials test lab, sometimes on a witness sample from the same cleaning and finishing process as your parts. Ask the shop whether it can supply witness samples or coordinate with a test lab, and confirm your program's actual acceptance criteria directly rather than assuming a shop's general cleaning process meets a specific outgassing spec.

Is outgassing cleaning the same as the packaging and handling covered elsewhere on this site?

No, they answer different questions. Packaging and handling protect a part from physical damage and corrosion during shipping and storage, covered in our packaging and storage guide. Outgassing cleaning removes or limits contamination that would release under vacuum once the part is in service, which is a surface chemistry requirement that applies before the part is even packaged.

What cleanliness level should I specify for an Invar bracket that touches an optical surface indirectly?

It depends on how close the part sits to the sensitive optical or thermal control surface and how much margin your program's contamination budget allows. A part with no direct line of sight to an optic and a lower outgassing sensitivity nearby may need only a documented solvent clean and bake-out, while a part inside a sealed optical cavity may need a formal cleanliness classification and verified outgassing data. State the requirement rather than leaving it to the shop to guess.

Reviewed against

Values in this guide are typical handbook figures for planning. The mill certificate for your heat lot and the shop's quote for your geometry govern.

Need a quote for this part?

Send the drawing. We match you with up to 3 shops that machine Invar and reply within 24 business hours.

Get matched with an Invar shop

Keep reading

All process guides