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PVC Foam Board Production Line: How To Eliminate Die Build-Up

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Introduction

Die build-up is a common problem that is synonymous with the PVC foam board production line process. It is a drawback that is associated with excessive stress at the die exit. When resin flow within the inner surface of the die suddenly changes its speed it triggers stress in the plastic melt. Due to the stress in the melt, polymer fractions with low molecular weight tend to separate. They end up being deposited on the surface of the die exit. If the buildup goes unchecked for long, it can result in blatant product failure. Additionally, it triggers product surface imperfection as well as a costly disruption in the extrusion process. There are various ways to deal with the die build-up that we are going to discuss in this article.

Plastic sheet extrusion die

Effects of die buildup on the PVC foam board production line

Die buildup has a negative impact on the efficiency of the PVC foam board production line. It also impacts the final quality of the extrusion process. Polymer build-up on the surface of the die is associated with defects and cracking in plastic products. That amounts to low-quality products. Other than that, die buildup causes missing extrudate surfaces along with process interruptions and prolonged downtimes.

Plastic sheet extrusion die

How to reduce die build-up

I. Clean the die

It is advisable to maintain the die at least once a year. Standard die cleaning happens through a process called splitting and cleaning where the whole component has to be taken apart. Splitting and cleaning the die helps eradicate any polymer buildup out of the die’s flow surface. It also allows you to inspect the die’s surface for any signs of wear and repair in time.

ii. Apply fluoropolymers

Applying fluoropolymers to die surfaces is one tactic that is commonly used to control die head build-up. Using metallic coatings which include fluoropolymers to the internal surface of the die will also minimize die exit stress.

iii. Increase the length of the die land

Increasing the length of the die land has proven to minimize die head swell as a result of stress. To achieve that, it is advisable to install a larger die opening which helps to lower stress at the die exit. In the same manner, it draws the extrudate size to a more desired proportion.

iv. Change the resin supplier

Every seasoned plastic extruder understands that the die build-up is often different depending on the materials in use. The eventual build-up could be anything from fluffy and thick to running and thin. A build-up that is runny and thin is always a result of utilizing fractions with low MW in the material formulation. Such materials tend to condense on the face of the die or volatilize at the exit of the die. On the flip side, buildups that are fluffy and thick are a result of high stress within the body of the die. Likewise, this sought of build-up can be associated with compatibility issues and partial formation in the plastic melt.  

Additionally, you need to ensure that the resin that is due for the extrusion process does not have an excessive amount of moisture. Along with that, you need to be on the lookout for inadequate venting as well as likely melt fracture.

Likewise, you need to consider the fact that resin quality may be different depending on the supplier. As a result, some suppliers may deliver resins that produce a higher amount of die build-up as compared to others. This may happen even when the resins come with the exact specifications. With that in mind, it might be prudent to opt for the same resin grade but from a different supplier and check out the outcome. If you happen to notice a difference it might be a good tie time to inform the original supplier regarding the resin performance problems encountered in the first place.

v. Check the viscosity

You will also learn that resins from different sources tend to have elongation viscosities that are quite different but a shear viscosity that is similar. In line with that, it is advisable to test the elongation viscosity of resins from different sources in the event that there is no other notable difference between them. In the end, you will realize that resins that happen to have an elongation viscosity that is high tend to experience more build-up problems. That is a drawback that is linked to higher stresses experienced at the exit of the die.

vi. Check the die swell

It turns out that resins that happen to possess high die swell exhibit die build-up at higher rates. On the other hand, resins that possess a molecular weight distribution that is narrow tend to have a die swell that is lower. Nonetheless, that does not imply that such resins have a lesser buildup rate. Still, resins that come with an MWD that is narrow are harder to process. Likewise, they put out low-MW components that trigger die buildup in the long run.

vii. Test the die build-up chemically

Testing the die build-up using a chemical procedure will help you to understand its components. In this manner, it is possible to understand the source of the problem. A good example is reclaimed scrap which often comes with low-MW fractions that come up because of thermal degradation which happens before processing.  A good way to chemically find out components of the die build-up is to utilize additives that reduce chain extenders and lower degradation. If that does not help, you might have to find another resin source.

viii. Add lubricants

The PVC foam board production line should be lubricated regularly. Incorporating more lubricants in the extrusion process may help to deal with the buildup problem. However, you have to be really careful about the amount of lubricant used since too much lubrication can also heighten die buildup. In the same manner, you have to keep a close eye on the chemical compatibility of the roles played by different components in the formulation. If you happen to melt blend polymers that are highly incompatible, the outcome will be a heavy build-up at the die. The ultimate solution in such a scenario is to utilize compatibilizers. In a similar fashion, fluoropolymer processing agents can be incorporated at the die exit in little quantities to minimize stress around the area.

ix. Include antioxidants

Including antioxidants in the material formulation can aid in reducing the rate at which die buildups oxidize to black or brown. Although the trick will not curb the die build-up problem, it will be helpful in making the ugly resin build-up that might be lodged on products more invisible.

x. Adjust the melt temperature

The melt temperature affects the efficiency of the PVC foam board production line. One way to mitigate die-exit stress is by adjusting the melt temperature and optimizing a higher die. Die build-up is often worsened by plastic degradation that comes as a result of a hike in the fractions that are low-MW. In order to rectify the extrusion process, you need to tackle the die temperature and melt temperatures independently. On some occasions, an extremely low-temperature environment within the die can result in a cool resin layer that forms on the die’s inner surface. The resultant resin will gradually creep to the die’s exit consequently separating from the bulk plastic flow and eventually forming a buildup.

Therefore, to solve the die build-up problem, you need to begin by ascertaining the actual melt temperature. When that is done, adjust the temperature of the die body to complement the true melt temperature. In order to find out the true melt temperature, you need to confirm it manually by hand. Traditionally checking it by use of melt thermocouples can be misleading since they are easily corrupted leading to wrong temperature readings. As much as checking the temperature is hard, it is worth the while during the troubleshooting process. In a like manner, the surface on the die’s exit might be relatively cooler in comparison to the die body itself. Thus, check the temperature of the die’s exit using a reliable thermocouple. 

xi. Utilizing air sweeps

Moving on, another alternative that can help to keep the die buildup in check is to utilize air sweeps. Air sweeps are basically wands that are contoured in the shape of the extrusion line. They also feature holes that are strategically drilled to evenly distribute any compressed air. Installing the air sweeps at the exit of the die helps to redirect any condensables and smoke away from the body of the die. In addition to that, the air sweeps aid in cooling any die build-up and prevent it from oxidizing and finally turning dark. The best way to curb oxidation of the buildup is by incorporating nitrogen.  Yet, you have to be careful when blowing nitrogen into the die. Cooling the die on the PVC foam board production line too much or blowing it too hard may potentially distort the extrudate.

Plastic sheet extrusion die

How to disassemble the die head off-line

After placing the die on a working surface, proceed by removing the seal plate and endplate components. You can achieve this by the use of Allen wrenches. Allen wrenches will come in handy when dealing with bolts on the die’s body. While undertaking this procedure, it is vital to remember that the die will be super hot. Thus, your insulated gloves should come in handy. Similarly, you need to understand that every bolt on the die includes a washer that should be handled carefully. Upon removing the washer and bolts, place them on a surface that is clean and heat-proof.

After removing bolts, you will move on to separating the two halves of the die body. To do that, you need a lifting device. Place the lifting device onto the eye bolts while ensuring the lifting chain is tightly drawn in a position that is vertical to allow the upper die body to come up in a straight motion.

Onward, you need an anti-seize agent that you will insert into the holes of the jack-bolt and the outer surface of the jack-bolts themselves. When that is complete, slightly tighten the bolts by hand. Thereafter, get rid of the cover protecting the die’s front. In the cover’s place, insert a plastic shim shock that is high-temperature. You can also utilize aluminum or brass in place of the protective cover. The purpose of substituting the die’s protective cover is to provide adequate protection to the lip surfaces. Lip surfaces tend to brush against each other while the die is being taken apart. 

Separating die bodies

Upon replacing the die’s protective cover, you need to work on the jack bolts. Tighten the jack bolts until the lip gap opens up. While doing that, you need to check of the splitting of the die bodies is even. If one side seems wider you have to apply more pressure on the narrower side for the jack bolts to open up. You will need to readjust the lifting chains periodically since they will become loose as you apply extra pressure to the securing jack bolts and try to separate the die bodies.

Continue evenly turning the bolts while observing the gap between the two die halves making sure that they are even and parallel. Keep on working on the bolts until the upper die body comes off.

Die head cleaning procedure

Before cleaning the die, you need to raise it. Thereafter, find a way to cut off any strands of molten polymer stretching between the two halves of the die body. When that is done, completely separate the two halves of the die body by removing and carefully ejecting the jack bolts that seize the two parts together. After that, take the die’s upper body and lay it down on a surface that is clean and protected before turning it over.

You need to remove the polymer as quickly as you can because the two halves of the die will begin cooling quickly once separated. Basically, it is easier to clean the molten polymer while still hot. Use help if possible. For this part, you will need brass scrappers. Carefully scrape off any polymer starting at the sealing surface all the way to the lip edge front and the flow surface. Take time to work on the die lengthwise, thoroughly scraping off any molten polymer as well.

Moving on, gently apply soap to all scrapped surfaces focusing on one section at a time. The die soap will melt as you apply. To clean away the die soap rapidly, you will need to utilize copper gauze. Be extra cautious at this stage as hot soap is dangerous. Also, the die soap should not penetrate the bolt holes on the threaded body since it is harder to clean. That aside, you have to switch to fresh copper gauze every short while. 

Polishing die head

When the surface of the upper and lower die bodies have been cleaned perfectly and the polymer residue is fully eliminated, use a cotton rag to dry out the surfaces. The cotton rag will also help to eliminate any haze or film that may be left behind. Make sure to utilize abrasive pads while cleaning the die’s seal area. Nonetheless, the die pads should not be used on flow surfaces as the outcome can be damaging.

Once the surfaces have been wiped clean use compressed air to blow out all the die-body holes. Next, remove adjusting bolts on the die-lip after which you will need to reset, lubricate and clean them.

After working on the adjusting bolts, proceed onto the inspection stage. Take a close look at the front edges along with the flow surfaces of the die to find out if they have any dings, dents, or nicks that require repair. That will help to mitigate damage. If the damage is extreme, you might have to get a qualified technician to polish and rework the surfaces.

How to reassemble the die

Ideally, the die is composed of the lower and upper body. Therefore, it is important to first apply a thin lining or coat of a quality anti-seize compound between the lower and upper body. To do that, you need to carefully apply the anti-seize compound on the dowel pins then put the shim stock onto the die-lip lower body’s surface.

You will need an extra handy when lowering the upper part of the die body onto the lower one. While the first guy holds onto the upper body to keep it steady and straight using a straight metal bar, the next one will carefully monitor the positioning of the upper body as it descends onto the dowel pins.

To direct the upper body on the dowel pins of the lower body, utilize a sturdy steel handle. This will help to ensure that the two parts of the die – lower and upper body – are parallel to one another. If the two die bodies are not parallel to one another, it will lead to unequal length. Along with that is a resultant difference in thermal expansion. When that happens, it will be much more difficult to reassemble the two bodies back together. Apart from that, you will waste more time. You will have to wait for the two die bodies to cool down and contact entirely before it becomes possible to reassemble them once again.

Upon lowering the upper body onto the pins on the lower body, you will realize some sought of looseness on the lift chains. You might also observe a gap between the lower and upper die bodies. Any gap or looseness between the upper and lower body could compromise the functionality of the die. Fortunately, there is a remedy to this problem.

How to fix the loose gap

All you have to do to counteract the looseness or gap between the die bodies is to incorporate an anti-seize coating. Apply the anti-seize coat onto the bolts on the two bodies. To be precise, apply it on the thread and head of the bolts without forgetting to apply it on the washers’ sides. Thereafter, proceed by inserting the coated bolts into the die’s center. To tighten them into place, you need to use Allen wrenches. While tightening and squeezing the two die halves together, ensure that the lower and upper bodies are parallel to one another.

Note that you might need more bolts in the event that the die happens to be longer. After the two halves have been locked in a parallel position to one another, insert the rest of the body bolts beginning with the center of the body. Of course, each of the remaining bolts needs to be coated with an anti-seize compound. 

Torquing body bolts

The purpose of torquing the bolt is simply to tighten them. Every extrusion line manufacturer has specific instructions and sequences on how torquing of body bolts should be done. You can always refer to the user manual before getting to work. Nonetheless, the torquing process ought to be conducted once the die has been restored online. Prior to getting on with the procedure, it is vital to cover the lip area with some protective elements.

To begin with, you will need a torque wrench. Apply gentle movements using the torque wrench starting with the four bolts at the center. You must use a crisscross pattern when working the four bolts. When the four bolts are tight enough, proceed to the outer bolts located on either side tightening them firmly until every bolt is in place

To top it all off, install the necessary end plates back in place. Only when you are the two die bodies have been fastened together in a parallel position, the bolts tightened firmly and the plates installed can the die get back to production once more.

FAQs about cleaning the die head for the PVC foam board production line

How do I use the die soap?

It is advisable to clean the PVC foam board production line regularly. Die soaps are composed of abrasives that are ultra-fine. The ultra-fine abrasives effectively scrap any polymer buildup on the surface of the die hence cleaning it in the process. They are also engineered for use under high temperatures of up to 150 degrees Fahrenheit. To utilize the die soap, you need to chop the soap bar into fine pieces. After that, apply it to the inner surface of the die after splitting the component. When the die is closed, you need to put the soap pieces into the hopper. Thereafter, lower the machine’s rpm allowing it to push out the soap. In the course of that, carbon residue and plastic will be eliminated.

What tools are needed to split and clean the die?

The tools you need are mostly utilized for splitting the die body. That includes jack bolts to help in splitting the die as well as Allen wrenches to take apart the die bolts. You also require end-seal gaskets and body seals. Cleaning supplies include heat-resistant gloves, abrasive pads, brass scrapers, die soap, and copper gauze. Anti-seize compounds with high-temperature capabilities should also be on board to coat guide pins and threads. Moreover, you also need a torque wrench for even torquing that mitigates leakage. Along with that, you should have cardboard that is wedge-shaped to offer a protective cover for the lip area. Finally, you ought to have a sturdy lifting device.

Conclusion

Splitting and tidying your die head should be easy with this guide and valuable tips. Cleaning the die head goes a long way in boosting the efficiency of the extrusion line. In addition, it helps to amplify the quality of the final products. Together with that, eradicating resin and plastic buildup from the die eliminates disruptive downtimes that might limit your production rate. FANLYPLAS is a seasoned manufacturer of quality die heads for the PVC foam board production line. Contact us today to make an order. If you need a new die head or you simply need a reliable die head servicing partner, FANLYPLAS is at your disposal.

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