Blog Posts by Timothy Womer : Plastics Technology

Timothy Womer

Tim Womer is a recognized authority in plastics processing and machinery with a career spanning more than 35 years. He has designed thousands of screws for all types of single-screw plasticating. He now runs his own consulting company, TWWomer & Associates LLC. Contact: (724) 355-3311; tim@twwomer.com; twwomer.com

Screw Rebuilding

By: Timothy Womer 16. May 2012 10:41

Many processors don’t realize that the screw flight diameter clearance has a significant impact on the performance of their screws. You need to periodically pull your screw, measure the flight diameter, and record the information. Engaging in this regular maintenance routine will help production personnel predict when they may need their next shutdown so that they have can their screw repaired to buy an altogether new one.

Typically, it is not recommended to rebuild any size screw more than three times. This is mainly because of the dilution of the base metal that occurs between the hard surface weld material and the 4140 H.T. or carbon steel base metal. Also, geometric dimensions start to deteriorate, such as flight widths, channel depths and channel radius. The geometry of the screw changes because every time that the screw is rebuilt it is either milled or flight ground to clean the over weld on the sides of the flights. Then the root is polished to remove possible root wear or scarring if a foreign object had entered the screw.

If the root of the screw in the metering section is polished deeper, then by deepening the screw channel the pumping or melting capacity of the screw can be impacted . Also, if the screw is rebuilt and the concentricity between the flight outside diameter and the root is not maintained, then the variation of the channel depth could cause the stability of the screw. This is more of an issue with small screws (2.5 in. and smaller) than larger ones.

Also, if a screw is rebuilt too many times not only is the metallurgy of the base material degraded but the narrowing of the flights can cause an even more increase wear due to less flight land width to support the screw.

The best tool when it comes to screw maintenance is to do regular measuring of the flight OD, record the data along with the production rate as a means to predict a preventative maintenance shut-down.

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Tim Womer is a recognized authority in plastics processing and machinery with a career spanning more than 35 years. He has designed thousands of screws for all types of single-screw plasticating. He now runs his own consulting company, TWWomer & Associates LLC. Contact: (724) 355-3311; tim@twwomer.com; twwomer.com.

Solving Splay

By: Timothy Womer 11. May 2012 09:47

Moisture and shear are the two primary causes of splay on an injection molded part. Let’s first discuss moisture, as most of the time splay is due to the fact that the resin has not been dried properly.

Resin drying is a time/temperature element. Most drying specifications for various resins are at temperatures where the resin will not agglomerate, and held there for a given amount to time for which the moisture in the resin will be reduced to a processable level. Splay caused by moisture can typically be easily spotted because it will move around from part to part; it will never be in the same place.

Splay due to shear can come from two sources. One source can be from the shearing action caused by the screw. This is due to high screw speed, or from a mixing section that has too tight of clearance on a mixing section. It can be determined if the splay is due to screw shear by slowing the rpm of the screw. By reducing the screw speed, the shear rate in the screw will be reduced and therefore should eliminate the splay seen in the part.

The other type of shear splay is due to shear at the gate of the mold. This splay will typically always show on the part directly in front of the gate of the part. This is caused by the fact that the gate is improperly sized, and during the injection of the part, the shearing action on the resin occurs due to the high shear rate at the gate area. It can be determined if this is the cause of the shear by reducing the injection speed of the resin into the mold. If by reducing the injection speed and the splay disappears, then the mold needs to be reworked to have the gates modified by either fanning the gate or possible adding additional gates into the part.

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Tim Womer is a recognized authority in plastics processing and machinery with a career spanning more than 35 years. He has designed thousands of screws for all types of single-screw plasticating. He now runs his own consulting company, TWWomer & Associates LLC. Contact: (724) 355-3311; tim@twwomer.com; twwomer.com

Poor Solids Conveying?

By: Timothy Womer 3. May 2012 09:34

Molders quite often call me and say their screw is “windmilling” or isn’t picking up the material. This phenomena is due to poor solids conveying. Solids conveying occurs when the material “sticks” to the barrel and “slips” on the screw. This has to occur in order for the material to be moved forward.

Most often this problem is occurs when processing high-temperature engineering resins. You can solve it simply by changing the temperature profile from the commonly used “flat” profile to a reverse temperature profile, where the barrel temperatures are higher in the rear and lower at the discharge end of the barrel.

Say, for example would be as follows, your profile, starting at the feed zone of the screw: 450F, 450F, 450F, and 450F. Try this instead—500F, 480F, 460F, and 440F—and see what happens.

Some molders believe that running hotter in the rear zones will increase their melt temperature. Not so; higher temperatures in the rear will not affect the melt temperature because the plastic is still in pellet form. But it will improve the coefficient of friction at the barrel wall between the pellets and barrel and therefore improve the solids conveying and eliminate “windmilling.”

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Tim Womer is a recognized authority in plastics processing and machinery with a career spanning more than 35 years. He has designed thousands of screws for all types of single-screw plasticating. He now runs his own consulting company, TWWomer & Associates LLC. Contact: (724) 355-3311; tim@twwomer.com; twwomer.com

Processing Regind

By: Timothy Womer 19. April 2012 14:52

Almost all processes require use of regrind. The amount of regrind consumed by a process depends largely on the process itself. In injection molding, for example, since a relatively minimal amount of scrap is generated in the first place, there isn’t much regrind used. The story is quite different in sheet extrusion-thermorforming operations, where the entire skeleton has to be fed back into the process; or in blow molding, which almost always generates large amounts of regrind.

The particle size of the regrind has a direct effect on the bulk density of the material being blended with the virgin pellets. The percentage of regrind also has an impact on the overall bulk density of the feedstock. Both of these parameters will affect the overall plasticating rate of the screw, whether it be extrusion or molding.

Best practices for reprocessing regrind hold that the overall bulk density be close to the bulk density of the virgin pellet, and that the size of the regrind be approximately the same size as a pellet. Typically, if the screens in the granulator that is being used to produce the regrind are 3/8-in. or 10-mm in diameter, the particle size of the regrind will be close to the size and bulk density of the virgin pellet.

Finally, the other important issue in using regrind in the plasticating process is that the regrind be “dust free.” This requires that the granulator knives are kept sharp, and that a good dust collection system be part of the process. “Fines” in the regrind will cause problems such as black specks in the finish part, loss in rate, and melt blocks. Melt blocks occur because the fines will melt before the virgin pellets and regrind particles, potentially of causing severe after effects.

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Tim Womer is a recognized authority in plastics processing and machinery with a career spanning more than 35 years. He has designed thousands of screws for all types of single-screw plasticating. He now runs his own consulting company, TWWomer & Associates LLC. Contact: (724) 355-3311; tim@twwomer.com; twwomer.com

Powder Down the Feed Throat

By: Timothy Womer 11. April 2012 11:14

In my career there have been a handful of times where a custom compounder using single-screw extruders have asked me to design a screw for mixing large amounts of powdered additives into the virgin base resin. But single screws do not have the capability to convey powder additives in volumes above 20% in a flood-feed condition. Twin screws are built and design for processing large percentages of powder at the feed, and this is typically done in a starve-feed condition and also at very high screw rpms...normally, two and three times faster then single screws.

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Tim Womer is a recognized authority in plastics processing and machinery with a career spanning more than 35 years. He has designed thousands of screws for all types of single-screw plasticating. He now runs his own consulting company, TWWomer & Associates LLC. Contact: (724) 355-3311; tim@twwomer.com; twwomer.com

Processor Tips

Timothy Womer

Screw Rebuilding

Solving Splay

Poor Solids Conveying?

Processing Regind

Powder Down the Feed Throat

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