extrusion - info please!! 1

[Elastomatt]

Does the method of curing extrusion (microwave, hot air or salt bath) have an affect on the quality of joint?

I have the option of shot joining and Normec film (Normec preferred).

Also does anybody know any good resources about extruding that I can get hold of easily?

I get the general principles, but would like to know a bit more about the science of the materials and the importance of the mechanics (screw speeds, temperatures etc) and how they can affect the stability of the process.

Also die design if possible.......at present most dies here are single plate and somewhat crude (in my opinion) for the profiles produced.

Thanks

Elastomatt

 

[GrahamBennett]

Sorry I can't help with your first question.

The mechanics of extrusion are probably the most important parameters for dimensional stability followed by variations in Mooney viscosity, which should be kept to an absolute minimum (certainly no more than ±4 or 5 mooney units).

Feed strip dimensions (width and thickness) should be as near constant as possible because variations in these will affect the head pressure, in turn this affects dimensional stability.

Screw design is important. If you are extruding EPDM or butyl-based compounds then a plain screw will be satisfactory. If the polymer is more nervy (NR, Neoprene, etc) then either a barrier or pin type screw is needed. Screw speeds should be about 90-95% of the rated maximum for your extruders - the limit is determined by the extrudate temperature, which should be at about 90 - 100°C.

Machine set temperatures affect output rates more than anything else. The feed pocket/zone 1 temperature should be around 60°C, zone 2 around 80°C, head/die at around 95°C and the screw set to around 80°C. These temperatures will vary according to the polymer being used so you need to conduct a series of experiments to find the optimum set up for your extruder line.

Take off speed must be very slightly faster (no more than +2%) than extrusion speed to minimise stretching prior to cure. Make sure the speeds are constant because any variation will cause dimensional instability.

Dies are usually cut from one sheet of metal only so what you have is not unusual. The complexities come in when you are extruding hollow sections such as weatherstrips, door seals, etc. These complex designs are cut into the torpedo that resides at the end of the screw and behind the die. If you are extruding multiple strands of rope for example, make sure you have the cavities centred on the same circumference to ensure each extrudate is travelling at the same speed as they leave the die.

Polymers and some ingredients from different sources will give compounds with varying degrees of die swell so this must be taken into account when designing dies. It would probably be best to cut a die for each combination for future efficiency of operation.

For anything else I would contact the manufacturer of your extruder and talk to its technical people.

 

[GrahamBennett]

Thank you for the star Elastomatt. Much appreciated.

 

[atreyu915]

GrahamBennett,
I am interested in what you are calling the "torpedo", is this a type of breaker? What purpose does it serve for complex profiles?
Also, centering multiple cavities on the same circumference in my experience has still led to the outer cavities moving slower, what can be done to improve this?

Any help is greatly appreciated


Thanks

 

[GrahamBennett]

The torpedo sits between the breaker plate and the die, and protrudes into/through the cavity. It carries the "internal" shapes of the profiles. In it's simplest form, it looks like a torpedo, hence its name.

To increase the speed of flow in multiple cavities you need to work on the reverse of the die plate. This might involve simply changing the shape of the rear of the cavity by chamfering the edge or, more complex, by adding baffles to change the speed and direction of flow across the back of the die.