Polypropylene Welding 1

[nlj]

We currently have a rectangular shaped water tank made of polypropylene. The tank is a large tank designed to hold 3,000 and 4,500 gallons of water. The majority of the tank welds are welded using extrusion welds.

We are looking to redesign the tank and the new tank would have more complex weld joints (v-groove, double v-groove, lap welds). We don't fully understand why the current welds meets our needs, but they do. I'm trying to understand what occurs when you weld PP and how the strength of the weld will change with the new weld configurations in efforts to predict that the new design will meet our requirements.

I have taken some cross sections of the current weld design and am unable to see any effect of the weld on the parent material, like you would see in a metallographic sample. Are there other ways to evaluate polymer welds?

Any information on welding PP and how to design proper joints would be appreciated!

Thanks in advance for your assistance.

 

[Demon3]

Polymers get their strength from entanglements between polymer chains and from the crystals that act as entanglements. If your hair is too short you will not get any tangles, the same for polymer chains. Short polymer chains (low molecular weight) cannot tangle so they have poor strength.

When you weld a polymer you need to heat the polymer long enough and hot enough so that the polymer chains can flow together and re-entangle in the new configuration. Full strength will not be achieved quickly even if the weld looks fine because polymer chains move slowly (viscous). Heating more will help to a degree but too much heating will shorten the polymer chains and lessen the strength.

Filled polymers tend to have problems at the join because the filler forms weld lines with low strength. Therefore, I would expect an unfilled polymer to have lower strength at the weld, far lower than for the rest of the part (up to 80% lower strength for example). So, use unfilled polymer if you can.


Chris DeArmitt

 

[nlj]

Thanks for the response Chris. We actually have a current tank in which the welds are sufficient but I'm concerned that if the joint geometry is changed the welding procedure may not be sufficient. I was wondering if there were specific methods to test polymer weld joint design. Is it similar to metal weld joint design? I am planning to pull tensile samples on the current weld design and potentially do the same for the new design.

I've attached a hand sketched drawing of the current tank and the new proposed design. Is there any information out there that suggests one weld joint is better than another for polymer weld design? Sorry the drawings are so crude.

 

[Demon3]

I think your plan of tensile testing is a good one.


Chris DeArmitt

 

[nlj]

Thanks for the suggestion rotadesign. I'm not familiar with the process, but perhaps it would work. I'll take a look at the link you provided. I'm sure this website will have all of the answers, but are there large tooling costs for rotational molding?

 

[patprimmer]

Rotational moulding tools are relatively cheap due to the low pressures involved, but cycle times are long, distribution of wall thickness is difficult to control accurately and mechanical properties are substantially lower than the same polymer processed by other methods.

The shapes obtainable are also limited to some degree, but the material strength should still be stronger than a simple weld of PP sheet.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers