Moulding and dyed Lexan 2

[bassnut]

I have a supplier who seems to be a little over his head with the polycarbonate thing.
He has produced some excellent parts with ABS but I was having problems with cosmetics. i.e. scrathes and dings. There was also a need to use a higher temp. resistant mat'l.
The part in question is dyed black and is a commercial, consumer product.
I requested that he switch to Lexan.
Since he's been trying he has produced very poor results in the 1'st trys due to moisture and dye problems and has been increasingly hard to get a hold of....
He may be over his head here. What problems might he encountering?

anybody? anybody?

Thanks.

Fred

 

[patprimmer]

There are a number of problems which may apply here:-

Firstly you do not say very much about what you are after and how you are doing it.

Are you injection moulding from granuals, or thermoforming sheet.

Maybe acrylic would be a better choice than Polycarbonate.

Wether injection moulding or thermoforming, moisture and drying is a real problem with PC.

What do you mean by dyeing. Dyeing normally means appling colour by absorbing it from a solution of the dye. Is this what you mean? Dyeing PC black by imersing the parts in a solution would be difficult. A better option would be to buy the material already coloured black.

Regards
pat

 

[bassnut]

OK Good.
I believe he is now getting the already dyed black granules.
Before he was using a clear pc and adding color which is the 1'st pieces I saw. They were badly streaked.
The process is injection moulding.
Before I put him through further stress and waste my time and money, why do you think acrylic would be a good choice?
My only requirements at this point is that the parts a) look good in black. b) are scratch resistant and c) can take more heat than ABS.
The part is a coil bobbin for electric guitar pickups(transducers) They have terminal pins stuck into the body that will have lead wires soldered to them. To much heat for too long in the ABS causes melt around the pin base. They will also be immersed in melted parrafin to "pot" or encapsulate the fine wire coil. The ABS warped at 150 deg.

Thanks for you help.

Fred.

 

[bassnut]

I forgot to mention that these bobbins will have some holes machined in them. They are being circular interpolated with an endmill as opposed to drilling. I've heard that acrylic is difficult to machine.
They will also be glued to chrome and brass covers in the final assembly.

see photos

http://www.basspickup.com

Thanks again.

Fred

 

[plastx]

Bassnut,
Lexan or polycarbonate does have higher heat than ABS but is more sensitive to molding due to moisture. The reason is that when you mold ABS with moisture you will get bubbles due to steam but with polycarbonate there is a chem reaction which will cause brittleness and appearance issues. If you are running small production runs the molder may want to mix clear polycarbonate with black concentrate rather than buy a quantity of black polycarbonate. This can lead to color streaking if the molding process doesn't mix enough during melting/conveying. Alternatives would be a high heat ABS or a PPE material like GE Plastics Noryl or BASF Luranyl while keeping the mold dimensions the same as the ABS.

Mike

http://www.plastxpi.com

 

[patprimmer]

Acrylic has exceptionally good scratch resistance as well asexcellent surface gloss. it is used extensively in backlit outdoor signs for this reason. Care does need to be taken when machining it as it can be brittle, but millions of signs are made each year useing woodworking type tools.

Re heat deflection temperatures, high heat acrylic is good to about 100 deg C which is slightly less than High Heat ABS which goes to 120 Deg C, but not so good as PC which goes to about 140 Deg C. These are HDT under some load. They will go a a fair bit further without deforming if there is no load.

The tension from the wire might be creating a load though.

If you really need 150 deg C under some load, you will need nylon, or maybe something glass filled. Nylon will shrink more, and cannot easily be glued.

Does this need to be flame retardant?

Melamine flame retardant types of nylon 6 if moulded and moisture conditioned correctly, don't shrink all that much more than ABS.

Regards
pat

 

[bassnut]

Whoops!
I'm sorry. That was 150 deg. F not C.
My wax pot runs between 150 and 160 F. The ABS bobbins are under a little stress with the wire coils and are warping even under this low heat.
Because these go on a musical instrument the faces have to be pristine and scratch free. The assembly process requires a lot of handling. It's easy to inadvertly scratch them and require protective taping etc. A real headache. Then there is the soldering heat problem as I mentioned.
I'm still feeling dubious about machining acrylic. I have no experience with it. Again the face has to look pristine meaning no chiping around the holes etc. I suppose I could give it a try.
Can anybody suggest some more materials?

Thanks .

Fred.

 

[Pudding]

Several points:

Poor colour in PC when using colour additives (i.e. masterbatch) is caused by the masterbatch carrier not being compatible. Even so-called "universal" colourants. Try either PC specific masterbatch or dry colour.

PC needs drying at 120 deg C for at least 4 hrs in a dehumidifying dryer. Lack of drying is the most common cause of problems when moulding PC.

Tool temperature is also important. To reduce moulded-in stresses the tool needs to be at least 80 deg C at the tool surface (not what it says on the tool temperature controller! - measure it)

PC, although being very tough has quite low surface hardness and is prone to scratching. A previous posting mentioned Acrylic. This has good hardness and can be polished with proprietry compounds. Acrylic also needs particular care to ensure it too is dried well before moulding. (80 Deg C 4 Hrs min)

Machining of acrylic is quite straightforward - do it in a warm room!

 

[gambro]

It is absolutely essential to dry polycarbonate at 120°C for at least 4 hours immediately before injection molding. If you use masterbatch, you must dry it as well. Please note that PC is sensitive to stress cracking in contact with chemicals. It is important to test the stress cracking resistance with all chemicals (for example cleaning agents, detergents etc) which may come in contact with the finished part.