Best method for welding of large Polypropylene part

[StevenMorrow]

Good day,

I have been tasked with determining how best to make a shallow version of one of our existing products (injection moulded in UV-stabilised, impact modified PP, L x B = 1m by 1.2m). I think that the best way to do this would be to cut out the middle section and to then weld the top and base pieces together.

I have searched the Eng-tips forums and also Googled fairly extensively and have encountered some conflicting information. I have narrowed it down to laser, hot plate or linear vibration welding.

(I doubt it is possible, but could the same laser be used for both the cutting and welding operations?)

Requirements:
1) A neat and strong join (or will some form of finishing be required?)
2) Reasonable equipment costs
3) Short cycle times (if possible, as we are looking at producing 10,000 per annum)

Any suggestions as to which method is most suited to this application?

I would appreciate any information that you can provide. Let me know if any other information is required.

Thanks and regards

Steven Morrow

 

[patprimmer]

It will be difficult.

I am not a welding specialist, but PP can be welded with a hot air gun and hand held filler rod just like oxy welding steel except there is no colour change with the heat.

I guess a laser could be used to supply the heat.

I guess the same laser could be used if there is enough control of the heat.

I guess you could automate it with a robot or a device to accurately control the speed and position of the laser and the rod, modulating supply of both as required.

Some out of left field problems I would expect are:-

Getting a suitable filler rod.

The part may warp even before you weld it as the cut may relieve frozen in strain.

The heat of welding might induce stress and the joint may move before it is fully welds. Strategically placed spot welds might control this.

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

http://eng-tips.com/market.cfm

for site rules

 

[MikeHalloran]

At 10,000 parts per year, you can't justify an injection mold of its own?



Mike Halloran
Pembroke Pines, FL, USA

 

[StevenMorrow]

Hi Pat -

I do agree that it will a challenge to achieve an accurate and strong weld. At this stage, I am trying to determine the most suitable method.

Mike - we have already received quotations for a mould. Personally, I would prefer to use a purpose-built mould, but I have been given the task of figuring out the costs involved with alternative methods.

 

[patprimmer]

The cost per part of doing something that does not work is infinite on a per part basis.

I guess part of what I was trying to say above is that PP is one of the most unstable plastics there is re dimensions. Some people call it polywarpolene.

I am sure hot air welding equipment is available commercially and is used to repair car bumpers among other things.

I would be surprised if you got 10,000 second rate parts out for anywhere near the cost of a mould and two 5000 off production runs or even four 2500 off production runs if storage is an issue.

If you already have a mould for a pattern, a good Chinese mould maker should be the cheapest due largly to their low cost of steel.

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

http://eng-tips.com/market.cfm

for site rules

 

[Demon3]

At those volumes can't you save by using an aluminium mold?

P.S. not an expert in molds

Chris DeArmitt PhD FRSC CChem

 

[StevenMorrow]

Hi Pat and Chris,

I have managed to find some quotes that were prepared last year for producing this mould (two EU and two Chinese). We are seeing if we can create something with a decent finish for less using welding techniques. I am also leaning towards a new mould but I like I said, I have been asked to come up with a proposal for an alternative.