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Relative Humidity and Plastic Raw Material Moisture

J

JPDaniel

#1
Dear forum Members,

My query comes out of the need for setting the right preheating time for plastic raw material before injection molding for various seasons for a tropical climate near sea coast.

Would the plastic granules have low moistue content during summer when RH is lower compared winter where RH is high. The absolute humidity must be high during summer, which means more water vapour in air and I am unable to decide. :confused:

Is it true that low RH means lower moisture content in plastic granules?

Please give your thought on this.

Best Regards,
P. Daniel
 
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Ronen E

Problem Solver
Staff member
Moderator
#2
Hi P.,

Absolute humidity is irrelevant. In general, if the barometric pressure is constant, the higher the air temperature, the more moisture air is able to hold. The fact that there's more moisture in the air (in absolute terms) doesn't necessarily mean more moisture will be transferred to the plastic granules. There's more moisture around, but also a higher ability of the air to keep it.

On the other hand, as relative humidity (RH) goes up, the air is less and less capable of keeping it, up to the point of RH 100%, where you will see fog forming. the air is no longer able to hold the moisture as gas molecules and the water will start forming droplets. So basically you should pay attention to RH.

Considering plastic resins for injection molding, you should be strictly adhering to the raw material manufacturer's instructions concerning moisture content / drying. If critical, get a moisture content meter and use it for making decisions. If the bulk raw material packaging is adequate and storage periods are not excessive (proper stock management), then this should be less of a concern, but you may want to establish the facts based on actual measurements.

Cheers,
Ronen.
 
J

JPDaniel

#3
Hi Ronen,

Thanks for the information.
I agree, measurement should be the basis for this decision. :yes:

We might have to test samples with different initial moisture content and record the time it takes to reach the required level.

We have Precisa moiture testing instrument and would carryout the tests.

Again, I presume the above test might give different heating times based on the RH on that day. Hope it would not be significant.

Thanks,

P.Daniel
 
B

Bjourne

#4
Very nice post there Ronen :) I agree there that aside from paying attention to the RH, the proper handling/storage/dispensing of the mold material is very critical also. And please also consider the kind of mold material you have. I am not that familiar with injection molding plastic resins but in semiconductor molding our EMC (Epoxy Molding Compound) follows strict stock management based on (as Renon stated earlier) it's characteristics and manufacturer's instructions. They may differ per manufacturer such as in Henkel they advice us to:

1. Store at 5deg or below (cold storage)
2. Remove from cold storage thaw from 16-24hrs prior usage.
3. Handle with lint free gloves.
4. Keep away from heat.
5. Keep desiccant bags with the pail/box until used up.
6. Replace lid on pail after each use.
7. Consume within 24 hours of opening.
8. Unused --unopened material which has been already thawed and exposed to room temp min 48hrs (with within spec RH)can be restored/refreezed to cold storage only once(at 5deg or below). Thawing and other conditions apply.

Floorlife (72hrs): Thawing time(24hrs) + Exposure Time (48hrs)
RH: 40-65% (SPC recorded)
Temp: 20-27deg C (SPC recorded)

Samsung EMC's differ from Henkel as they allow

1. Store at 5deg or below (cold storage)
2. Remove from cold storage thaw from 16-24hrs prior usage.
3. Handle with lint free gloves.
4. Keep away from heat.
5. Keep desiccant bags with the pail/box until used up.
6. Replace lid on pail after each use.
7. Consume within 24 hours of opening.
8. Unused --unopened material which has been already thawed and exposed to room temp min 72hrs (with within spec RH)can be restored/refreezed. Refreeze can only be done once.

Floorlife (72hrs): Thawing time(24hrs) + Exposure Time (48hrs)
RH: 40-65% (SPC recoreded)
Temp: 20-27deg C (SPC recorded)

Refreezed materials can be used for production within 24hrs. If unopened it will be scrapped. If opened and used it must be consumed with the 24hr period otherwise it will be scrapped.

Also I'd like to add that we have observed that the time of preheating EMC shortens as the summer months come (despite the proper control of humidity with the proper storage and handling). That short pellet preheat time is within pellet temp requirement. I do not have a copy of that particular study that our engineers have done as I am not privy to it but I was involved with it. Mold EMC exposure over time (in the production floor) during the summer months and the cold months. Humidity, stock management/handling, machine settings/parameters(which include preheat time) etc affecting quality.

Good-luck ;)

Some links if it may help in a small way,

http://www.pharmtech.com/pharmtech/data/articlestandard/pharmtech/022005/141826/article.pdf

http://www.asse.org/professionalsafety/docs/Alan McCartney Article.pdf

http://www.toray.jp/plastics/en/products/amilan/technical/tec_007.html
 

Ronen E

Problem Solver
Staff member
Moderator
#5
Hi,

Just wanted to mention that thermoplastic injection moulding is quite different from thermoset (cross-linking, gelling) moulding such as with epoxy resins. In thermoplastic resins flow is enabled by heating, then the object solidifies when cooled. Humidity is controlled to avoid the water vapour interacting with the resin while hot and under pressure. With thermosetting resins the material is essentially a fluid while cold and then it solidifies (gels) when heated. Solidification of thermoplastic resins is a physical process which is essentially reversible, while in thermoset materials it is a chemical process (chemical bonding) that is mostly irreversible. That's why thermoset materials will usually burn before they melt, if heated sufficiently.

I'm not an expert on thermoset moulding but I assume strict RH and temperature control is required to prevent spontaneous gelling (chemical reaction) prior to moulding.

Cheers,
Ronen.
 
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