Okay, what have you considered so far?

How do you plan to add anything to HDPE?

I'm curious. Why have you chosen HDPE? Why do you need to increase the tensile strength of the material? What do you envisage happening if you overload your structure?

A.

Talk to a supplier of prototypes. Common material type available is a type of glass filled nylon.
Polypropylene type also available.

Politicians like to panic, they need activity. It is their substitute for achievement.

well we thought about talc but it turned out that they need a polymer and talc isn't so we don't know what to do

If we add anything it will be during melting it in order to mix them together

after researches it turned out that hdpe is a good material that can hold load, waterproof and doesn't react with a lot of chemicals , the minimum used materials should be 2 and they both should be polymers so we must add something to hdpe and if we must add something to it better be something that increases the strength of it in order to carry the load, most likely if we overload it will break since HDPE is sensitive to stress cracking

ABS is three polymers.

Politicians like to panic, they need activity. It is their substitute for achievement.

yes but will it increase the tensile strength or it will be affected ?

Have you worked out how much additional strength you need to add?

Do you know whether you're in a load regime where HDPE stress cracking is actually an issue?

Are you happy that you don't need to worry about stiffness (that your plastic will break before it buckles)?

Is mixing everything together to make a moulding material an absolute given, or are you allowed to laminate your structure?

If you need tensile strength, you won't find anything with more of it than gel-spun HMPE fibre. I've seen it laminated into an LDPE matrix, though that takes some pretty delicate temperature control.

A.