Looking for info to get started with injection molding

Hi all,
I'm currently looking into getting a small injection molding machine to make consumable sandblasting fixtures in bulk. They will be for use inside the company, so I'd like to see if I can make the molds myself. The fixtures could be modified to have draft angles very easily, and ABS seems to be the closest analogue to the PLA+ I've been 3d printing the molds in, but each pair of printed parts takes approx 3 hours. Those numbers are especially inconvenient when I'm being asked for 24 pairs of fixtures, and each individual part has a different sandblast fixture. From my 3d model, the sandblast fixtures are in the neighborhood of 50cm^3 to 70 cm^3.

Would a BOY 50E be a good starting mold machine (76.5cm^3 shot size according to website)? Or should I err toward a larger shot size in case I need a larger part made someday?
We do have cnc machines in the factory as well as a tool room, so simple fixtures in aluminum should be feasible.
Would molding simulation software be worth it?
Is UHMWPE an option for injection molding? The sandblast fixtures were originally machined out of that before I switched to 3d printing in PLA+.
Would I need some kind of plastic pellet dryer to feed the machine from? If so would it be best to buy or design my own?
How do you decide how large to design your ejection pins?

I'm aware injection molding is pretty complex, but I'm also looking at this as a learning opportunity, since my company does do injection molding in a different one of our locations, and it would also be an exercise in machining.
Any info you could give me about getting started would be much appreciated.

Thanks,
Max I'm still just learning injection molding myself, so I won't try to answer things I don't know, but I can address a few of your more basic questions.

But first, does your part need to be solid? Injection molding works best with parts with uniform wall thickness. Non-uniform and thick walled parts are subject to defects such as warping, voids and sink marks. Can you redesign the part to be shelled out? This would also reduce the material used and size of the injector required.

Typically you want to use 25-80% of your shot volume for your part for standard plastics (PP, PS, etc) and 25-60% for engineering plastics (PC, PA, ABS, etc). Also, in addition to shot size, the length to diameter ratio of the screw is important to ensure the material is fully mixed and melted uniformly.

You'll need to calculate the clamp force needed based on your part dimensions.
Clamp Force Required = Molding Pressure x Sectional Area x 1.3

The 1.3 is a typical factor of safety. For ABS, a typical molding pressure is between 300 - 500 kg/cm^2.

You will need a drier for ABS.

Making the molds is another can of worms entirely. There are several guys on here with heaps of experience with that so I'll let them chime in. I imagine you'll get a lot of "you're in over your head".. which is probably true (it certainly is for me).. but don't let that discourage you if you have the drive, resources and opportunity to make it happen.

There is surprisingly little useful information about injection molding on the internet. Here are some books that I'd found to be helpful.

Injection Molding Handbook by Donald Rosato
Injection Molds and Molding by Joseph Dym
How to Make Injection Molds by Menges/Mohren

Would a BOY 50E be a good starting mold machine (76.5cm^3 shot size according to website)? Or should I err toward a larger shot size in case I need a larger part made someday?

As mentioned if shot size is close move up to next size machine, don't go way oversized, polymer heated in barrel for too long can degrade.
But larger machine can run a mold with multiple parts instead of one.

We do have cnc machines in the factory as well as a tool room, so simple fixtures in aluminum should be feasible.

Then why dont you machine them from solid?

Would molding simulation software be worth it?

No, and its very expensive, if you don't know what all the technical parameters are its worthless.

Is UHMWPE an option for injection molding?

Yes

Would I need some kind of plastic pellet dryer to feed the machine from? If so would it be best to buy or design my own?

ABS?, Yes, any hydroscopic polymer

How do you decide how large to design your ejection pins?

The larger the better, then it reduces the chance of part pushing issues, there are a few. Some say you can never have too many ejector pins, I disagree.

I'm aware injection molding is pretty complex, but I'm also looking at this as a learning opportunity, since my company does do injection molding in a different one of our locations, and it would also be an exercise in machining.
Any info you could give me about getting started would be much appreciated.

Thanks,
Max

This is a doable task, you can just buy a pre fab Aluminum mold and go to town, if it gets messed up or needs changed, that's what inserts are for.

Hope this helps

each individual part has a different sandblast fixture.

If you go the injection molding route that sounds like lots of molds and setup time.

What about casting these mask fixtures from urethane or silicone? Like @Houdini said, rubber stands up better to blasting than hard plastic. A silicone or flexible urethane mask might last a long time.

As a first step you could try making a silicone mold using a drafted version of the 3D printed mask, pull the print part out of the silicone once it's cured, and then pour resin into the cavity. I haven't tried the 3D printed aspect of this, so not sure about getting the printed part smooth enough to release. Vapor smoothing might help if you're set up for that.

A more advanced version could involve machining either an aluminum mold or an aluminum version of the mask to make a silicone mold.

It looks like you're clamping the 2 masks onto the parts with a pair of tie bolts. A metal bar on each outside end would prevent bowing if you move to a rubbery material for the masks themselves.

I'd try a silicone mask first and see how it stands up to sandblasting. If it wears out quickly then move on to urethane. Urethane is much cheaper than silicone, but it's a bit toxic and sensitive to moisture exposure during storage and mixing. The internal injection molding we do is for solenoid coil cores, which are off the top of my head about 1" x 0.5" x 1.5", and those molds are made by an outside company for about $25k last I checked. I don't think their machine can fit a mold for a part that is about 8 inches long in the longest direction.
As for the parts, I believe my coworkers would like each part to have a set of 24 pairs of fixtures, and at a glance there are dozens of parts we already have machined fixtures for.
The fixtures are "clamped" together with rubber bands, but most of the gripping is a tight fit on the bores gripping the parts just so that sand doesn't get where we don't want it.
I'm not sure I need a brand new machine, is there any reasonable way to verify a used machine will work other than just seeing it run a part?
Can injection molding do any stiffness of rubber? It seems to me that if I got the flexibility right a thin walled fixture could be intentionally undersized to grip the part by stretching slightly and then not require as tight tolerances as a hard plastic part would.

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