Mold cavity insert question

[dav612]

I've been doing some research on mold making lately and have a couple of questions I could use some help with.

Shut-off land - for sealing of the cavity, right?

How wide should the shutoff land be? Do I make it the whole insert? If so, how big should the insert be compared to the part?

Should the shutoff land be .001" above the A and B plates for ABS plastic? What about the runners? should they have their own shutoff land? or just keep them flush with a and b plates and use the .001" to vent the runner?

I have attached a pic to give you an idea what I'm talking about.

One more question. Can I make have the mold out of steel and the inserts out of something like QC-7 to use for runs of 5000 or so? The parts are small (2"x3".5") electronic enclosures.

What are the advantages/disadvantages to having the base out of aluminum vs. steel for this application?

Thanks,

David

[VWSatOz]

The shutoff of the whole die must be strong enough to take the whole clamp force of the molding machine. The inserts can be flush with the partline & vents made where needed to let out any trapped air that might cause a burn when compressed by th incoming plastic. The runners dont normally need venting unless very long and branched to multi cavities. I dont know what Q7 is but die cavity blocks must be sometimes have 10tons/inch squared of injection pressure, so for instance aluminum would be too weak unless extreme care taken by the moulder not to "flash" the die with too high injection pressure etc. I have never used Aluminum as a base holder block, but have used ii as the riser support blocks each side of the ejector plate with water cooling holes to conduct heat away from steel die plates. The A&B die plates must not bend apart when the injection pressure is applied to the cavity area, this is the critical thing to calculate, the thickness vs the length vs the force applied & distance between supports.
Hope this helps!

[Dexloy]

Yes you can use QC-7 for the insert, we do it all the time, in your pic it looks like you are using a pre made mold base and your part size looks about right for that size base. "Guessing a 10"X10"" it is fine to preload your inserts by.001 per-side, don't worry about runner flash. I have been making molds for 20+ years and simple short run tools are easy don't over eng them just make sure you get it to stay on the EJ side and can fill it, then eject it without distorting it. Put cooling in the base and the QC-7 will disapate the heat fast. I don't see corner radii for the inserts, you need to add that so they can cut the pockets in the base.

[Garypl]

I did not see any Return pins shown, There should be 4 Ejector pins say 1/2 " Dia. that are located in the corners of the Ejector bar (the Ejector pin retainer plate) that are located long enough to touch the Cavity side of the mold in the closed position (plus .002 -.003 in length beyond the mold base stack up height if you use .001 clearance as mentioned). This is to positively return the ejector pin plate into position and not allow the actual part ejector pins to be closed against the cavity, which will cause damage to the cavity. There are also usually springs between the ejector pin plate and the core retainer / support plate but these usually are not the only source to return the ejector pins.
I have used QC-7 quite a bit for molds exactly as you mentioned with production of up to 2 million shots with simple parting line poly pro molded parts. Qc -7 is quite hard compared to soft steel and is harder than 7075 ect, it gives a great finish too. I really think that the cost these days might make any steel worth another look though. Along those same lines there is Px-5 which welds nicely and one other which the name eludes me now that IMS Company makes provides great mold cavities. There is also another aluminum that I believe to be harder called “Fortal” from Superior die sets but costs even more.

[dav612]

thanks for the replies.

As far as the ejector return pins in the above pictures. they were not finished in any sort of way. I only drew them so you could have an idea of what I was talking about.

let me give you an idea of my plan and then ask some q's if you don't mind sharing some of your wisdom with me.

My family and I own a small company which manufactures electronic devices for a niche market. We are currently machining some enclosures out of 6061. I designed the enclosures and mill them at our shop. There are about 4000 units a year. I have designed 4 other plastic enclosures that I have had made by some other people. It usually doesn't go well. It either costs too much or takes way too long or this or that. We have more parts that need to be made in the near future. I want to make them in house. I have about 5 years experience CNC machining aluminum, mostly self taught, but not any experience machining steel.

Most of the parts I would be making would probably be a tunnel gate through an ejector pin.

I am machining on a Fadal 40/20 with a 7500 rpm spindle. I will be getting an EDM and a molding machine. I will get training on the molding machine when I get it.

Will that kind of a setup work with the fortal or qc-7 insert into a steel mold base?

Does a molding machine of say, 110 tons clamp with 110 tons every time or is it adjustable up to 110t?

Do I calculate clamping force from the shutoff area or vise versa?

I have used QC-7 quite a bit for molds exactly as you mentioned with production of up to 2 million shots with simple parting line poly pro molded parts.

What does poly pro mean? So your saying qc7 is ok for possibly a few million parts? I only need a few thousand a year.

[dav612]

Along those same lines there is Px-5 which welds nicely.

Oh yeah, when you say welds nice... what are you welding? Repairs? I read a little on px5 and about the room temp welding, but for what? do you tig it? what filler rod?

thanks

[DJPLAST]

For 5000 pcs you can use 60-61 T6 aluminum. We do it all day long for low production runs. Moldbase and cavities.
Our molding department knows the rath of the toolmakers and the owner for not notifying them in the event of a stuck part.
The molder is where you will have the biggest disappointment. When operators are left to unstick parts, they usually get carried away with screwdrivers and punches, aluminum will not tolerate that action at all.

[DJPLAST]

Regarding your clamp force question and calculation, it has nothing to do with shut off area. The cavity and core create a hydraulic cylinder. The plastic is the "fluid" that exerts pressure on this hydraulic cylinder (even though it is not round). You will need 3 to 5 tons of clamp force for every square inch of surface of plastic at parting line (or in that plane). example: a 5 sided box 1" X 1" x 1" , would need 3-5 tons of clamp.
Keeping the bottom of the box at 1" sq and lengthening the vertical walls to say 4" will still only require 3-5 tons. (the 1" X 1" "floor" is still 1" sq.).

If you make a box (5 sides) and the floor is 5" X 5" and the walls are 2" tall, you would need 75 to 125 tons of clamp ( 5 x 5 = 25 x 3 - 5 tons). The vertical wall play minimal part of calculation.

You are calculating on projected area in the parting line plane.
Hope this is not too confuseing.
Later....

[dav612]

You are calculating on projected area in the parting line plane.
Hope this is not too confuseing.
Later....

That makes sense. And it's great to know I can use 6061 for playing around while I figure all this out. I have tons of 6061 cutoffs that will be about the right size.



How would you finish the surface in green that is in the images above?

[VWSatOz]

Ok thats how how you calculate the force reqd to keep the dies shut during the injection. The green areas are the only shutoff areas to take this clamp force PRIOR to the injection coming in. If the green areas are raised above the blocks at all then they must have enough area to take all of this clamp force compressive stress. Usually other outside areas take all this inital clamping force prior to the injection pressure trying to push the cavities apart. You can set your moulder clamp force to a lower than maximum when running smaller jobs in order to protect the dies and machine platens from unrequired stresses, ie a 110ton mc mc becomes a 75t mc etc
You can sneak up on the packing pressure setting so as only push as hard as is required to fill the mould and not damage the mold materials. With a modern machine it is great fun tuning up all the control parameters and alarms, just tread cautiously so as not to prang anything.

[Garypl]

About the welding, yes TIG is the choice for welding for any material, but even though PX-7 is suggested for room temp welding, I still pre heat and post heat it.
About the Poly Pro, Poly propylene is more flexible than ABS and usually easier to mold but it is not as good for high heat environments.
One thing I might suggest, if you have been having difficulties molding other parts, the design could be the issue. Get other help about design there are sources for consulting that can really help initially. A few points that seem small are usually the killers. Draft, Uniform wall thickness and correct cooling are focal points. A saying that I use, you don’t know until you know, which holds for so many things. I built molds for years before I learned about plastics, then I started to really learn and understand that I did not know much about what I was doing and how I got by without that .
Tunnel gating for ABS works but requires very specific conditions; I would suggest tab gating unless you get more advice.
Glad to see that others have made clamping force clear, I have seen so many molds slamming together because this is not understood. I use so many materials for molds, even plastic like resins for cavities, to make the point the molds halves do not have to push / slam into each other!

[hotgrips]

We've been using the fortal aluminum and love it. It machines nice, is not "sticky", and has excellent compressive strength. The remark about bringing up the injection packing, or "shot size" with metering of the injection is extremely important. Take your time, slowly increasing the injection pressure and shot size (amount of material that is injected) and at the slightest sign of "flash" at the parting line edges of the plastic part, back off on either the injection pressure or on the shot size until that flash stops. Run the molding machine on semi-automatic cycle, not fully automatic cycle, until you get the settings right, and inspect each part that is molded, line them up on a table beside the machine in order that they are molded, so you can see what changes in the machine settings cause in the plastic part.

Another factor that can affect the flash is mold temperature. Injection molds can be run at 0 degrees F or anywhere all the way up to 200+ degrees F. Warmer water will cause flash to occur more easily, colder water will cause the injected plastic to freeze up before the cavity is fully filled. It is a delicate balancing of various adjustments on the molding machine to get it right. Then you have to deal with surface blemishes, "blushing", streaking, flow lines, short shots, etc.

If your "gate" (the entry point from the runners to the mold cavity) is too small, that can cause short shots, or burning, since the plastic is forced thru the gate it raises the temperature of the plastic. If the gate is too large, it creates various problems. The plastic resin manufacturer will have molding guidelines usually available online at their website, and they will recommend pressures, molding machine barrel temperatures, injection speeds, mold temps, gate suggestions, etc. They will get you in the ballpark. on their particular plastic resin, which can be very helpful.

Depending on the part, some parts are more or less tolerant of "regrind", which is granulated reject parts and runners, sprues. Some have troubleshooting tips on plastic parts and runners, sprues. There is a lot to learn with injection molding, but it's all readily available on the internet. You have to have patience and always sacrifice your time to prevent mold cavity damage. Always think in terms of "metal-safe", i.e. once you machine too much metal away, you cannot easily put it back.

If you ever get your plastic part stuck on the right side of the mold (the "A" side, or non-ejector side) then never use tools to pry out the part. The screwdrivers will leave marks or scars on the aluminum mold cavity surface. If you have to, remove the mold and use a propane torch, softening the plastic enough so you can gently pull out the plastic. You may even have to disassemble the mold base from the cavity depending on how much the plastic is stuck in the cavity.

There are things such as "sprue pullers" which are devices that will make sure the sprue pulls out on the ejector side of the mold when the molding machine platens open. Sometimes you purposely put little undercuts in the runner to make sure they stay on the ejector side of the mold. The ejector pins in the runner troughs will just pop the plastic out with small undercuts.

Don't be surprised when you first run your mold if you have to pull the cavities out and make adjustments. Sometimes you may pull it out a number of times to get it to run on an automatic cycle. Watch the mold run for several hours and be sure it is not going to flash before leaving it unattended. Make sure your mold metal temperature has stabilized before leaving it too.

There are a lot of accessories you may need with your new injection molding machine. You may end up buying a granulator to chop up runners and short shot parts, a temperature controller to heat up or cool the mold, a small chiller to run very cold water, a dryer to get the moisture out of the plastic resin if it is the type that must be DRY to mold properly. Modern Plastics is one magazine (there are several) that will educate you monthly on plastics, molds, machinery. Their website is a good source too.

When I first was introduced to injection molding 30 years ago, I worked with a custom injection molder, serving as a no-pay "helper" to learn the ropes, and picked up information just being present when my parts were being molded. It was a shop where they ran 3 shifts, and I had to run the night shift from 10 pm to 6 am. I would sit beside the machine taking each plastic part and inspecting it, advising the molder what was changing. Taking your mold to a custom injection molder to get it sorted out may be the way to go, rather than risking damage to your aluminum cavities. Although I have 7 injection molding machines now, I would recommend not just jumping into buying a molding machine for 4000 parts a year, until you understand fully what you are getting into.

Are you planning to join two halves of a box together, or a box with a lid? fasteners? radius corners? wall support features? holes or bosses molded into the box for mounting your electronics?