Information required after injection molding.

[ShaneDoe]

I have some limited knowledge here so please feel free to que me in. Due to the tools only being able to be so complex and how complex inner cavities can get in plastic parts I often see plastics molding by the halves.

I am looking at having a part made that needs to be air tight and was wondering what is the best way to put these halves back together. Is there a process that can combine these in such a way that would provide me with an air tight single piece with a complex inner cavities (over molds and inserts etc.)

 

[MikeHalloran]

It depends on the plastic, the differential pressure, the direction of the differential pressure, the stresses not associated with pressure, the environment in which the part lives, the production quantity, the production rate, the environment in which the factory lives, and to some extent the skill of the experienced plastic part designer whom I suggest you rent for a while.


Mike Halloran
Pembroke Pines, FL, USA

 

[ShaneDoe]

Thanks for the If's. And while I agree I will end up hiring out for specific specialized jobs I still like to do my research.

If there was a way of putting these halves together outside of a gasket and tightening screws I would like to know about it before I dive into setting in a specific design etc. Just doing my homework to see how plausible and feesable things are before I start dumping money into people where the end goal might not even be possible.

 

[Compositepro]

Ultrasonic welding is common method of joining and sealing plastic parts.

 

[ShaneDoe]

Thank you very much Compositepro. This was the information that I was looking for. I will do some more research on this form of sealing plastic parts.

 

[MikeHalloran]

We can help you here, but only to the extent that you are prepared to answer the questions implicit in my prior message. ... and this is a global, public forum, so if you need and don't have patent protection now, you probably shouldn't say any more.

That said, it is possible to join >some< kinds of plastics with solvent bonding. The details of joint design are covered in the design manuals you can get from the resin suppliers.

It is also alleged to be possible to join >some< kinds of plastic, with very specific joint details, by ultrasonic bonding. I personally have been disappointed by this process twice, so I do not trust the ultrasonic equipment vendors, but I think some high volume items use it.

It is also possible to join >some< kinds of plastic, with very specific joint details, by thermal bondng, which amounts to inserting a hot soldering iron between two parts, then withdrawing the iron and mashing the molten surfaces together. Of course, consistency requires pretty decent temperature and timing control, so the process has to be automated, and the tooling is expensive. I think it's almost universal now, or at least very common, for heater/defroster ducts in automobiles.



Mike Halloran
Pembroke Pines, FL, USA

 

[ShaneDoe]

Great information Mike, I will take your warning as I have no current Protections and do not want to disclose any product information rather just gain knowledge on the different types of processes available. From just reading your specific post I can see that I will more than likely be guiding my project around the type of joining process rather than the process around my product. At least that is what I am thinking as of now. Still in the starting stages of what I am doing.

Thanks for that help.

 

[ShaneDoe]

Edit: I did not see an edit option so I posted this, sorry if I missed the option.

Once I got a few answers from you guys I found my way to Plastic Welding.

http://en.wikipedia.org/wiki/Plastic_welding

This covered the basics of the different types of ways to join plastic together. I just needed help coming up with the terminology to help me find my answer. Im putting this here to hopefully help others.

 

[patprimmer]

I know nylon in line fuel filters for cars where solvent bonded with formic acid. Due to OH&S issues and a more consistent process I think they went to spin (friction) welding then for increased production rates and even more control of the process I think they went to ultrasonic.

PVC plumbing fittings are solvent bonded with I think an acetone or MEK based glue.

Polycarbonate surf board fin boxes where solvent bonded with methylene chloride.

Polyethylene plumbing fittings screw together because solvent bonding does not work for PE

Floating yacht winch handles are ultrasonically welded together to complete the box section for strength and to make a water tight compartment. It took some development work to get the details correct, but it now works well even with 45% glass filled nylon.

So, Mike was correct. It depends largely on the materials, expectations and design detail. Also all the projects mentioned above where largely based on the raw material suppliers data.

I have even seen nylon 11 gas pipe fittings solvent welded and PE gas line fittings electrically heated by directing current through wires moulded into the fitting.

Of course for spin welding, the joint has to be round.

Regards
Pat
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