PRACTICAL ASPECTS OF BARRELS WITH INTEGRAL FEED THROATS

I thought I had invented something, when presented with the prospect of replacing a damaged feed throat and barrel, I chose to design a barrel with an integral feed throat and get both for the price of one. Well I wasn't the first to think of this, there are O.E.M.s who do just that, Wayne Extruders for example. What are the practical aspects associated with integrating the feed throat into the barrel? In short there are a number of good reasons to do this.

Most extruders have a separate 'bolt on feed throat'. At first this appears to make a lot of sense:

- easy to swap out feed throat if worn or damaged

- easy to exchange different feed throat geometries, such as grooved feed

- water cooling jacket can be installed directly into the feed throat section

Sounds like the right choice! However there is a hidden serious engineering flaw in this design.

Barrels and screws are manufactured with very tight tolerances. A small extruder barrel may have bore tolerances of plus a thousands, minus nothing and the matching screw, minus 2 thousands below the nominal barrel size. One might assume then that these dimensions would also be applied to the feed throat bore. Designing a larger bore in the feed throat would introduce a radial step that would inhibit pellet conveying into zone one!

The problem however is one of alignment between the barrel and feed throat. It now becomes almost impossible to mate the feed throat to the barrel without some concentric or axial misalignment, and this leads to an interference fit between the feed throat and the screw. This causes "feed throat galling". Add to this the use of soft metals for the feed throat bore and you have a recipe tailor made for feed throat galling. Ever seen this?

Some extruder manufacturers have tried to get around galling by opening up the bore in the feed section, but in doing so have they impaired the solids conveying ability, due to the radial step that now exists at the entry to Zone one? Ever experienced poor feeding in your extruder? Axial grooves are fine, but radial steps or radial grooves in the feed section are a no no.

The only way to maintain accurate alignment of the barrel and feed throat is to manufacture them as an integral unit; essentially a long barrel with the feed throat port machined straight in.

What are the benefits of an integral barrel / feed throat design?

The downside? There aren't really any major downsides, you have to get used to the idea of swapping out a whole barrel when you want to change feed throat geometries or replace a damaged feed throat.