different parts with same molding conditions 1

[PravinP]

Hello everyone:

We are making molding NYLON 66 with 20% GF filled. We have two sites, one in France and one in Mexico. Both the sites are using same molding machines, same dimensions molds, same pressures, temps, times, etc. Parts coming out of Mexico site are bigger than those made in France! The measurements are made at the ambinet conditions at the repective sites. We checked every details but could not find the cause. Only difference is the capacity of molding machines. Mexico uses 160 tons and France is using 210 tons press.
Can someone points out the cause.
Thanks.

 

[Like2Strike]

From the information you supplied a number of variables could exist. I am not even sure how much variance you are seeing between parts.

The first thing I would check is to be sure you are using exactly the same materials (Nylon) from the same manufacturer.

Second, is the material being dried the same prior to molding?

Third, check your mold temperatures to be sure that the parameters are the same.

Fourth, the numbers that come up on the molding machine screen, to show injection molding pressure, back pressure, screw speed, etc. Are to be used as a guide for the present molding session. Repeatability from machine to machine using the same exact values is uncommon.

Hope this helps get you started!

Live Aloha

Frank M.
Tradewind Resources

http://www.tr-usa.com

 

[fjbalaux]

The ambient temperature and humidity are probably higher in Mexico than in France. Maybe you could measure parts at the same temperature and humidity in both plants.

 

[JRamirez]

Hi. I'd like to know if you found an explanation to your problem. We are facing a similar situation: One of our components was being molded at another shop, but they closed their facility and deliver their mold to us. We set it up and find that the parts (1.5in. x 1in. "U" frame)are way smaller than the parts we were receiving from them. We used the same material (virgin nylon 6/6) and tried to fine-tune the molding program but the closest we can get is .020 below spec. on the longest wall. is this something normal on nylon 6/6?

 

[ornerynorsk]

Isn't this fun ?!? Depending on the manufacturer, your 160 and 210 ton machines might have different screw diameters and different injection cylinder diameters. THEREFORE - you could be right on the twig with all of your INDICATED pressures, but in reality might have different PROCESS pressures. This can cause huge differences. If all of a sudden you have a 20% change in injection, holding, and back pressures, you are effectively changing your entire process parameters, along with minute changes in S.G. of the material, absorption, shrinkrates, etc etc etc. The other folks had very good points about the difference in ambient conditions on two separate continents also, but I would check machine specs first. Second, why don't you try "tweaking" the process on the unacceptable location to see if you can get the finished parts within spec?

 

[patprimmer]

Re machine conditions, as discussed by others here.

Identical conditions, only means identical to within the accuracy of the controls, and all controlls have an accuracy range. GF nylon 6.6 has a very narrow processing window, and therefore requires more accurate control than most other materials

Not all parameters are controlled, for example, ambient temperature, cooling water temperature, electricity supply, drafts in the moulding shop. Back pressure is often overlooked, as is moisture content of the granuals.

Slightly damp granuals leads to drool, the operator changes conditions to control the drool. This leads to freezing the nozzel, this causes reduced pack, this gives increased shrinkage.

Also, the moulds are identical?

What about section thickness and gate position, size and shape.

Given that cavity sizes are identical enough, are the waterways identical, are the runners identical and are the gates identical. Even if the waterways are identical, are the hoses hooked up with the same water flow pattern.

It is wise to regularly check surface temperatures at various defined positions of the mould if you are chaseing problems.

Are the steel blocks (read heat sinks) the same size and shape in both cases.

The shrinkage of nylon 6.6 is quite sensitive to effective pack. even small variations in conditions, eg the very same machine and mould, but with different ambient conditions can produce variations.

Variations in effective nozzel temperature, and therefore freeze off time, and therefore effective pack, can have a profound effect on the moulding of nylon 6.6.
I always recomend a heater band very close to the tip of the nozzel, with a variable voltage to control the temperature, so as to prevent drool, freeze, drool problems. PID electronic controllers are also suitable.

Having said that, I find the main variations in nylon 6.6 are more to do with moisture conditioning and or aftershrinkage. Moisture conditioning has two effects on dimensions.
1) It swells the nylon as water is absorbed.
2) It anneals or stress relieves the nylon at about 60 deg C if hot water is used for conditioning.

I have often seen moisture conditioning of nylon done in a hot water bath, at an alledged temperature, only to find that there is up to 40 deg C variation in temp accross the bath, with some parts being well conditioned, some being hardly conditioned, and some being anealed as cwell as conditioned.

Are the parts moisture conditioned
If so, at what conditions
How long after moulding, and/or conditioning are the dimensions measured.

Are you measuring a dimension that might be affected by warpage rather than simple shrinkage.

Not all shrinkage occurs imeadiately after moulding.
You will find variations in size due to aftershrinkage occuring while swelling is also occuring due to moisture absobtion. This somewhat complicates the situation as the two forces oppose each other to varing degrees, depending on a number of variables.

Thats all I have time for now.

Do some homework, then if necessary, ask me some more specific questions.

Also read up on all the shrinkage data you can for Nylons, Acetal and Polyesters, then read all you can on moisture conditioning of nylons, then think about what you are realy getting from each site

Regards
pat

 

[plasgears]

Packing parameters are key to the amount of shrink. This is one of the tricks of the trade in targeting final size of parts. To reduce shrink, develop pack pressure and pack time vs. final dimensions.

 

[Soupcat]

Good comprehensive thorough analysis Pat, I'll be keeping that post for reference.. Also thanks for the mailed info much appreciated.

Kieran

http://www.moulddesigner.com

 

[wonderer]

Hello,

all the previous replies seem to have thought mainly through processing. What about the origin of the nylon 6,6 you use? Even though it is from the same manufacturer (I assume it is) it doesn't necessarily mean it is the exactly same plastic. Only the same lot number would guarantee that, assuring storage has been properly handled. Ask your manufacturer the lot numbers you use in Mexico and France, and then check they use the same raw material from the same source. This may also give the answer to your problem.

Please let us know what you found!

 

[patprimmer]

Good point Wonderer

I let that one slip by to the keeper

There is rarely a problem with the same grade from the same supplier. Sure they have production tollerances, but it is rare to see these cause shrinkage variations, except in very difficult to pack moulds.

Most variations in shrinkage are due to the use of different grades, even if the supplier assures you they are equivilent. Variations in melt viscosity can affect pack, but the most likely culprit is variations in the neucleating agent, which can affect the degree and nature of crystalisation. Colours can have quite a marked effect on neucleation, and therefore properties and shrinkage Regards
pat

 

[Chr1s]

Who cares if the process is the same or not ?

The only thing you are aiming for is parts that are in spec.

Therefore tweak the process until you get good parts, the customer will not care if it takes two different set ups, only that they get good parts.

 

[patprimmer]

Chr1s

You are absolutely right if the parts are in spec and functioning satisfactorily.

However, I think the original question stems from the fact that some parts are not in spec, and concequently, what might be done, or tweaked, to correct this. Regards
pat

 

[hp48gx]

Have you verified that the Molds are exactly the same in both locations. Meaning dimensionally from cavity to cavity?

What about gating and runner sizes, are they the same for cavity to cavity and tool to tool?

Also, were the parts measured in a climate control room? Nylon will grow and shrink relative to ambient temperature.

Crystalline materials, you just got to love 'em. Have fun!

 

[JRamirez]

hp48gx:
In our case, we are talking about the same mold being moved to a different facility, and tryng to duplicate the results without success. We have tried adjusting drying temperatures, drying times, process and mold temperatures, process and mold times, process and mold pressures, and we just can't get the parts even close to spec. The closer we get, the britaler the parts become, and not only that, but the parts fail in assembly every time. I put a different material in the mold (cycoloy)and the parts come out ok, and functional, just one problem:the material specification is virgin nylon 6/6. If we had to suggest a different material, it would have to be one with dimensional stability, good mechanical properties and relatively good flexibility for ease of assembly. Any suggestions?

 

[patprimmer]

Your last post makes it sound very much like a moisture conditioning problem.

Are both plants conditioning the nylon equally before measureing and assembly Regards
pat

 

[ir12]

Put parts in vacuum oven @ 250 degrees F for 24 hours. Accurately weight the parts after they come out of the oven, from both mafg. facilities, then perform dimensional checks. Then you will have your answer. You have to get the water out of the part or you can't measure it. Then look @ processing conditions as previously suggested.
IR12