IEC 60601-1 (Cl.15.3.6) Mould Stress Relief Test

[SligoRover]

Hi All,

IEC 60601-1:2012 requires Mould Stress Relief testing to be performed on plastic enclosures and references Cl. 15.3.6 of 60601-1:2005. This clause appears to specify that the part be subjected to a minimum temp of 70C x 7 hrs. The primary question I have is, is it possible to avoid testing at 70C and instead perform the test at 50C -but for a longer duration (referencing an appropriate Aarhenius temp acceleration factor) and claim that the 50C test is equivalent to the 70C Mould Stress Relief Test? i.e. is the Aarhenius model valid for Mould Stress Relief Test?

As I understand it the test is a test of the design of the plastic part wrt the residual streses remaining after moulding. I note that only 1 sample is required for the test and that any damage sustained must not constitute an unacceptable risk wrt basic safety and essential performance of the final product.

My secondary question is; There is an alternative to doing this test as the standard states; [I]"Compliance is checked by inspection of the construction and available data where appropriate or by the following test." [/I]....and then it proceeds to list the 70C x7hrs test, (-so based on my reading of the standard anyway it would appear that if there is adequate data available it is not necessary to do the temp test.)

Can someone with previous experience of this scenario please advise what the "inspection" and "available data" referred to above might involve. And which approach most people follow ..the inspection/data approach or the temp test? I'm guessing that it would involve design validation activities, e.g. mold flow simulations, inspection & dimensional analysis for sink, warpage etc. verification of tool construction to incorporate tool design guidelines to minimise residual stresses, etc, etc, but I'm just guessing and I'd really value the experience of someone who has gone down this road already preferably with a medical device incorporating plastic parts.

In terms of background the plastics involved are a range of PP, HDPE, LDPE, TPE and POM parts. The product is a medical device and the Mould Stress Relief test will form part of the design verification testing.

All comments, suggestions and insights welcomed, (my primary question is the main one I want answered the secondary question is a "nice to know".)

Kind regards,


SR

 

[Ronen E]

Hi there Sligo dweller,

is it possible to avoid testing at 70C and instead perform the test at 50C -but for a longer duration (referencing an appropriate Aarhenius temp acceleration factor) and claim that the 50C test is equivalent to the 70C Mould Stress Relief Test? i.e. is the Aarhenius model valid for Mould Stress Relief Test?

(...)

In terms of background the plastics involved are a range of PP, HDPE, LDPE, TPE and POM parts.

Despite what you might read on the Internet, you must not assume that any Arrhenius equation derivative is applicable. Basically, Arrhenius model applies to chemical reactions of order 0 or 1; all other uses are empirical approximations (residual in-moulded stress seldom has anything to do with chemical reactions). Any time-temperature equivalency model has to be reviewed and substantiated for each and every polymer grade, and for the specific situation.

Can someone with previous experience of this scenario please advise what the "inspection" and "available data" referred to above might involve. And which approach most people follow ..the inspection/data approach or the temp test? I'm guessing that it would involve design validation activities, e.g. mold flow simulations, inspection & dimensional analysis for sink, warpage etc. verification of tool construction to incorporate tool design guidelines to minimise residual stresses, etc, etc, but I'm just guessing and I'd really value the experience of someone who has gone down this road already preferably with a medical device incorporating plastic parts.

I haven't gone down this very specific path, but I do have long experience of designing and verifying plastic parts for medical devices. I also don't know which route is followed by more people. In my view, "Inspection of the construction and available data" would begin with a detailed review of the part engineering design within the context of the whole device, understanding its performance / functional requirements and making educated conclusions regarding the risks arising from residual in-moulded stress. This is a multi-faceted analysis that requires some expertise in plastic materials, part design, tool design, the moulding process and the use environment. It may include some or all of the aspects you mentioned, and other supporting tests as applicable (polarized light, chemical stress rating etc.).

Cheers,
Ronen.

 

[SligoRover]

Thanks Ronen, food for thought there

(couldn't find an appropriate smilie but as we're in the middle of a horseburger scandal here in Ireland this one probably is the most apt.

Cheers,

Niall

 

[Peter Selvey]

You might be aware this test has been around for some time as a USA deviation in UL 60601-1:2003, which is used for all NRTL marks (UL, CSA, ETL, TUVs etc).

The test is routinely performed, and I never saw any adverse results for electrical enclosures, likely to be because the materials and processes are well established.

I did once see a surprise result for a part of the enclosure that did not enclose electrical parts. At the time it was not a failure, but under the 3rd edition it would have been because the failure affected critical performance. The result suggests that the test is still important, and not to be taken lightly.

 

[Ronen E]

Polyolefin (PP, PE etc.) parts might react adversely at anything above 55 deg C if under load, be it in-moulded residual stress or functional (assembly) stress.