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Locking fastners in plastic
7

Locking fastners in plastic

Locking fastners in plastic

(OP)
We currently use M6 knurled brass threaded inserts in an injection molded glass filled nylon base to mount a small electric DC motor, similar to types used in electric wheelchairs.

The threaded inserts are fitted using a heating element (basically a modified soldering iron). The problem is that the inserts do not go in correctly most of the time due to variations in the plastic - the plastic is quite thick and has a high glass fill content that sometimes results in rather large voids. This causes the inserts to go in skew or the heat burns the plastic. That, and the fact that the inserts are expensive. The product is very cost sensitive. We manufacture locally and are trying to compete with very cheap products imported from the east.

First prize would be to get away from using the inserts. Similar products mount the motor by placing a nut into the mounting hole from the underside of the base, to be supported by a step in the hole and then the motor is mounted using standard bolts. Unfortunately we do not have that luxury because our mounting holes are blind.

Our fastener supplier has suggested that we try thread cutting or thread forming screws. These, however, come loose over time due to vibration. The fasteners should stay put for at least five years of cyclic intermittent use.

Is there a way to use a thread forming screw in plastic more securely? If the pilot hole is made any smaller the plastic cracks because the high glass fill makes it brittle. We cannot change the plastic composition. We do have an ultrasonic welding machine but prefer not to use it. We have good relations with a company that can manufacture any custom type of fastener for us, so we are not limited to standard parts. Is there another fastener that would be appropriate? It would be beneficial to be able to remove the fasteners for servicing of the product without damaging the plastic base.

Thanks very much for any suggestions!

Mechatronics Engineer from South Africa.

RE: Locking fastners in plastic

Can you not mold around the inserts rather than "soldering" them in after?

RE: Locking fastners in plastic

Several thoughts/questions:

1. Have you investigated insert molding the brass thread inserts during the injection molding process, like ajack1 mentioned? This may be a relatively easy thing to implement without having to develop a new fastener.

2. What thread forming screws have you tried so far? Remform from Conti and PT® or DELTA PT® from Ejot are probably the best for clamp load retention in plastics.

http://www.taptite.net/taptite/remform.asp

http://www.ejot.com/ejot.de/PT®--5957.htm

http://www.ejot.com/ejot.de/DELTA_PT®--5062.htm


3. Have you investigated a self-tapping insert or wire thread insert? Here are some possibilities:

http://www.ejot.com/ejot.de/DELTAsert®_--5064.htm

http://www.spirol.com/library/sub_catalogs/ins-Ser...

http://www.specialisedfasteners.co.uk/cat-03-page-...

http://www.bollhoff-usa.com/ae/us/fasteners/thread...

http://www.industrialbolt.com/downloads/2009/promo...

http://www.afs-idg.com/docs/Smartsert.pdf

RE: Locking fastners in plastic

besides the excellent answers above.
it could be as simple as redesigning/modifying the heat staking punch to prevent the issue in the first place.

RE: Locking fastners in plastic

If you have room, you could make the blind hole deeper, and use a longer thread-forming screw. If you can get more than three diameters of engagement in the plastic, you shouldn't need the inserts at all. ... unless you need to remove and reinsert more than a couple of times, in which case you do need them.

Mike Halloran
Pembroke Pines, FL, USA

RE: Locking fastners in plastic

Your insertion equipment is not suitable. There are heat insertion machine and ultrasonic insertion machines, both which are able to insert parts without the defects you describe. A quality supplier is Spiril:

http://www.spirol.com

RE: Locking fastners in plastic

As CoryPad said; ultrasonic insertion process would be the best option. Overmolding process would produce weld lines near inserts which affect mechanical (vibration) properties of part it selves.

RE: Locking fastners in plastic

I have seen soldering iron insertion work in production, but it is skill intensive, and it takes a while to work the process out.

If you are having trouble with voids in the molded part, they might cause trouble for an ultrasonic inserter also.

One solution better than inserts is to dump the screws and change the base design so that the motor just snaps in place, with secondary locking tabs, perhaps further secured by a ty-rap.

Mike Halloran
Pembroke Pines, FL, USA

RE: Locking fastners in plastic

If you have the correct length of engagement and hole size and are using one of the fasteners developed just for plastic (Delta PT, Remform or Plastite) you should not have loosening in a high glass filled plastic. It just isn't a failure mode that we normally see in plastic. Creep, yes we see that, but not loosening. There are millions of electric power tools out there with Plastites in them and they don't come apart.

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