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Screw and Barrel Rebuilding Guidelines
These are the Recommended Maximum Wear Clearances between screw and barrel for best output performance:
Screw Size Clearance Clearance Clearance
1" .003-.004” .018” .020”
1-1/2" .005-.008" .020” .025"
2-1/2" .007-.010" .021” .030"
3-1/2" .010-.012" .030” .036"
4-1/2" .010-.013" .030” .039"
6" .013-.016" .039” .048"
8" .016-.019" .048” .057”
10" .018-.021" .054” .063"
Here is a typical payback example using a 4.5” dia x 24:1 barrier screw processing polyolefin:
· Nominal Rate – 1,500 lbs/hr
· Melt Temperature – 420 to 430° F.
· Output Stability - +/- 5%
Effects of diametrical wear factor of .025-.030 may result in a 60 lb/hr output reduction due to wear
· Nominal Rate – 1,440 lbs/hr
· Melt Temperature – 430 to 450° F.
· Output Stability - +/- 10%
Penalty Cost: 60 lbs/hour x 0.65= $ 65.00 /hour x 24 hours = $936.00 per day.
A typical screw rebuild for a 4.5” Dia X 24:1 Barrier Screw with Colmonoy 56 hard surfacing
costs $ 3,642.00 with a 2 week delivery.
Your Return on Investment Payback
is 3.89 days
If you are planning on rebuilding your screws, take advantage of Xaloy's screw rebuilding cell that features fast turnarounds (7– 14 days) on your screws. Streamlined using Lean manufacturing
principals, we can receive incoming screws for inspection, estimate costs, quote and release to the shop with your approval
within 24 hours of receiving the parts.
When to Repair an Extrusion Screw
wear the clearance between the screw flight and barrel wall increases, especially in the high pressure areas of the barrel. For example, a new 2.5” diameter screw will have 0.005” to 0.007" diameter clearance or approximately
0.003” on the radius.
Note: The above recommendation is assuming the barrel is new or has no wear. As barrel wear would have to be added to the screw wear to get the total clearance.
You pay for screw repair whether you repair it or not.
As barrels wear the clearance between the screw flight and barrel wall increases, especially in the high pressure areas of the barrel. Buying a new screw for an worn barrel does not solve the clearance problem. It may make it better, but it is not a fix for a worn barrel.
Since barrels only wear in the high pressure areas or where fusion of the polymer takes place, measuring the end of the barrel ID does not tell the whole story of wear.
A bore gage should be used to measure wear all along the barrel. Keep in mind that the screw wear has to be taken into account with Barrel wear to determine the total clearance.
you don't know what your wear dimensions are, we have the tools and can measure your barrels.
In the injection molding markets it is common to "re-sleeve" the high pressure end of the barrel. The last 20" or so of the barrel is re-sleeved. The screw is reciprocating back and forth.
Why don't we recommend lining barrels in extrusion?
The barrels are longer. Example: 3.5" x 24:1 L/D is approximately 80” long, not including the feed throat. In extrusion, unlike injection molding, the screw only turns one direction. When several sleeves (sleeves 12-15" long) are heat shrunk in the barrel. They do not match up evenly. To hone or grind out the inside diameter of the barrel would take hours and hours to perform, resulting in high cost. The hone will follow the highs and lows of the barrel and not yield a straight bore. All the time, the hone is wearing or reducing the thickness of the bimetallic liner that has been inserted.
Some customers in blown
film applications, have been led to believe they can get at least another 6 months to 1 year out of the barrel. WRONG!
Problems that may pop up later with barrel rebuilds:
The sleeves can shrink and spin.
The sleeves can weld to the screw flights if too tight. (caused by high and lows of honing barrel)
They can have poor heat transfer from heater bands, due to different metals and space between OD of barrel tube and ID of new sleeves.
All of the above is 60-75% of a new barrel cost. Do the right thing and purchase a new barrel!
Plastics Machinery, LLC
141 Cliffe Run
Franklin, TN 37067