Dear group members,

I have a little problem with some parts produced with PLA and after of some days and some different trials, I need help of someone experienced with this material and background of injection moulding this kind of materials.

The Problem:

Some stains on the molding part like a smooth smoke.

My feedback:

I've cleaned the surface of the mold many times (which is made of steel and polished mirror surface) and the result improves considerably but after of 8 to 10 shots come back again the same problem.

This is an aesthetic problem more than other thing, but the appearance of the part don't looks good as expected for this kind of surface and material.

The parts must be fully transparent and I'm not able to achieve the end result as I would like.

Any suggestions?? Any guidance to follow will be welcome!

Dear Raul,

we have injected some parts in PLA and although we didn't have this kind of problems with this polymer, we actually had with PSU. Our PSU part was also an aesthetic part and this kind of "smoke" was an important issue we had to deal with.

My advice is to try injecting at lower speed.

On the other hand, as you stated, when the mould is clean the problem diminishes, you may have a venting problem. After some shots, the dirt could be metering in the clearance between ejectors and mould cavity, PLA emanates some gases or something that becomes everything sticky; also it could be a problem of thermal dilatation of the ejectors (but with PLA it won`t be important as the mould temperature usually is low), you may try grinding the ejectors to facilitate the venting.

I hope I have helped or at least have given some ideas.

Regards and good luck!
Xabier.

Dear Xabier,

Many thanks for your answer and suggestions.

Unfortunately, I can't reduce the injection speeds due to the thickness of the part (0.35mm the lower thickness in the part moulded to 0.95mm the higher thickness in the part moulded).

I tried to reduce the speed, but with less of the current speed I'm not able to fill the part (at least with a goood finished surface) and though after of test this trial I can take parts to check the effect of this point, I can see the same smoke on this parts. So that means that probably though can sounds strange, isn't the main reason.

I'm agree with you that probably the gases have an important part of the problem, but at the same time, due to the part geometry, I can't increase the venting fit.

About to grinding the ejectors, I can't do it because the ejection of the part is by plate movement and not by ejectors. That means that the fitting is the upper part of the moulded part (like a cup) and the diameter doesn't allows me to increase the venting.

It's true that sounds strange that after of clean the core of the part, runs good for some shots and I'm thinking more in some part of the material that degrades and then sticks on the surface. But obviously, I'm not chemical and I haven't the resources to find this solution.

However, thank you for your kindly answer and I'll keep you informed of the progress with this issue.

Best Regards,

Raül.

Without seeing the tool design it is hard to understand why you cannot increase the venting on a stripper plate. Very often a gap is left all round for a couple of mm then shutting off but not all round to allow gases to pass and location to take place. Injection speed reduction helps if you cant fill then increase mould temp and/or material temp. You might lose a bit on the cycle time. Also a coating to decrease the ejection forces on the cavity and core can help to slow down a build up on the mould surfaces. There are a number of companies can provide a coating service and will advise.

I suppose the part Raul is talking about is a kind of cup like part where the edge must be smooth and without any kind of flashes, that is why, in my understanding, the venting gap cannot be extended in this zone.

Modifying the temperatures can be also difficult, the PLA degradation temperature is not far away from the (easy) injection temperature, and also, the in-barrel residence time has its importance as the PLA degrades quite quickly at high temperatures. Longer cycle times are not good neither for production rate nor for the material itself.

Raul, another point you can try, but it supposes to modify the mould, is to enlarge the gate, you can get smaller speeds and less shear and friction at same volume transfer rate. If you are not using hot sprue it will suppose a higher scrap volume due to the bigger feeding system.

When we worked with PLA, we used a special machine from DESMA which only melts the material you need to fill the cavity (for delicate materials). It was only for microparts and I don´t know if they offer this technology for bigger volumes. I regret not being able to give you more information about this, but I am more focused in microparts.

Regards!

Hi again and thank you for answering one more time,

Your guide is very helpful for me though you think the opposite!

Yes, you are right, is a champagne cup (but not the modern version like a flute is the classic version of the old times).

The sizes are the following:

96.0mm of total height X 96.0mm of diameter on the top part (where you have to drink) X 0.70mm of thickness is this area.

I can send you by email some pictures if you would like to receive it with the defect mark in the moulded part if you give me your email address.

Though after of have made some trials, I'm close to ensure that is only a venting problem more than others issues.

About the mould;

Yes, the mould have a hot runner system with valve (only 1 cavity) and the part is injected by the bottom of the part by a injection moulding point gate of 2.0mm. The cooling water circuit is with a temperature of the water at 15.5ºC with a Delta between the entry and out of the mould of 2.5ºC +/- 0.5ºC.
(I've tried to increase and decrease some different temperatures to check the end results, but honestly, when more cooler better is the ejection.)

The machine used for produce this part have the following characteristics:

200 Tones (obviously graduable and the mould is working with only 110) and the barrel is 60.0mm of diameter (too big maybe for this application but is my smaller machine here) the cycle time currently is 12.5 seconds (at least until now, because I have tried to reduce the time but the ejection of the part doesn't works good due to the cristallization).

Regards to both and I'll continue updating my progression.

Raül.

What kind mold steel temperature you have.Some time it will be killer.
Flow in surface with PLA a liitle bit different from other polimer material.
Special with High Polish Steel.
Please if you still have problems let me know.
Best Regards
Boris

Sorry I not saw mold temp, but 15.5 C I think too low. You need to be around 30C .
Please try and let me know. Low mold temperature can give some not transparent effect.

I agree with ken Adams. A decent carbon-like coating will reduce the amount of friction during injection and during demolding. Try the DLC family. If still the part leaves small particles onto the molding surface (oligomers or bigger), it could help to make the molding surface a very light vaporhone + repolish (Max Ra 0.2). This will help to reduce the friction when the part is unmolded, and thus reduce the risk of particles sticking to the mold surface.
Is your part filled nice and even?
To check the behaviour of the melt flow, make a short shot and see how even/planar the filling front is. If it's very skewed/uneven, you may have issues with high shear on one end of the melt, creating more degradation and more byproduct.

This sounds like a classical case of gas condensate, increase your mould temp above 50 C and the issue should be eliminated, otherwise it may be that you cleaning the mould every 8-10 shots is just a coincidence and you have actually got a melt/residence time issue, try leaving the guard open for the same time that you would have cleaned the mould.

Hi all,

The problem is that at 50 ° C the part behaves as "rubber" and demold it is practically impossible due to the geometry of the part.

I've been trying with some differents mould temperatures (from 10ºC to 65ºC) and the best range is around 15ºC to demold without problems and to maintain stable the part dimensions.

Still making trials to find the best process conditions with this material for this parts.

About the coating, could be a good solution but as the surface is a polish mirror surface, before to do it and to spend a lot of money in do it I need to be fully sure that could work better with this solution.

I'll be back with more details when I've achieved the optimal results.

Without doubts, the most difficult material with I've been working never before. :)

Thank you to each one of you for your collaboration.