Feed Screw Designing
Science Based? Black Art? Or A Bunch of Hype?
Feed Screw Designs Simplified.
You have probably heard all of the trade names and types.
Pular® , EfficientTM, VBET, StrataMeltTM, DM-2, FusionTM, Barr 3, MultiMeltTM, Z-Mixer, Eagle, ET, DSB®, Bald Eagle, DSBM-TTM, EasyMeltTM, V-Mixer, FusionTM II, Fluxion Mixer, Steward, Pulsar II, V-Mixer, MeltPro Barrier, Maddock Mixer, Spiral Mixer, Mixing Pins, Blister, Notches, Double Flighted, Variable Lead, Metering Screw, Mixing Screw, Single Stage, Two
Stage, Vented, Zero Compression, Constant Flight Depth, Constantly Varying Flight Depth, Pineapple Mixer, Slotted Mixer and on and on.
In answer to the questions above there probably is some truth in all three. While many of the screw designs above are very good and made by excellent companies, many times these designs are pushed as a cure all for ALL of ones processing problems. Only to have a disappointed processor who has spent big money for small improvement,
no improvement or worst off than he was to begin with.
The truth of the matter is new feed screws should be designed for a specific application, process, machine, materials and product. While many of the designs above are excellent for specific applications, many are
not very forgiving should any of the operating parameters be different or changed from what the original screw designer based his assumptions on. Plus many times the screw design is not the problem. Screw, barrel and feed throat wear can be the problem. So these should be measured first.
The feed screw has four important jobs, feed, melt, mix and pump. Just a few things that can affect a feed screw performance.
- Feed length
- Feed Screw Depths
- Screw root finish
- Barrel surface finish
- Barrel temperature profile
- Feed section cooling
- Screw coring
- Pellet geometry
- Resin coefficient of friction
- Resin temperature at feed port
- Grooved or smooth feed throat
- and of course the screw design
The material being processed is being pushed forward, dragged against the inside of the barrel and is sliding and slipping on the screw root. If the machine is producing a well mixed material, at a good rate of production
and the material hasn't been damaged ex. burned etc. you are home free. You have the feed screw you need at least until it wears out. If not you probably have one of these problems.
- Not enough output
- Un-melted material
- Damaged material and/or product (Melt temperature too high)
For low output a hi-performance screw can be used, typically a barrier design. Un-melted material probably requires some sort of mixing section. Basically, there are two types.
1. Distributive mixing. Think of this as turbulent mixing. Such as a blender, mixing melted and un-melted material together as well as other additives or colors. This is obtained by material flow disruption and material
2. Dispersive mixing. Think of this as targeted mixing. Shearing the material to eliminate gels and un-melted material. To give added homogenization and break down the particle size of any fillers. Then evenly
distribute this material throughout the fluid stream.
Feed screw processing problems related some mixing sections to be aware of.
- Excessive pressure drop
- Material burning, hang-ups or dead spots
- Problems with cleaning it with a purging compound without removing it
- Reduction in pumping action
- Delayed residence time
So the decision to buy a high priced expensive name brand or a generic feed screw, depends on the job at hand and how much money one has to spend and how much you trust the sales pitch in the fancy brochure.