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mc-motorsports
02-05-2009, 10:35 PM
I have a used mold base and I need to make inserts for a part that will be optically clear plastic. What material do I use? I know it's going to be tool steel, or mold steel, aluminum is out, has to make 600 parts without being repolished if possible (don't think that's asking much, is it?).

What grade steel should I use for this application?

Would A2 or O1 work? Needs to be .750" thick and 1.5" wide.

TIA,
MC

plastibob
02-06-2009, 07:31 AM
420 SS polished to an SPI#A1

plastibob
02-06-2009, 07:34 AM
One other note: gate and gate location are important as well as venting for lenses.

mc-motorsports
02-06-2009, 10:46 AM
420 SS polished to an SPI#A1

And if told you I was making this on a bridgeport? I'm limited to 2600 RPM. TIaLN coated carbide tooling?

plastibob
02-06-2009, 11:04 AM
If you don't want to mess with stainless I've made lens molds with P-20 ultra which is tougher than regular p-20 but still machines easy and has excellent polishabliltiy.

mc-motorsports
02-06-2009, 11:13 AM
do you have a good online source for online tool steels such as P20 ultra? I went to mcmaster and P20 is only available in rod and bar? and there is no listing for P20 ultra.

And is P20 prehardened 4140?

mc-motorsports
02-06-2009, 11:18 AM
Just a question, what about 52100? I have coniderable experience with making tooling out of 52100 which is my prefered tool steel. Just wondering.

Thanks for the help!

plastibob
02-06-2009, 11:38 AM
P20 is essentially a modified 4140. You can get the super from Dunn Specialty steels www.sunnsteel.com. The P20 is easier to machine than the 52100. I wouldn't recommend using the 52100 for a mold application. If your only talking 600 pieces you can make it easy on yourself and use 7075 aluminum.

mc-motorsports
02-06-2009, 11:57 AM
??? 600 pieces is an estimated 1 day run which will keep the customer busy for a while. I would guess a 600 piece run, then the mold will sit for a while, 3 months to a year? I can only guestimate.

If I used 7075, would that be ok for an optically clear plastic molded cover? Right now the parts are made from lexan, milled and cold formed, basically some tabs bent 90*. The customer is pickey, no scratches, he's also my father, trust me, he's anal about that stuff!

If money and machine time didn't matter, and you were limited to 2600 RPM, up to 100 imp feedrates though, what material would you use?

Thanks for the link, I'll check that out.

MC

plastibob
02-06-2009, 12:06 PM
Since your not using a filled material and considering your machine I would use the 7075. I buy my 7075 material through yarde metals dropzone. Most of my molds are aluminum and you can run thousands of parts off them. Right now your machining polycarbonate I would look at acrylic which is more optically clear and easier to process than the polycarbonate.

plastibob
02-06-2009, 12:12 PM
I missed you calling it a cover I thought it was a lens, the same applies but another molding material you may look at is SAN as it's pretty tough, scratch resistant, inexpensive and molds easy it also has better chemical resistance than polycarbonate. I'm not sure what environment this cover is in.

mc-motorsports
02-06-2009, 12:30 PM
It's NDA kinda stuff, but it's a cover to keep your arm and hands off of nicely engraved or plated parts. Will be exposed to sunlight, normal UV rays. Right now the parts are just cut out of lexan sheet, I don't even take the plastic off, just drill some holes to mount them to a fixture, bolt them down and cut them out 6 sheets thick. After that, my job is done, I charge $1 each, but then they have to be lightly deburred and the tabs bent 90* so there is a lot of man hours in handling time plus material costs...$$$.

The molded part should cost $.45 each according to the mold shop, they donated an obsolete mold base, should only have to clip sprues and lightly deburr any edge flashing.

For the aluminum mold, 7075-T7351 or 7075-T651 or does it not matter?

plastibob
02-06-2009, 12:38 PM
Both materials are ok, if you had a choice T651 is better. Be careful with polycarbonate and UV exposure it will start to yellow.

mc-motorsports
02-06-2009, 12:55 PM
Thank you very much! Greatly appreciated!

MC

plastibob
02-06-2009, 01:01 PM
no problem, good luck on your project.

mc-motorsports
02-07-2009, 01:49 AM
As far as gates and vents, are you guys running simulation software or do you take an educated guess? My problem is I'm not educated enough to guess!

Is there anyone I could send a CAD file to for simulation for optimal gate and vent placement?

I still have to draw the part, but I need to know if it's a good idea to consult a genuine mold maker and or engineer so that I can pass along the cost. No worries though, it's for my father and he knows I'm not a mold maker and have some learning to do in order to complete the job. I wouldn't quote a mold for an actual customer being that I'm not a mold maker. I know enough to know I don't know enough, if that makes sense.:)

MC

plastibob
02-07-2009, 10:29 AM
In most cases it's an educated guess although on tricky parts I will do a moldflow analysis. It sounds to me like you have the machining capability, and you just need to know what to machine. I'm a plastics engineer who does product design/mold design moldflow analysis and builds prototype injection molds. I can take a look at it for you and let you know about the vents and gate. I would make the following recommendations:

1) definitely use aluminum - shut offs are easier and more forgiving in AL
2) have a good mold design that will give you your best chance for sucess.
3) make sure your part design is optimized to make the tool build easier for you.

I can help you out on all those fronts, PM me if your interested.

mc-motorsports
02-07-2009, 12:50 PM
Yes, very interested.

Would you need a drawing of the part it's self, or the proposed cavities? And I guess I should find out exactly what plastic they plan on using?

plastibob
02-07-2009, 02:53 PM
Yes, very interested.

Would you need a drawing of the part it's self, or the proposed cavities? And I guess I should find out exactly what plastic they plan on using?

You would need a drawing or rough sketch, the number of cavities would depend on the size of the part the machine its run in and the number of parts you plan to make. The material should be driven by it's use and environment.

mc-motorsports
02-07-2009, 03:18 PM
This is going to be easy for you :)

It's just a single cavity, single part mold.

It's some kind of little German machine. I'm sure if I could remember the name, you would know them. The mold base is only 5" X 8.5", roughly.

The mold shop has 20 of these machines, the only product that he makes that I recognised is bondo spreaders, just that plastic knife used by body shops to spread bondo for body work. Boxes and boxes and boxes of them, mostly for the chain auto parts stores like Autozone and Napa.

Then he has the larger machines that are all enclosed using multi cavity water cooled molds and such. But this will be a relitivly low volume part, a 1 day run on a single cavity mold will make enough parts for months.

I'll send you a PM when I get the part drawn up on CAD, and I'll send pictures of the part that the cover goes on and pictures of the mold base. It's probably one of the simplest molds you can make. But I also don't want to underestimate things especially since the part is clear plastic. Being clear plastic is my major concern. I know you guys have terminology for this stuff, but I'll just say I'm worried about the material jetting through the mold or making visisble lines where the plastic is injected from 2 ends and the plastic meets in the middle.

mc_n_g
02-13-2009, 04:02 PM
You will need a good tab of flash-type gate so the material will not jet and swirl into the mould. Do not allow the runner to enter directly into the mould without some sort of gate system on clear items. Make sure you allow for cold wells. I don't know how much material you will have to inject but you will need to make sure the runner system can handle the injection before freezing. If the part is thick you might also have shrink cavities.
http://www.glscorporation.com/resources_im_md.php only a reference
http://www.rtpcompany.com/info/molding/ only a reference

plastibob
02-13-2009, 05:06 PM
You will need a good tab of flash-type gate so the material will not jet and swirl into the mould. Do not allow the runner to enter directly into the mould without some sort of gate system on clear items. Make sure you allow for cold wells. I don't know how much material you will have to inject but you will need to make sure the runner system can handle the injection before freezing. If the part is thick you might also have shrink cavities.
http://www.glscorporation.com/resources_im_md.php only a reference
http://www.rtpcompany.com/info/molding/ only a reference

Jetting is the result of an unobstructed long flow path, incorrect gate design and is the result of higher viscosity flow which creates the visible defect. Cold slugs are always a good idea. It is very important to size the runner and gate properly as well as the type of gate.

jetski
06-29-2009, 10:54 AM
p-20 for that amount will work fine (Alro steel). Acrylic is the material for the best transmission of light (GE Plastics). If this is a lens and has to be perfect pay a polisher who has done tail light lens molds to polish it or get ready to recut it.