Nozzle VS Drooling/Stringing effects

[Wiseco]

I'm working on a hot runner prototype for our injection-blow molding process and now I must find a way to make nozzles that will not drool or at least, that will reduce it. And we cannot use shutoff nozzle.

With my experience, I saw that if the nozzle have an angle gate, it tend to reduce drooling. But why? I know that the melt flows from the inside to the outside like rolling but how can an angle gate make less drooling?

Here is some design of cavity nozzle to figure what I'm talking about. Nozzle have been split in two for a better view.

[Wiseco]

Also, who have experience bad effects using pullback motion of the screw to reduce drooling? For us, it makes bubbles in thw polystyrene melt... which is wierd I think.

[Wiseco]

So nobody can explain it to me???... or nobody knows?

[HuFlungDung]

I have no experience in this area, but, is it out of the question to make a spring plunger check valve inside the nozzle?

[Wiseco]

No because I already have something in the manifold that take the space for a mecanism like that.

I know that a reverse taper does the job, but I want to know why.

[One of Many]

I have had some experience with glue nozzles that work somewhat similar to your description. The reverse taper I suspect is in effect reducing the amount of surface the pressure coming into the nozzle will act upon.

Activating the nozzle shaft to open it is done with air pressure on a piston at the rear of the assembly, which must overcome a spring that closes off the nozzle(pulling a ball or cone into the reverse taper seat) . There is also a diaphram within the fluid cavity connected to the nozzle shaft which provides the suck back feature. Depending on the fluid viscosity, there is some adjustment to the piston/diaphram/nozzle valve stroke. The pressure and volume in the fluid cavity verses the ability of the nozzle to cut off flow and drool takes some trial and error. We use Ketone based adhesives in a pressure pot that feeds the nozzles. There are many factors that can change the flow due to drag on long hose runs, glue age, temp, thickness and slight air pressure fluctuations. This in turn can also influence how well the valve can close off quickly since that (in this instance) is set by simple spring return working against fluid pressure to hold it open.

Converting this application to hot plastic may be very similar in operation with different materials for temp and flow characteristics. Norgen or something like that makes units for hot glue applications, so it cannot be beyond impossible if willing to R&D the works.


DC