Classic recycling Question, how many times a material can be used ?

[Sustainability]

Hello All,

Probably many have gone through this question some point in their career.
I still don't have a clear answer though, How many times a material can be recycled without property loss? In this case, my product is non-engineering product, such that non-functional, just a packing box for electronic products, so no worries on time dependent property of the material.
Assuming I take 100kg of material, subjecting it to melting and injecting, how many time I could do the same thing with out losing its properties, without any addition of vigil material. Obviously at one point due to thermal history, the polymer chains will start giving up, but when, after second time or third time or fourth time?
If I am using the runners of the pervious production lot to the material of next lot of production, what is the best ratio? should it be added with another additives?
Cascade recycled material content, such that, Ist lot runner in 2nd lot, and 2nd lot runner in 3 lot, and so on, what ratio and the impact of 1st lot material in the material matrix, how is it impacted.

I will constrain this with few material spec and product performance spec, for example, tensile strength and product shouldn't break when dropped from certain height.

Please don't say it depends on the product & application & material, all I am trying to do here is not to reinvent the wheel,I am sure someone out there had down it.

I greatly appreciate if some one could guide in this. Also if you could refer me to any journal articles, I deeply appreciate it.

Thank you

Regards

[brentb]

Good to cascade! PP and PE can be used almost indefinitely in this manner.

KOM

brent

PS It depends upon application!

[Suhas]

Hello Sustainability,
A common question as you say.
There is no such number nor can one be calculated for a given material since it depends on:
> The product's requirement to sustain load, heat, chemicals, etc, etc. FDA will not allow you any.
> The product's design. If the design is 'right on the edge', the material may fail even with a little regrind. If it has been designed with a substantial factor of safety, you could probably run even 100% regrind.
> The process that was used from where the regrind (sprues, runners, rejected parts) was generated. This is very important. For example, if you are molding a PP at 600 deg F with high injection speeds in a poorly vented mold, you probably are degrading the material right there. Regrinding this product is as you can see worthless. But if you have molded the product at say 450 deg F with decent injection speeds, then you can probably use a 100%.
> If you are using the batch process or the continuous cascading process. For a batch process the above is all important. For the continuous process, you must consider the ratio of the runner to part weights.
With all due respect, if a material supplier tells you you can use 'up to 20% regrind' - not sure what they mean.
Hope this helps.
Regards,
Suhas

[Sustainability]

Hello Sustainability,
A common question as you say.
There is no such number nor can one be calculated for a given material since it depends on:
> The product's requirement to sustain load, heat, chemicals, etc, etc. FDA will not allow you any.
> The product's design. If the design is 'right on the edge', the material may fail even with a little regrind. If it has been designed with a substantial factor of safety, you could probably run even 100% regrind.
> The process that was used from where the regrind (sprues, runners, rejected parts) was generated. This is very important. For example, if you are molding a PP at 600 deg F with high injection speeds in a poorly vented mold, you probably are degrading the material right there. Regrinding this product is as you can see worthless. But if you have molded the product at say 450 deg F with decent injection speeds, then you can probably use a 100%.
> If you are using the batch process or the continuous cascading process. For a batch process the above is all important. For the continuous process, you must consider the ratio of the runner to part weights.
With all due respect, if a material supplier tells you you can use 'up to 20% regrind' - not sure what they mean.
Hope this helps.
Regards,
Suhas

Hello Suhas,

Much appreciated. I was wondering why this issue hasn't found any generic closure yet though its been there for long years. Wish more people could share their experience here.

Thank you

[Suhas]

Hello Sustainability,
This is a grossly misunderstood term and practice in molding. I think we can save a lot of landfills if people used regrind correctly.
Thanks for the question.
Suhas

[jnewmanco1]

Hello All,

How many times a material can be recycled without property loss? Regards

Of course the answer to that question is zero. You cannot mold it one time without losing "some" properties. However, if you cascade it, as was mentioned earlier, you will be basically out of material in 3 to 4 passes so you really don't need to worry about it anyway.

Say you have 100lbs of virgin material for example. You are running a single cavity, cold runner part, where the runner is 20% of your shot weight. Now for the sake of keeping it easy, say there is no startup or production scrap at all and you are running good parts immediately. You run through your 100 pounds of virgin material and end up with 80 pounds of parts to sell and 20 pounds of runners to regrind. Then you take that 20 pounds of regrind and run it through your mold. 100% regrind won't hurt you in your example. Again, you get 80% parts (16 pounds) and 20% runners (4 pounds). Now, once more, run the 4 pounds of regrind through your mold and you get 3.2 lbs of parts and are left with .8 pounds. Depending on your shot weight, you could run this little bit through again or you could just call it good and toss the remaining little bit of regrind in with your next batch of virgin (or the waste can or recycling box). Basically all of your material is now used up and you can start all over again with a fresh batch of virgin and not have to worry about keeping up with any heat histories.
James