Using Your 3D Printer to Injection Mold Tiny Objects




Introduction: Using Your 3D Printer to Injection Mold Tiny Objects

About: Old inventor, reverted back to my 10 year-old self. A shop full of tools, a boat, race car, 3D printer and a beautiful wife who wants me to invent things for around the house... Now how cool is that?
This is an offshoot of another Instructable that documents the development of a previously unrecognized capability of fused deposition 3D printers.

Rather than subject readers to endless babble, dull research and boring experimentation, this Instructable will take you step-by-step from making a mold to the production of an extremely small, detailed part on your own 3D printer.   A part, well beyond the normal resolution of your printer, moving it closer to the rarified atmospheric capabilities of DLP and similar technologies.

This new capability does not give you the ability to print small and detailed parts on your printer directly from stl files, but it will allow you to reproduce an existing small and detailed part, using PLA plastic, with your own printer, at will... Over and over, any time you want. 

Extraordinary claims require extraordinary proof... Here's a video, taken in 1 shot of me producing an "Injection Printed" part on my printer:

Step 1: What You'll Need:

1. 3D printer:  Any fused deposition printer will work.  You will NOT be modifying this printer, so don't be afraid of ruining it.  Also, you'll need a reel of PLA. -  ABS might work, but it's higher melt temperature makes it more difficult to keep fluid.

2. RTV Rubber:Room Temperature Vulcanizing Rubber is one of the most popular mold making materials available.  It's a stretchable rubber that can be fast curing, forgiving about undercuts and readily available.  You'll also need the catalyst to cure it, a rubber to rubber mold release, mixing sticks, mixing container and a brush.  I get my RTV materials here.  You can also pick up good quality polymer clay here as well.

3. Knives:  Assorted knives to cut, clean and sometimes separate the mold halves will come in handy.

4. Dowel:  I use plastic blocks to build small RTV molds and make my sprews from those, but a short piece of 1/4" dia. wooden dowel will do just as well.

5. Modeling Clay:  You can use clay made specifically for mold making, or the clay your kids use at school.  Both will work, but The better the quality clay you use, the better your results will be and the easier time you'll have.

6. Something to make a mold with:  For this application, you'll need a ceramic container to make your molds in.  A few years ago, I picked up several boxes of Ikea candle holders for $4.04.  At the time, I hadn't a clue as to what they were, but knew I'd be able to use them someday.  I was right.  If you can find these, it's like they were custom made for the job... More on that later.  

If you can't find them, a ceramic demitasse cup should work.  Be creative.

7. Something to make a mold of:  Any small object you'd like to duplicate.  Keep it small... Money is a no-no.

8. Silicone Lubricant:  If you want your molds to last longer, silicone lubricant will help.

9.  Patience:  RTV takes hours to cure.. And you'll need to cure things twice.  There's also a high chance of an "iffy" result.  Depending on the quality of your mold, the item you choose, the length of time for the extrusion, the temperature and the equipment doing the work.

Excellent results are possible, but patience is an absolute requirement.

Step 2: Making the Sprue

The first thing we should probably make is a sprue.  A sprue is the pathway between the extruder on your 3D printer and the cavity in the mold.  It can be made out of nearly anything.

I use the snap together plastic pieces found in toy building blocks.  I also use building blocks to build the containers for the clay, my masters and pouring the RTV into, but that's another Instructable.

If you don't have toy building blocks lying around, a short piece of 1/4" dowel will work just fine.

1.  cut a short piece of dowel approximately 1" long.

2. whittle or sand one end into the shape of a cone that's roughly the size and shape of the nozzle on your printer's extruder.

3. Flatten the tip about 1/16" as shown in the picture.

That's it.  Your sprue is done.  You may want to cut it down once you've determined how long it needs to be, but for now, set it aside.

Step 3: Making the Mold - Part 1

This will be a 2 part mold.  2 part molds show detail completely around the object being molded.  Each half of the mold will cover half of the object you chose to copy.  


1. To start, begin by making a wire ring that fits the container you're using.  Nothing fancy, it's only there to help pull the clay and rubber out of the cup after it's cured.  A wire coat hanger will suffice. 

The ring isn't totally necessary, but makes things easier when removing the first half of the mold.

2. Next, take a small lump of clay and drop it into the container.  Press it down, getting it deep into the corners and all around the ring.  Make it as flat as you can on top.  I modified an old coffee tamper to fit my candle holders and that seems to work quite well.

3. Place your object in the center, on top of the clay.  Press it in about half way.  If your part has protruding appendages, you'll need to build the clay up a bit under the parts that stick up.   Make sure you don't cover anything over.  Ideally, you want to be able to look straight down at the clay from above and see the entire perimeter of the part.  

If the clay is covering a portion of your master, push it back so the part is exposed, even if it's below the surface.  Be sure the part is snugly surrounded by clay, that any openings or raised parts are exposed and supported and there aren't any large undercuts.

4. At this point, using a pencil eraser or similar object, press a couple of locating holes into the clay.  These will keep your mold halves lined up when it comes time to inject plastic.

5.  if you're unfamiliar with the container you're using, now is a good time,to determine what the volume of molding compound you'll need is.  Don't worry, there's no math involved... Just coffee.  Because coffee is dark, it will be easy to see in light colored molds.

6.  pour coffee into the mold container until it's level with the top.  Then transfer it all into the measuring cup you're going to use to mix your RTV.  That's it.  Note the level and mix the same amount of rubber when it's time to fill your mold.

6. Now carefully wash the mold with a soft brush, soap and water.  Because it's dark, it'll be easy to insure you've gotten all of the coffee out before pouring RTV.  Clean the dirt and grease from your hands as well as long as you're at it.

7.  The half of the mold you're pouring first, will determine when to mount the sprue you made.  The gun carriage I'm making will have the top half of the mold poured first.  That means I'll have to mount the sprue to the master before the first pour.  You can see where I mounted it, using super glue, pointing straight up and somewhat centered in the container.

Note:  If you're using your great aunt's priceless gold and diamond earring as your part, don't ruin it with superglue.  Use blue tack, or whatever they call it in your neck of the woods that parents use to hang all the stuff their kids bring home from school without ruining the walls.  This stuff is incredible. I'd be lost without it.


Mixing silicone RTV is a sorcerer's mystical combination of weight, volume, time and luck.  I never bothered too much with it all, discovering early on that "more is better".  We already know how  much rubber we'll be needing.  The problem comes when attempting to guess how much catalyst needs to be added to cure it.  My method seems to work pretty, pretty, pretty well.

The instructions say "add 10% by weight"........................        Ya, Right.  

I totally guess the 10% part.   Then I add half as much again.  Seems to work every time.  Of course, the RTV sets up pretty fast the way I do it, which means the bubbles have to be removed ASAP.  No problem.  I have that covered as well.

You need to keep bubbles to a minimum.  You can help by pouring the mixture from the side, allowing the thick liquid to flow into and around your part.  Don't pour it directly on top of your part, whatever you do.  You'll carry a lot of air along with it.

Once you've filled the container with silicone, you'll want to get as many bubbles, trapped in the mixture out, or at least up from the bottom where they can't do any harm.  You can repeatedly slam the mold down on a hard surface if you're OK with possibly breaking things free.  I do that a couple of times just to break some of the more stubborn bubbles free.

Then, I use a hand sander to vibrate my mixtures.   Placing the sander upside down on the bench (with no paper in it), turn it on and balance the ceramic cup on top, close to an edge, letting it bounce around like a happy goat.  You'll see bubbles come to the surface and burst open.  A few minutes of this and the bubbles will drastically reduce both in size and number.

Now, if you measured the catalyst according to the instructions, you'll have to wait here a little longer while your rubber cures...  It should be ready in about 24 hours.

The rest of you, read on...

Step 4: Making the Mold - Part 2

In a few hours, your mold will be firm to the touch.  if it's still slimy feeling, you didn't add enough catalyst, so you'll have to go wait in the hall with the others.  Conversely, if it started setting up before you got all the bubbles out, you'll have to tell everyone that those little warts all over your parts were planned, and then re-evaluate your concept of 10% + 5%.

Now, I know someone is going to point out that adding too much catalyst shortens the life span of the molds.  The same kind of someone who once told me that I was apathetic.   I told him that I didn't know what that word meant... And I didn't care.

Yes, it does shorten their life, but these are silicone molds, and they shouldn't be used for long-term production anyway.  When I make a mold, it's for a job.  After the job is done, the mold is no longer needed.  I can only use so many cannons anyway.

OK, enough horsing around. Back to work:

Pulling and preparing for the second pour:

Pulling a mold with the wire is easy. Clay has a tendency to cling quite strongly to the vessel its in.  the wire makes short work of pulling it up and out.

1. Using a sharp knife, cut around the perimeter of the mold, cutting straight down along the wall of the cup, going as deep as you can.

2. pull gently on the wire and the parts will come out cleanly, or at least more cleanly than they would if you didn't have a wire.

3. Peel the clay away from the silicon.  The sprue, if it was a part of this half of the mold and the part will stay in the silicon portion of the mold.  The wire needs to come out.  You can either thread it back through the silicone or snip the silicone around the wire.

4. Toss the clay and thoroughly wash the silicon, getting it ready for the next pour.  If the sprue sticks out beyond the surface of the mold, cut it so it sits flush with the silicon, or completely remove it and fill the hole with clay.

5. You won't have the luxury of the wire this time, but after you read the next step, you'll understand why you won't need it.

6. Warning:Paint Rubber to Rubber Mold Release on all surfaces of the silicon and part before placing the silicon mold with the part attached back into the ceramic container.  Silicon does not attach to any surface (the reason you won't need the wire).

Correction... It does stick extremely well to one type of surface... Silicone.  Be sure you use the rubber to rubber mold release... if you don't, you'll end up holding a solid chunk of silicone, with your part securely locked inside.  It'll completely ruin your day.  But the good news is; because silicone doesn't stick to ceramic, it'll slip out of the mold container with no trouble at all.

Pulling and preparing for "Injection Printing":

1. With the exception of the mold release, you'll be filling the second half of the mold the same way you did the first.  Only this time, when you're ready to lift it out, cut around the sides as you did before, then gently pry the part upward.  It will slide out of the cup, pretty as you please and if you have used mold release, as you should, you'll be able to find the seam and peel the two halves apart.  Your model and sprue (if you left it), will be embedded in the silicone, but because silicone is so flexable, you'll be able to pull the part out with no problem.

2. If you replaced the sprue with clay, you'll want to wash the mold before you take it to your printer.  Let it dry and get ready to experience a new way to make parts with your 3D printer.

3. Speaking of sprues... It's important that the sprue be as short as possible, so the tip of the printer's nozzle is just outside the cavity of your mold.  If yours is too long, don't dispair.  Take one of your sharpest knives and simply slice away some of the top of your mold, making it thinner.  The top only needs to be 3/8" thick at most.

Step 5: And Now, Off to the Printer...

Congratulations... You've just made a two part, injection molding die for your 3D printer.  Injection molding is a process invented over 100 years ago and is probably how the majority of the plastic things you've ever owned were made.

We're going to use that same process to make a plastic part on your 3D printer, using the mold you just made and your printer's extruder.  I don't know if this can be called "injection molding", so I'll just call it "injection printing" instead.  Maybe he term will stick.

True injection molding machines would destroy your ceramic and silicone mold in a split second if you attempted to use it with one.  These machines extrude plastic using tons of force, which is why they require steel dies (molds).  

Your 3D printer also extrudes plastic.  Far less powerful, it sends out a plastic "thread" at about 20mm per second onto your build plate.  We're going to use that weaker force, along with heat, to keep the plastic flowing long enough to fill the hollow cavity inside your silicone mold.

The shorter the filling sprue is, the better your results will be.  Use the drawing to check your work.  If your die is too heavy on top, a razor knife, like the one you cut your mold out of the ceramic container with, should work.  Slice the top of the mold around the outside as I show in the picture.

Here are the steps you'll take to make your first "injection printed" part:

Note: These steps will vary from 3D printer to 3D printer.  Mine's a MakerBot Replicator, but whatever printer you have, you'll have the equivalent controls for your printer.

1. Turn on your printer.  This process works best with PLA material, so make sure that's what's loaded on your machine.

2. Turn on your preheat.  If your printer has a heated build plate, you might want to turn that off.  In a minute, your hand'll be in there and it wouldn't be right if you burned it.

3. Bring the extruder up to temperature (mine is set to 225c, but it might work better if I raised it a bit).  It's a good idea to test the "load" process for a second or two to insure the plastic will flow, as well as clearing any hardened PLA out of the nozzle.

4. Lower the build plate or raise the extruder a couple of inches to get it out of the way.  

5. Place your mold under the extruder and align the printer's nozzle up with the hole on the top of the mold.

6. With your hand under the mold, push up, sealing the nozzle against the silicone mold.  The quality of this seal seems to be what makes the difference between a successful "print" and a dismal failure.

7. Now, start the load function.  The time it takes to fill the mold depends on the size of the cavity.  Times will vary from 15 seconds to a minute.  Experimentation is the only way to find out the correct timing for each mold, however, it's usually better to over-fill the mold than under-fill it.  If you over-fill, there'll be a lot of flash to clean up, but that's better than having only half a part.

9.  Patience... Don't attempt to open the mold until the plastic in it has cooled and the part isn't rubbery.  Carrying the mold to the sink and running cold water over it speeds the process up.  If you try to open it up too soon, you'll only burn yourself and ruin the part... Patience.

10. When it's cool, pull the part out of your mold and rejoice, knowing you have exponentially improved the capability of your printer from this moment on.

11. Every once in a while, possibly every time you're done using it, spray your mold with silicone lubricant.  It rejuvenates the mold and will make it last a bit longer.

12. Sign on to where I'll be keeping everyone up-to-date on the results of my own experimentation.  There you'll find the latest ways to help guarantee great parts.

Step 6: What's Next?

The process of using a 3D printer's extruder to create injection molded parts is still being explored, developed and experimented with.  If the silicone mold works for you and you'd like to experiment with more things, you might want to keep up to date via this  Instructable.  Check it out, add your thoughts and expertise to the knowledge base.  As what we know increases, so will the capabilities and value of our 3D printers improve.

If you went through all the trouble of making a mold and it didn't work for you, you're in good company.  there are a lot of variables to this process.  Even plastic manufacturers will admit it's sometimes like a black art.  Try adjusting your heater's temperature, the time, the heat of your mold.  I'm having difficulty with one part because I'm experimenting with the mold, trying to eliminate all the flash points.  I was having no luck.  Instructables member quixel suggested using a hot glue gun... I did, and the part cast.  Now, I'm trying to replicate the same conditions with the printer.  I know it prints, I just don't yet know how to do it with PLA.  My advice? Keep trying, keep experimenting and have fun.

We are standing on the front porch of a new era, with the rising sunlight of innovation, filtering through the trellis and comfortable bamboo shades of yesteryear.  

Just as the printing press, radio, automobile, airplane and industrial revolution lit up the grey floors of our ancestor's front porches, so will the dawn of digital technologies spread new light across ours.

Welcome to our house.  Come on in.
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    QJ Neo
    QJ Neo

    8 years ago on Introduction

    Maybe you can try to put the mold on the printing bed and then modified the code of the 3d printer so that it can lift the mode to match the extruder. So that you can do other works when waiting the extrusion process. Furthermore, you can put more than 1 molds on the bed and then tell the printer to extrude on each mold automatically. That's my idea :)


    9 years ago on Introduction

    I am absolutely blown away. Your bio is AWESOME. It sounds like you've had a wonderful life and is paying off 10 fold! I have no idea what you did except the end result which I am in awe over. I know in 2-3 years, heck, maybe less...3D printing will be in all homes! I saw them create an ear out of stem cells and 3d. BRILLIANT to me...probably expected by you...keep having fun and creating!


    Reply 9 years ago on Introduction

    Wow... You make my head swell. Thank you for that, but you're waaaay off base about me... Just ask my wife :)

    I'd written that within 2 years the technology will be available in big box stores, but I see a 3D printer can be picked up at Staples right now ( Staples is a large North American office supply chain). I'll bet it won't make much of an impact because of limitation and cost, but it's a first step. And unless another, more efficient technology happens to come along, layered home cmanufacturing will most likely be the means of future instant shopping gratification and a center of medical advances.

    Thank you again for your undeserved comments and open-minded clarity of what the future will be.



    9 years ago on Introduction

    Sorry, don't know what the problem is... My computer is down so I have to look at the world through my iPod for a day or two. I'll straighten it out then. Until then, here's a direct link:


    9 years ago on Step 6

    Seems to me, you could skip the expensive part of this build and just use a high temp hot glue gun to inject into the mold for similar results.


    Reply 9 years ago on Step 6

    Hey Qwixel:

    Have you tried this? I know there have been attempts at hand-held extruders for 3D printing on the fly, but they use motor power to be able to produce an even thread of plastic. If you use, as you say, a hot glue gun, modified to accept plastic filament, then a hand-powered version, while not very good for 3D printing, may work exceedingly well as an injection molder. This should be looked into. Great idea.... I think you're my hero.


    Reply 9 years ago on Step 6

    No, I have not tried a hotglue gun with PLA or ABS filament.

    I would just use the high temperature hot glue. It can set up pretty firm, and since you just appear to be injection molding a plastic bit, it seems like using high temperature hot glue would be just as effective, a lot cheaper and a lot simpler.


    Reply 9 years ago on Introduction

    Turns out, Reactive urethanes have been out for about 5 years and is the technology I was referring to. Go figure. These LOW temperature, air curing resins would probably work great, if I could afford the equipment to use them... However, I came across a polyurethane, 1/2" gun loading stick that just might do the trick. I don't know if this is the same stick you use, but it appears to have attributes I need. I'm trying to find a source where I can order a few for experimentation.


    Reply 9 years ago on Introduction

    Thanks for your response.
    You're the first person to give me a valid critique. I need more responses like yours to help me improve this idea.

    What you suggest certainly is true and would also allow the pressure to be controlled. The problem for me is, I make models for museums (the reason for those guns) and hot-melts are wax based polymers. Not so good at longevity. Wax based things, especially small ones, are also susceptible to large amounts of shrinkage (heat-shrink tubing is wax based). Wax is also difficult to paint with water based paints, which is what I use, for a long list of reasons.

    I may try your suggestion just to see what happens with one of my silicone molds. If you know of a non-wax based hot melt, please let me know.

    And now that I've thought about it, I might attempt to modify a hot melt gun to accept PLA filament. That would be the best of both worlds, don't you think?

    If someone else has or has had the same thought, it would be interesting to see what's been done in that direction. I know there's at least one product being prepared, but that has both the heating element and extruder... I like the idea of a hand controlled extruder better. Hmmm.

    Maybe this is something you could do as well? Seeing as it's your idea.


    Reply 9 years ago on Step 6

    I think you are I get much different hot glue sticks. My hot glue sticks are thermoplastic, not wax.

    I have not used them for injection molding, but I have succesfully used them in a number of structural places including car body repair. I would skip the low temperature materials and go straight for the high temperature sticks - they have a LOT of mechanical strength once cooled.

    As for a hot glue gun using PLA -

    already exists.


    Reply 9 years ago on Step 6

    I believe this gun (I remember when they were developing it) uses an extruder that's motor driven. That's how they can get it to produce a consistent thread of material coming out of the nozzle. The idea of using hand pressure, as the glue gun does is what's different and unique. Hand pressure as a means to produce a consistently "perfect" thread would be impossible. but to fill a mold with plastic, there is no need for precision... Only pressure.

    I'll check out the glue sticks, thank you. if they are as you say they are, then I'll try them. The nice thing about PLA is, it's hard and tools like nylon. I'll order a different color other than clear, which is giving me fits under the magnifier trying to clean parts up.


    Reply 9 years ago on Introduction

    Sorry... I just re-read the first two paragraphs in your comment above (I was going out the door and just caught your last point). I know there has been on hot melts with modified polyethylene waxes that "cure" after being heated allowing for stronger bonds. Eastman Kodak is one of the companies that was involved in the experimentation. I'm not a chemist, but the extremely low molecular weight of polyethylenes is probably their big weakness, and may be what gives them their wax-like properties. I don't think natural wax has been used much since the 1970s, but man-made wax still has all the characteristics that prevents me from using it for models. I'll check around to see if curable polyethylenes are being used. Possibly in those high temperature sticks you use for body work. If they are curable, I'm going to thank you again, make some models for the Parris Island Museum and put them to use on my fiberglass car as well :)


    Reply 9 years ago on Introduction

    Sorry to have the reply above your comment oldmicah. AutoDesk's new instructable app seems to only allow new comments and no replies. Back at my computer so I can correct that... Thanks for the kind words :)