posted October 19, 2004 05:00 AM               

I want to extrude a new grade of PVC into a tube profile.

But I am getting problems that I cannot identify. They could be "fisheye" or "void" or "bubbles" or "sink mark".

How on earth do I tell the difference? And what are the recommended solutions?

posted October 19, 2004 08:43 AM               

A traditional cure for unmelted resin is to increase the screen pack density to increase backpressure. Be careful as this cure can lead to high head pressures very quickly. Make sure your rupture disk is in good shape. Annual replacement is recommended.

The better cure would be to have a screw designed for the resin. Even if you currently use a barrier screw or have a fluted mixer, some soft grades of resin can get over the barriers unmelted.

posted October 19, 2004 08:23 PM               

How then do bubbles and voids look like? Is it that bubbles and voids will appear as a string instead of sparodically? Like the defects I'm experiencing now?

Lowering the screw RPM to remove unmelted resin? I had the impression that increasing the screw RPM will increase frictional heat and promote melting instead. May I ask, Whats the science behind lowering the screw RPM?

I've tried increasing the screen pack density already and it didn't quite work. And purchase of a proper screw will be too costly.

In terms of the orginal pellets. Is there anything I can ask my supplier to do to the formulation? Or to the pelletisation process?

posted October 19, 2004 10:52 PM               

Slowing the RPMs will move the melting completion point towards the throat.

Adding screens slows the output rate, again moving the point of melting completion towards the throat.

Preheating the resin improves melting.

Bubbles like you describe could be from entrianed air (a melting + internal pressure issue). Sometime small bubbles in streams are caused by resin buildups in the die which are degrading and gasing. Therefore try cleaning the die and see if the condition improves.

posted October 22, 2004 05:12 AM               

Thank you so much for the information,
Suddenly it all makes more sense!

posted October 22, 2004 06:24 AM               

If you find that cleaning the die resolves the bubble issue, eventually the bubbles may come back when materials build up and degrade again. This is caused by dead spots in the die.

A simple way to resolve the dead spot problem can be to just fill in the areas where you find degraded material with metal. The burned material tells you where the process has low flow, and those areas are not needed anyhow.

Sometimes, like in the case of spider dies, it is a streamlining issue. 2D FEM flow analysis has been valuable in designing flow channels without stagnation points, and therefore resolving buildups and burning.

posted November 05, 2004 04:50 AM               

This is regarding another grade of tubing that I have.

I've tried everything on the list, I've lowered the screw speed, I've tried changing the temperature profile, preheating, and adding screenpack as well, but still, I'm getting this fisheye problems!

Or maybe it's something else that looks like fisheyes?

Is there anything I can do besides the previous recommendations?

This is causing my production quite a bit of problem!

posted November 06, 2004 09:24 AM               

Examine the defect microscopically to try and determine what is causing it.

Check the screen pack microspocially to see if similar material is getting collected on the screeen pack.

If the screen pack is clean, then the source is after the screen pack, and visa versa.

The possibilities are too numerous to discuss further, so please report back.

posted November 11, 2004 06:49 PM               

Anyway, my extrusion run has failed again!
I have decreased my screw speed so much that the production ability is too uneconomically low.

Another question to pose:
Does the extruded tubing thickness affect the incidence of Fisheye occurance? Will thicker tubing have a higher chance of having the defect?

posted November 11, 2004 08:16 PM               

You need to look at the defect microscopically in order to determine the source.

My take on "Gels";

Extrusion Gels and other Contaminates

Gels and contaminants (referred to as gels in this text) in extrusion can cause disruption in the extrusion process and quality problems. Here we hope to discuss and address the problem by listing sources and cures. Of course Extrusion Technical Services would be happy to help you improve your extrusion process.

Unmelted resin:

Since the early days of extrusion unmelted resin has been the number one cause of extrusion and quality problems. Unmelted resin occurs when the extruder pumps the resin faster than it can melt it. This is a common situation. Unmelted resin can be difficult to test for and to resolve. If your gel problem seems to have the same color as your base resin, then this is most likely the source of your problem. The quick fix is to slow down your extruder until the problem goes away. Sometimes additional or denser screen packs can be added to increase backpressure and improve melting. A better solution is to have a screw designed to melt your material at the desired output rate.

Degraded residue:

This is a common source of gels. Resin that stagnates on process surfaces can polymerizes, cross link, degrade and/or oxidize. The resultant materials can then release from the surface and contaminate the flow. These contaminates can have a huge range of color. They can also have a large range of physical strength from soft to hard and non-flowing. Solutions to this problem involve screen packs, formulation stabilization, equipment cleaning, screw design, adaptor design, die design, and checking for wear and corrosion. Some extruders run a highly stabilized blend through a clean extruder first to coat the surfaces. You can analyze these contaminates for size and strength to see if it came from before the screen pack, or after.

Degraded resin and additives:

It is very possible for resin and additives to degrade in the extrusion process and for the degradation products to form carbon and other compounds. If the gel problem your having involves a lot of brown or black specs which are not sourced from other contaminants, then there are two courses of action; 1) Evaluate the extrusion process to lower the melt temperature and residence time, 2) Evaluate all raw materials for temperature stability. One method of determining temperature stability is to compression mold a plaque of the resin (with compounded in additives) and hold it at high temperature for an extended period (example HDPE at 550F for 15 minutes). If there is a stability problem many dark spots, or charring will occur. Of course be watchful for smoke or fire during this test. Some resins can degrade at high processing temperatures and can cause gassing and foaming. These gasses are usually highly dangerous so please be careful. Vary the time and temperature to see what the system can tolerate. Some systems, like flame retardant ones, can tolerate very little time at temperature. If the formulation shows signs of instability, work with your resin supplier to obtain a more stable formula.

True gels:

A true gel comes from the resin producer. Some resins are more likely to produce gels in the reactor. A true gel is defined as a very high molecular weight version of the same resin, and may also be somewhat cross-linked. The viscosity of a true gel is so high that while the gel will flow with the melt stream, it will not disperse into the melt stream. A true gel will normally have similar optical properties as the base resin, but it will clog screen packs and distort extrusions. True gels also have the unique property of being able to flow through tight mesh screen packs and end up with a particle larger than the screen mesh hole on the other side. Solutions to this problem should always be the responsibility of the resin producer. Find out whether the resin you are using is subject to large gel concentrations when produced. One way to check for gels is UV light as many gels will fluoresce under UV. Other tests involve thin films, various light sources, and counting techniques. A standard test should be developed with the supplier and material should be certified before shipment. Most resins do not have this problem so it is best to look elsewhere for the source of your “Gel” problem. If you are a resin producer or a user who has to deal with a true gel problem yourself, solutions come in the form of “don’t create-em”, “break-em” or “trap-em”. Not creating gels has to do with reactor operation, design, and maintenance. Breaking gels is a proven method of controlling the problem by using an extrusion process that can stress the gels and reduce their size so they are no longer a problem. Recent work with extensional mixing concepts has proven useful to in gel reduction. Trapping works fairly well with gels, and should be used in conjunction with breaking. Trapping involves using filters with depth, like stacking five 325 mesh screens together. While gels can flow through a single screen, multiple screens seems to stop them. If you have even a mild gel problem, the stacked screens can be quickly clogged. When you take the screen out however they will seem completely clean because of the gel’s optics being similar to the base resin.

Foreign contamination:

Wood from pallets, paper, plastic bags, dust, higher temperature resin, pellets stuck to shoes, transfer pipe cross contamination, can all cause problems in extrusion. These contaminants can enter the feed stream at any point. Screen packs can stop a lot of these material from ruining the product or process. When foreign contaminates are suspected examine the extrusion and screen packs microscopically to help in determination of the type and sort. One additional suggestion is to check your screen packs for the possibility of material flowing around the edges.

Un-dispersed additives:

Additives are added to resins in powder or masterbatch pellet form for inclusion in the melt. The additives need to be melt mixed; dispersed and distributed. In production of concentrate master batches, or when using powdered additives directly, the powdered additives often experience high pressure and form agglomerates. These agglomerates are very difficult to break down after they are formed. The results will be white or colored specs in the extrusion. Agglomerates that are visible to the eye are normally not supplied by the additive powder producer, but a screen mesh test of the powder can be performed to rule the additive supplier out as the possible source. If you are purchasing pre-compounded pellet masterbatch it is the responsibility of the masterbatch producer to make sure your masterbatch is agglomerate free (down to a certain size). You, as the extruder, normally can not disperse an agglomerate once it has been produced, otherwise the masterbatch producer would have been able to disperse it. If you are using powdered additives, a dispersion aid (usually a wax, surfactant, or fluoropolymer) finely blended onto the powder will help prevent agglomerates from forming. Also a screw design which keeps pressures low while the resin is solid will help. Additionally new screw mixer designs can help break up agglomerates which have formed.

Un-dispersed modifiers:

Typically materials added to the resin above 5% or so are now considered modifiers and no longer considered additives. Modifiers can range from minerals, to organic, to other polymers and rubbers. It is a mistake to attempt to modify the resin and produce an extruded product at the same time unless you have a specially designed process. Typically modifiers are added in the reactor or in an extrusion compounding step. It is the responsibility of the resin producer or compounder to make sure the resin is free if gels. If you have a modified resin, and seem to have gel problems, contact your resin supplier to see if the resin is subject to improperly dispersed modifiers.

Moisture:

Moisture is a very common source of extrudate defects. Your resin supplier should be contacted to determine the maximum allowable moisture content for your resin. Some resins can be processed with higher moisture as long as the extruder is vented, or vacuum vented. Different resins react differently to the presence of moisture, and to different level of moisture. Some resins will degrade when moisture is present, which generally will result in a low viscosity melt, and will not produce gel-like defects. Other resins will form bubbles that may be elongated or not, and may look like gels.

Air bubble or Void?

Often an extrusion is made where large bubbles are present. If these bubbles are randomly dispersed, then air entrapment in the screw may be the cause. Increasing the back-pressure, slowing the RPMs, or a change in screw design may help. If the air bubbles seem to be generally centered in large cross sections, and semi-continuous then product shrinkage could be at fault. In this case slowing down the product cooling rate should help.

posted November 17, 2004 07:04 PM               

Thank you so much for the information!
It's been something that I've been searching high and low for but could not find!

It's really helpful and I'm trying to get my production staff to understand this as well.

It seems to me that my problem could be that of "True gels" and I will be talking to my supplier again some time soon!