Press nuts on an ABS boss to retain panel

[kjoiner]

Hello,

We are looking at retaining an ABS injection molded faceplate on a small sheet metal drawer. The sheet metal drawer is only .030 Aluminum so snap features aren't really an option and must avoid screws. We have 4 bosses protruding from the faceplate that will pass through holes in the drawer. The two options we are exploring are heat staking the bosses into rivet heads or possibly using press nuts. I've seen many applications for heat staking but don't know much else about the process (e.g. temperature to properly "flow" the plastic without it melting execessively and making a mess) and how to go about obtaining tooling. The 4 bosses are 3/16" OD with a 3/32 core. Press nuts are commonly used on low cost metal shafting (garden cart axles, toys, etc) and seem simpler than heat staking.

My questions are:

Does anyone in this group have some expertise in heat staking they could share with me?

Has anyone used press nuts on ABS? I'm a little concerned about creep or the nut digging into the plastic and subsequent loosening of the faceplate over time.

I apologize in advance for the long winded post but wanted to provide as much information as possible up front.

Thanks,

Kyle

 

[dwightdixon]

Seems like you could go either way and there is also spin welding to do the upset or ultrasonic upsetting. Branson is a ultrasonic machine supplier who has been doing this job for years. Give them a call or other ultrasonic suppliers. Would require you to buy a machine but in the long range would be cheaper. push on bellvile washer type push on nuts would require only a press. Contact supplier of nuts for available designs and performance.

 

[kjoiner]

Dwight,

Thanks for your reply and input. I'm glad to hear you have seen applications for the push nuts on plastic. My main concern is creepage but ABS seems to have good creep resistance. I've done some experimenting with the push nuts and they seem to work well. I've talked with some plastic welding equipment suppliers and have some preliminary ideas on how to heat stake the bosses. They recommend a heat process rather than an ultrasonic process since they can cool the plastic before removing the tool and promote a tighther fit. My plan of action right now is to use the push nuts on the prototypes and pilot runs and if we end up building the main production run, we'll invest in a machine - about $5K with tooling.

Kyle

 

[dwightdixon]

The ultrasonic assembly is faster and therefore less costly but equipment expense is higher. Which is better depends on your volume and machine/operator cost per hour and your availablity of capital. I have also seen spinwelding being the lowest cost for larger bosses. There was an orbital machine developed which also did a great job.

 

[kjoiner]

Dwight,

Our anticipated volume would be about 8,000-10,000 stakes per month. Some suppliers will be providing quotes on the equipment/tooling to perform the stakes using heat and ultrasonic for comparison. Spin welding would probably be less feasible since the space is very tight. With either heat or ultrasonic, we will need to create special tooling with offsets to reach into the drawer.

Kyle

 

[dwightdixon]

This is pretty small volume, so the saving potential is not great and it would take a long time to achieve a pay back breakeven. Better to go with no investment and no risk.

 

[kjoiner]

Dwitght,

I think what we'll do on the pilot run is use the bellville type, toothless press nuts. I've done some mock ups and these types of washers seem to do very well for retaining the faceplate. What I like about them is that they apply even pressure around the boss and grip tightly once installed. With 4 of them in place, I feel that the installation is more or less permanent-which is what we want - and like you mention, there is little tooling involved except a punch or something to press the nuts over the shaft.

Kyle