Injection molding a part so it needs no paint, currently painting 3

[dennisbernal91z]

Hi I am new to the forumn and am a product engineer with about 3 years exp.

I am currently haveing a part molded for me that is quite big, it is a case to a medical instrument. It is currently made of ABS+PC (Hybrid S570 or similar) w/ chem. foaming agent (Hydrocerol or sim.). This material is OK, it just looks like white chocolate and the parts needs to be pantone cool gray 1 U.

Here are my questions:
1. How would I spec out a material that can be shot in this color.
2. Is it possible to add dye to the current material?
3. Paint hides sink and mold flow lines, the bare part currently has a lot of these. This would rule out using this material correct?

This is a good start if anyone can help. Thanks so much for any assistance.

-Dennis

 

[tomwalz]

Suppliers can be a tremendous help.

You have one supplier now who is not delivering what you wish to have. Contact them and see what they can do. Then look for other suppliers. Your question above appears to be very well written as an initial question. Send it to prospective suppliers.

With a little work you will find someone that can guide you though the whole processes, including helping you write specifications.

Tom


Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

Good engineering starts with a Grainger Catalog.

 

[MikeHalloran]

I recall a flurry of activity, or at least publicity, maybe a decade ago, about the subject of 'in mold finishing', or some such buzzphrase.

I think the essence of the technique was that the mold cavity was coated with a release agent, then painted, possibly with a dry film, and then whatever resin you wanted to use was shot inside the paint. ... and magically adhered to the paint without substantially distorting it.

ISTR photos of stuff like lawn tractor engine covers provided as examples. Hint: They were painted in that specific bright green that clashes with everything other than bright yellow.

I have no idea if the process is actually in use now.


Mike Halloran
Pembroke Pines, FL, USA

 

[dennisbernal91z]

I am looking to do something far more simple. Just having a minor textured finish on the surface of the instrument. We currently paint, then apply a "splatter" layer to give it some texture. I want to make the texture uniform, and to avoid painting. Just one simple gray color that is all. I started talking with my molders and they are looking into what they can do. One says there should be no issue as long as they can still get the part out of the tool. Added texture to a tool makes the parts stick to it.

 

[patprimmer]

PC/ABS is available from a few suppliers in a range of standard colours. Many suppliers of polymer can also custom compound a colour for you.

Another alternative is to have the colour matched and pigment supplied in the form of a master batch, although with ABS or ABS blends this can have some variation from batch to batch due to variations in the base colour of the ABS. It depends on just how fussy you or more importantly your customers are about aesthetics.

Also be aware that a Pantone chip has a different surface texture and reflection. This can have a significant influence on the appearance of a colour. Colour is a very subjective thing and the appearance of the same colour can vary a lot with circumstances. For instance, a glass of beer. The beer and the froth are the same liquid and therefore the same colour, but they look very different. I mention this because you are foaming your part, presumably to reduce weight and/or control sinks.

The more glossy your surface the more obvious sinks will be. A foamed moulding will exaggerate the appearance of flow lines.

Paint will mostly cover flow lines, but not so much sinks.

If you have tried to discuss this with your moulder and he has not provided a significant amount of the info above, maybe you need a new moulder, or you at least need to push him for information.



Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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[dennisbernal91z]

I just started the process of investigating the issue. I will be able to press my molder harder when they are back from break, next year. I will keep in mind what you said about the foam making flow lines more evident, that makes total sense. We definitely use it to reduce sink. The part in question is very prone to sink, it has many large reinforced bosses on the inside that are quite tall and thick.

In a bit of conclusion, it doesn't sound like color matching is a different type of problem than any other material selection problem. I thought these materials would be a different class of plastic. Not sure why I thought that since many other parts I make are a specific color and are not painted.

I will post updated as the issue advances. This would save my company a huge amount of money.

 

[MacGyverS2000]

Can you apply the texture to the mold instead of spraying it on? Would be one less step in manufacturing.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[dennisbernal91z]

Yes you can texture the mold. This is something I have done in the past. My molder(s) have no issue doing this once I tell them what texture specification I want. Only problem with texture is that it makes the part harder to release from the mold. One of the parts I want to do this to is already a pain to eject, so much so that a release agent is already used.

I sort of don't see this happening in the my current mold for the part. I may be adding a touch screen to the instrument in the future, so I may be developing a new mold, in that case, I will without a doubt, engineer in texture and the use of no paint.

 

[patprimmer]

And thinner ribs even if it needs more of them and a thicker main wall and smaller radii where the ribs meet the main wall I think.

Regards
Pat
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[dennisbernal91z]

Thats right, it all about reduction of mass with retention of strength.

 

[patprimmer]

And uniform wall thickness.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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[dennisbernal91z]

Right. I feel like all the golden rules need to be adhered to much more stringently when molding something that will not be painted.

 

[btrueblood]

Does adding pigment (going darker from a nearly white if I recall what the OP is saying) make the appearance of flow lines more pronounced? Or does darkening a part help to hide flow lines? Just curious.

 

[dennisbernal91z]

I think there is no rule that ties so color darkness. I have some black parts that you can see the flow lines very well. While I have other parts that are white where you can't see them at all, and vice versa. I think it really ties into tool design, material selection and setting used during the production of the part. A longer screw in a molding machine will provide more homogeneous mixing of the plastic while increasing the risk of "baking" the plastic (clear comes out looking more like ice tea).

The more things you need to control, the higher the price goes. One of the things that scares me about this project is how high the price may end up going.

 

[MacGyverS2000]

I think surface reflectivity/uniformity would affect visibility of flow lines more than color... but this isn't my field. When an item is tilted, you'll see non-uniformity in the light being reflected from a depression in the middle of an otherwise uniformly flat surface. A uniformly dull surface would present less of an issue compared to a more textured reflective one.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[patprimmer]

Black is usually the worst for flow lines, but it depends a bit on the nature of the lines. Finish has a mucg larger influence than colour with mirror finish being worst, but other things like additives, sparkle or pearl additives, fillers, MFI of base resin, mould temperature, injection speed, melt temperature can all influence it. Foaming agents are particularly prone to it and are sensitive to temperatures and speeds.

PC/ABS is particularly sensitive to shear during moulding.

A longer screw in general should be easier on the material so long as the residence time is similar. More important is screw profile or compression ratio. PC/ABS does not like high compression screws, particularly ones designed for nylon or for PP and high speed packaging applications. It also does not like to much back pressure. It particularly does not like small gates.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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[btrueblood]

Well, stars all around, good info. Probably forget it in a week, but someday it may come in handy...