Having spent a number of years inspecting worn screw and barrels, I know from experience that people go well beyond ‘a few thou’ of wear before deciding it’s time for a replacement or refurb. A good setter can often work with little more than a stick rattling around in an oversized tube, but there is always a price to pay……
How it’s supposed to work and what happens when it doesn’t.
Plasticising components are designed to melt polymers through the process of frictional shear, heater bands only really being needed to get things started and top up the thermal energy needed to get a ‘good melt’. If material is free to pass over the top of the screw’s flights, (this happens more readily with less viscous materials such as un-filled nylon), then one or more (or all) of the below will happen.
- You are injecting or extruding material that is not homogenous, for example, cold slugs of material that can cause surface defects and component weakness.
- In the case of injection moulding, cycle times are progressively extended as a result of having to increase material residence time or a prolonged screw recovery phase. In continuous processes such as extrusion, there may be a reduced material throughput.
- Screw R.P.M. is increasing to compensate for the above, leading to increased power consumption and wear of pumps, screw motors etc., as well as the possibility of ‘burning’ the material.
- Heater bands have to supply more of the required thermal energy, leading to higher power consumption and reduced service life.
How quickly should plasticising components deteriorate?
The most common combination of plasticising components found on modern equipment involves a bi-metallic barrel and nitrided screw. Nitriding produces a very hard but thin skin of material that is initially highly resistant to wear, but once breached, exposes relatively soft steel.
A bi-metallic barrel has a much thicker lining of abrasion resistant material that wears more evenly, typically lasting up to three times longer than the `semi-sacrificial’ (and cheaper) screw. Screws can also be bi-metallic, in that they can have their flights `hard faced’ with materials such as Stellite 12, (mainly used for injection moulding), and Colmonoy 56, (used for continuous processes such as extrusion). Smaller diameter screws can also sometimes be `through-hardened’.
The type of process and material processed will have a big impact to both the extent and position of wear. For example, heavily glass filled materials are more abrasive, and because the semi-protective polymer begins to melt part way down the barrel, exposing relatively long glass strands, a `balloon’ of wear can occur half way down the barrel.
What to do when you spot a problem.
Options come down to replacement components (either from the OEM or from a specialist provider), or refurbishment. In making your choice, you should consider the following.
- How long you can afford the machine to be down for, do you have spares on site already?
- The position and extent of the wear? A screw with virtually no flight left, or a barrel worn in the middle rather than the output end will be more difficult to refurbish.
- Do you want to upgrade? For example, the suppliers on the PlastikCity section for new plasticising components can offer wear resistant alternatives, or in some cases, an improved performance. Also, if you refurbish a screw, this is typically done with Stellite or Colmonoy, giving you a `better than original’ wear resistance.
- Your budget? A refurbishment is often the most cost effective option, followed by the supply of replacements from a specialist, then typically the O.E.M. (By some margin). The specialist companies on PlastikCity each have extensive libraries of component drawings, so when you submit details of your machine through the site, at least one of them will probably have a drawing to work from.
In summary, either throw that stick away and get a new one, or make sure it comes back `good or better than new!’