Help me...Magnification lens

[sulkair]

Can anyone help me here. The part I'm having manufactured has a bi-convex lens built into the top. Imagine an upside-down cup with a uniform wall thickness of .079" (2.0mm) On the top of the cup I want a magnification lens molded in so it will go from .079" to .173" (4.4mm) at the apex of the convex lens. The lense is about 1.5 inches in diameter. We want to use clear Poly-carbonate.

The manufacturer has called me with serious concerns about potential sink marks in the lens, and drop in pressure there. The industrial designer who drew up the CAD files is comfortable it wont be noticeable.

We're thinking about getting a mold flow analysis done to help determine the best course.

My question to you all is this. Do you see any fixes that we could implement that would mitigate any risk with changing wall thickness, and wouldn't break the bank at this point. the Tool is going to cost 22K -I am really at the top of my budget already. What would you guys do to mitigate risk in this situation, or do you even foresee a huge problem. See the whole project at this site: http://kck.st/LJDy94

[oldtool]

The problem is that your gating into a thin wall and going to a thick wall. Not good. Your ideal gate location is going to be in the center of that magnifying glass. But you probably already know this... I would try to move the gate closer to the mag glass and a thicker wall. Possibly use a flow runner, or several, to accommodate this. Personally I'm not a big fan of pin gates, I try to use cashews instead, but you can't use a cashew with PC. I'd edge gate with a fan gate, wide and as thick as possible to pack out. Again using flow channels to get to the thicker areas.

[oldtool]

you might try using acrylic instead. I'm not a material expert but I think you might have a better time packing acrylic than PC

[sulkair]

oldtool... Thank you so much - I appreciate the detailed advice.

If we put the gate right into the middle of the lens, wouldn't that mar up the lens? Or do you think it could be accomplished with only a very tiny blemish?

You got any Grand kids? As long as you don't have 20 of them, they're each getting one of these guys!

[oldtool]

Nick,

I have an idea that might get you a better fill even with PC but it's on an excel spreadsheet and I can't attach it here. I would need your email to send this. There also might be another way as well. You could use a 3 plate mold and gate at the top, middle of the mag lens with a .030 dia. gate. This would leave a portion of the mag lens inoperable as you would also want to make a .07 divet so your vestage would not protrude the surface. If you want the other gating idea please send your email address.

Ps. I do have 10 grandkids but the older 6 would much rather have cars than bug traps

Thanks

Tom

[Louis]

Gating aside, Polycarbonate is the resin of choice if you expect the item to be used over a long period, while Acrylic might be a better solution for such a project IMO, due to costs and the likelihood that the novelty will wear off, or the kids will outgrow it. Of course, there is always mom catching and releasing those nasty critters that manage to get into the house.


Any gate in the center of that glass will leave a blemish.

[oldtool]

Any gate in the center of that glass will leave a blemish.

I agree with Louis a gate in the center is not the way to go with this product.

2 flow leaders in a "V" shape pointing to the mag lens, make them look like they are part of the design, and tunnel into the flow one of the flow leaders would save a bunch of wasted injection pressure that the pin gate would create. Make the flow leaders .08 tall and .08 wide on the outside of the part either side of the hinge area. This would direct the injection pressure quickly to the thicker area, mag lens.

[pjhall]

I have molded very large PC lenses in the past some with wall thicknesses in the .500+ range. Hot tool, VERY slow injection, and long cooling. We had one lens with a 10 min cooling time. Not fun having to go work on some flowlines on that part!!! Where ever you gate the part you might consider making "Overflows" that can be trimmed off post molding. This will help End of fill venting and other flow issues.

[sulkair]

Gentlemen!

Thank you guys so much for the detailed solutions. I'm going to run all this by the guys building the tool.

Would a "mold flow analysis" be warranted and worth the money on a deal like this do you think?

[oldtool]

Nick,

Moldflow analysis is always good if the person doing it knows what he is doing. As the saying goes ... garbage in garbage out. Personally I'd talk to a couple old curmudgeons in the field before I would trust anything you might get from Moldflow. But ... it is a good confidece builder

[greg]

Timing is everything, but check out the news headline we ran from PlasticsToday this week:
http://www.plasticstoday.com/articles/i ... -061420121

Rick Fitzpatrick's relatively inexpensive injection machine, Plasaver, uses low pressure (less expensive molds if not too late) and the process specializes in multiple thicknesses. Clarity should be good.

Rick is a start-up entrepreneur like yourself, sensing synergies, maybe he'd help, possibly mold for you to start. But you could make nice quantities in your garage to get going! Check it out! We'll help publicize it.

On acrylic: it is better except it scratches easier than polycarb, BUT maybe just use a heightened rim around the lens to protect it from most scratches?

Best,

[M&M]

Hmmm not seeing how that rig would be good for long hold times. Looks like all the injection pressure is generated with screw rotation, which causes lots of heat and shear if the melt is not allowed to exit the barrel as it is extruded. Further how much melt pressure can you get from a screw turning and not damage the melt? A few thousand psi max?

Not knocking the work put into the machine but gee does that really make sense?

[Louis]

Hmmm not seeing how that rig would be good for long hold times. Looks like all the injection pressure is generated with screw rotation, which causes lots of heat and shear if the melt is not allowed to exit the barrel as it is extruded. Further how much melt pressure can you get from a screw turning and not damage the melt? A few thousand psi max?

Not knocking the work put into the machine but gee does that really make sense?

Pressure depends on the machine and material, but blow out plugs or rupture disks may be sourced at ratings from 1,500 to 50,000 PSI.

I've seen extruders blow off heads and split barrels, so generating pressure may not be an issue. Isn't there a machine on the market that uses both extrusion AND traditional screw injection to get larger shot sizes?

I've always been amazed that Injection molding works at all, considering that the proper melting and plasticating of a melt depends so much on proper screw design and L/D ratio. Given that the effective L/D ratio is shortened as the typical molding machine moves plastic forward as it retracts, it’s a wonder that a proper melt is ever achieved.

Of course, “squirting” a less than perfect melt through a very small channel will increase sheer to the point that it hides any flaws in the original melt as processed from the screw

[sulkair]

How much more on average would a 3 plate mold cost to make vs a 2 plate? So for examply my 1+1 two plate was quoted at 22K. Would i be looking at another 10K for a 3 plate? Thanks guys. The reason I ask, is I'm considering having the gate put right into the lens, but at the very edge of the lens (not the center). This should pack out the lens. But would need a 3 plate mold.

[rickbatey]

I will bet you that MOST of these in current production are molded in acrylic. Two reasons:1-Easier to fill due to lower viscosity. 2-Lower resin costs. You can get PMMA (acrylic) that is scratch resistant, but many of these probably only get used for one to two years, then replaced.
Lenses require high packing pressures to meet their clarity and light transmission. Now with that said, you should perform the Mold Flow. You can use 'coining' or injection compression (software change to a typical machine) to make the part easier to mold. You fill the mold to near full volume with low clamp tonnage, then the clamp tonnage is increased to compress the melt in the cavity to keep the molded in stress low and achieve the clarity/flatness. CD's are normally molded in this fashion.
The gate should be as close to the lense as possible, to keep the cavity presssure close the lens area. With either resin, you will need a tab gate on the edge of the part and cut it off. You must gate into the thickest wall section and flow the melt to the thinner areas. The thinner areas don't require the packing pressures. You will need to cut the mold from good quality steel as anything less will either NEVER get the required surface polish or be a bear to maintain the polish. This mold will be a mirror finish for that clarity. I have molded PC, PMMA, and even some crystal PS resin lenses. You will more than likely when making them from PMMA eject cloudy parts from the tool. The reason being the thick areas will not be 100% cooled, but can cool and be clear afterward. BTW-Clear resins are tinted blue to offset their tendency to yellow after melting.
Rick.