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.
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.
