During a discussion with his fellow employees about screw tips, this blog reader was asked and forwarded this question to me…
We were discussing the different types of screw tips: sliding ring (check ring), poppet valve, ball valve and smear tip. I was asked, under what circumstances would you use each type and I feel I could not give a good answer. I was wondering if you maybe able clarify this for me?
When discussing non-return screw tips in general, there are two competing aspects: backflow vs. material stagnation.
In general, tips with high backflow are not very well suited for high precision molding or materials with very low viscosity. Adversely, tips which cause material stagnation have dead-spots where material can become caught and remain indefinitely. Material stagnation is avoided when processing heat sensitive materials or products which require frequent material or color changes.
Smear Tips – High Backflow & Low Stagnation – These screws are ideal for high viscosity, heat sensitive materials such as PVC.
Ball/Poppet Valves – Low Backflow & High Stagnation – These tips are ideal for low viscosity materials which are not heat sensitive such as polyolefins.
Check Ring – Moderate Backflow & Moderate Stagnation – These tips are the most widely used non-return valve because the versatility of applications which can process using check rings.
Keep in mind, the discussion above focuses on the general uses of these screw tips. There are a wide array of screw tip designs for each of the general categories above. I have seen some check ring designs which have very little backflow, and some ball valves which have very little stagnation.