PS Foam Extrusion

[chloemarylouise]

we are encountering varying density during our run of PS foam. we are using a tandem type of extruder. our blowing agent is LPG (liquified petroluem gas).
for example, we are extruding a PS foam with specs: GSM (gram/sq. meter)= 180 and th=3.00..we have a problem maintaining the GSM of the extruded foam...our set tolerance limit is +/- 5 GSM...

what is the normal tolerance limit for PS foams? Is our tolerance limit too close?

What factors affect the GSM or density of the extruded PS foam?

[Len]

Some questions come to mind. Are you having trouble on the high or low side of you specification? Were you able to achieve your 180 +/- 5 in the past? What parameters/materials have changed? Are you able to maintain tolerances on other lines using the same feedstock and blowing agent? Is there a noticeable size/shape difference between bubbles when examining foam samples that are w/in tolerance and out of tolerance? Are you using a nucleating agent?

Len

[chloemarylouise]

Sir Len,

most of our off-specs are on the higher side of our specifications...In the past we were encountering off specs but just around 2% of the total rolls produced. However, now we're accumulating up to 10% offspecs rolls.Our raw materials have not changed (GPPS-Tairirex(from Taiwan) and Hi-Flo(local,Philippines), HIPS-Hi-Flex(local), scrap(which we pelletize) and talc). The material formulation have not changed at all. We only have two extrusion lines in the plant. One is Sencorp, the other is China made. We don't run the same specs in the two lines so i cannot compare. But if we examine the output of the two machines, Sencorp has a higher offspecs. (the two machines don't run simultaneously and around 5 specs were run on the China extruder, the output of China extruder is 1/3 that of Sencorp). We are a bit on the low-tech side so i haven't really examined the size/shape of the bubbles. (to tell you the truth, nobody even paid attention to that). But i will do it after this reply (hope i can find a lens here). We are using talc as nucleating agent...around 0.5 to o.7 percent (we tried 1.2% but we had trouble in our thermoforming lines using those rolls-the product becomes soft). As for our parameters, basically nothing has changed. The heater settings of the primary ranges from 190 C(feedthroat) to 225 C (barrel), secondary-110-120...At first we thought it was because of air supply (the line is shared with the printing line so the air supply is constantly changing). What we did was we dedicated two compressors solely for extrusion...but still the problem is there.... then we tackle Blowing Agent Flow, since it was going up and down +/- 2 lbs/hr of the specified value. We changed the regulator, but still the problem exists. We already checked the heaters, cooling system...The screw has been changed over a year ago...My next suspect is alignment (maintenance will go over this after the run). Since its run last March 25, 2008, we have accumulated 12% off-specs (Whoa) and our most problematic specs is GSM=250g/sq m; th=3.7mm...
I'm just wondering if other PS foam manufaturers encounter the same problem like ours. Also, I'm looking into our tolerance limit...i'm just thinking maybe our tolerance limit is a little tight...Im wondering what is the tolerance limit of other PS foam manufacturers.
Hope we can lower the offspecs this April but up to now, I still don't know what the real cause is...We cannot find an expert in PS foam here in the Philippines, so we are still at loss...
Hope you can help us sir. Thank you!

[Tom]

I recieved an email from you directly through my website, but i don't have very much expereince with this process.

There are a couple of basics I thnk you can check. Are your melt pressures consistant out of both extruders? Is the amperage draw consistent in both extruders? Is the melt temperature consistant?

[Len]

Thank you, CML. A detailed description of your set up and materials, is always helpful in trouble .

I'm trying to digest all the information. But, the variation in blowing agent sticks out as a concern. What % of the set point or specification is +/-2 lbs/hr.? Since I'm a materials kind of guy, I'd look at your talc. Is it dry, have you changed suppliers or grades, what was the GSM for the material made w/ the 1.2 % increased talc?

I don't mean to have you chasing your tail, but this sounds to me like a feeder is drifting off spec, a weighing error, or a slight change in a raw material has caused you foam density to dift.

Len

[DwightDixon]

I hope you are using weigh feeders rather than volume feeders for all ingredients. The volume feeders are the biggest cause of this type of problem. The liquid propane feed should be slaved to the extruder output rather than a fixed setting. A the liquid feed should be controlled by a variable speed gear pump slaved to the pound feed rate of polymer. If all you are using is a pressure regulator for the propane it may be affected by the ambient temperature and may be you source of varience. Are you using starve feed twin screws or flood fill singles? You may have a problem with variable melt temperatures. This is the second most common problem, which results in inconsistant amount of blowing. You need to start collecting all the process variables and readings over time and then look for a coorilation to the amount of off grade you are making. The statistics will tell you where to look. It is likely a out of control process that is leading to your problems.

[chloemarylouise]

sir len, sir tom, sir dwightdixon,

Thank you so much for the inputs. There are so much technical terms here that i have never encountered before. Luckily, the head of our maintenance department expained them all to me.

sir tom,
the melt pressures of the two extruders are not the same because they have diffent specifications running in them (meaning the product specs in Extruder 1 is not ran in Extruder 2). Same is true with amperage and melt pressure. With regards to amperage draw, I will ask our maintenance department to make an hourly check on amperage draw to check the consistency of RPM of the screw...

sir len,
In our control panel (we are on a manual control not computerized), there is a BA(Blowing Agent) Pressure Controller and BA Flow controller. For example, if you have to change product specs from a density of 55 to 100, we need to lower the BA Flow from 24 lbs/hr to 16 lbs/hr....we set the BA flow and the BA pressure. the % in the set point of the erratic/non-consistent change (+/-2 lbs/hr) is 8-15%.
About the talc, we use talc in powder form (we are still proposing to management to use talc in masterbatch). We havent change suppliers for a couple of years now. Visually, its dry. This is how we prepare the materials...the talc and cooking oil (yes, we use cooking oil in very small percentage to prevent early corrosion degredation of the screw) is weighed manually using an ordinaly weighing scale (not the digital one, i think its called a dial scale) which is calibrated once a year. Last january we changed our formulation and increase our talc to 1.2%(to decrease roughness of our rolls, giving a smooth surface) on all our product specs...the feedback coming from the thermoforming department was, they had difficulty in thermoforming the rolls,the product is soft eventhough parameters were adjusted. Also, there is rampant ID/OD (inside damage/outside Damage) in the products.So by 3rd week of february, we returned to 0.7% talc.since then, we havent heard a problem from the thermoforming dept except for off-specs rolls. Back to preparation of raw materials, after weighing talc and oil, it is mixed with bags of GPPS, HIPS and scrap (GPPS, HIPS and Scrap (all are pellets) are by bags so the proportions are per bag, we do not weigh them).They are mixed in the mixer for around 5 mins. then it is transported to the feeder by screw conveyor.
Sir, what factors greatly contributes to GSM and thickness?
Here are my initial findings: For GSM - amount of blowing agent, RPM of Primary and Secondary, Thickness- the Speed of Take-up rollers, die opening..


sir dwightDixon,
We are using weigh feeders (gravemetric). Right now, we are on manual weighing ( we use dial scale). However, we have a colortronic system which will be operational soon (they have fixed the motor and we will just have to calibrate it first before using). As for our LPG (BA Agent), we have BA Flow controller (lbs/hr) and BA Pressure controller. We will check the effect of the ambient temp (ambient temp here is 35 C) to the Blowing Agent (thanks for the tip). Maintenance department will see if the BA feed is slaved to the extruder output (we don't know yet, i'll give you an update later).Here's how we control/set BA feed.
Method 1(before the old regulator was replaced) - We set a value to the BA flow controller. The BA pressure is a fixed value (no matter what the product specs are, while the BA flow differs for every product specs). After setting a value to BA flow we adjust the metering pump (stroke/min) until set BA flow is achieved. The play in the flow of BA with this method is +/-8-15 % (BA flow is going up and down throughout the run)
Method 2 (after installing the regulator)-Prior to run we fully open the metering pump, and then we set the regulator to certain BA flow..then, when we need to change to another product specs, we just set a value to the BA flow regulator and then we adjust the BA pressure (you can set a value) until the set BA flow is achieved. (BA flow and BA Pressure can be set to a value, and also reads the actual value)
With regards to the type of screw, we are using a flood fill single. About melt temp, i will monitor it hourly to check if its changing (will give you an update) thank you so much for the insights!

[DwightDixon]

You may be experiencing sifting or settling of the talc with the method you are using now adding the oil to the talc and then mixing with the pellets. If it is an open mixer, I would recommend sprinkling the oil onto the tumbling pellets and then adding the talc and continue tumbling. This might get you a better distribution. It does not sound like the BA piston pump can be slaved to the extruder output or RPM. You might try to add some mixing pins to the extruder screw in the metering zone to make the bubbles more homogenious. This will not help your roll to roll variation which seems to be your problem. As an experiment you should weightand chart each roll as it comes off production to see if there is a pattern that may be related to something that is going on in the plant such as ambient temperature or melt temperature. To increase the density of the roll you should reducing the melt temperature or decreasing the % of blowing agent. It looks like you are possibly are having a longer time period variations in these two key parameters.

[Tom]

If you are using a tandem type of extrusion setup, it ususally consists of a melting extruder and a cooling extruder. I was asking about the parameters of each of the two extruders. For example, the melting extruder could be surging, and therefore your product will be inconsistant just from that. In such a case you may be able to match the variation in the product with the extruder surge. This type of variation will cycle in 5 minutes or less. If you have longer cycles it could be temperature control or feeding consistancy, as previously proposed.

[Len]

After reading the above comments, suggestions, and your observations about the process, I strongly suspect that your are seeing a variation in BA delivery versus extrusion throughput. If your BA feed is varying, along w/ extrusion surging and/or fluctuations in your raw material feed then your process is out of control.

Either level out possible surges (as tom suggests) in your extrusion process and remove the 8-15% variation in BA feed delivery, and/or slave the BA feeder to the extrusion out-put (as DD suggests).

Len

[Fard]

Dear Louise

It's for years that I'm working with PS foam, and I'm realy interested to know if you have solved the problem or not. And what was the exact reason of your problem. It would be highly appreciated if you describe the results.

[chloemarylouise]

Mr. Fard,

Are you engage in PS Foam Extrusion? We have controlled the variance in density to around 2% now. We have so much work done especially in the pull roll unit and in centering the machine...I have sent you a PM..hope to exchange more ideas in PS Foam with you

[Fard]

Dear Cloemarylouise

Thanks to your reply. I got your PM and I sent you the reply to your email two times. It's good news that you were succesful in reducing the weight variations. However, if you have solved the problem by working on pull roller device, then certainly you have had thickness variations during your process, no density variations. The density of the foam in comparision to density of polystyrene, only depends on the volume of gas which is injected to the plastic. The volume of gas injected to the plastic, may depends on many factors such as temperature profile, extruder pressure, rpm of screws and so on. But the pull roller, just determines the general thickness of the sheet, so if you have had variations on the thickness, so you had got variable roll weights.

Further, there is usualy a metering encoder on the pull roller which is metering the sheet. If there is any problem with this device, for example a weak touch of its weels with the roller, then you will got variable results about the lenght of your rolls and so their weights.

Good luck and looking forward to hear from you more & more.

[luckyman]

Dear All
we are engaged in PS FOAM Extrusion and finding difficulties in ASTM standards for thickness, and shrinkage tolerances is there any ASTM or other international standard for these things

Best Regards

Javed