How to bring a 3D printed 2-cavity injection mold to life – Part 1

Making the most inexpensive injection mold

After having tried DIY injection molding with my 3D printed single cavity mold and some softer, rubbery-style thermoplastic with some good results, eg mold resisted the heat quite good and the cavity did keep its sharp edges as well as did not degrade over time I wanted to see if I could reach a next step.

I wanted to go for a 2-cavity mold and inject real ABS (the Playmobil/Lego plastic) into the mold.

Injected ABS

This is far more complicated than my first mold single cavity mold due tot these points:

  1. I needed to understand the required diameters of the sprue, runner and gate system
  2. Specific shrinkage of ABS during cooling
  3. get black ABS pellets in a diameter of 5-6mm
  4. higher temperature of 250 deg C (482 Fahrenheit) is required to melt the ABS
  5. higher injection pressure is needed because of
    1. material density and flow
    2. longer injection ways as the sprue is not sitting directly on top of the cavity – instead the molten ABS needs to travel a certain distance before it reaches the 2 cavities
  6. an ABS mold is normally heated to 40-80 deg C (104-176 Fahrenheit) to ensure a good material flow

All of the points are interesting and somewhat hard to solve if you have limited time at hand and can only follow them up occasionally.

And once you have learned how to solve one point you find out that there are more topics to study and to find out how to make it right.

In the end I have taken the old 80/20 approach for my DIY experiment and I distanced myself from the scientific industrial scale approach that always comes up with injection molding (steel molds, ejector pins, automated cut-off of the runners and sprues, automated cooling systems, etc) to reach my goal with the limited budget.

But lets look at the points one by one.

Understanding the required diameters of the sprue, runner and gate system

Unfortunately if you want to have good results from your DIY injection molding you need to make yourself familiar with the requirements of getting the molten ABS from the injection molding machine without any kind of interruption through the sprue, the runners and the gates into the cavities – not to forget that you need to plan to have some venting channels in your mold where the hot, compressed air can escape through. Otherwise this air will burn you plastic inside the mold and create burn marks and damage you mold as well.

After doing some extensive googling on the topic I found these two webpages that I would like to recommend to you which will solve this topic for everyone completely

Even though the website dates back a couple of years it contains the basics in a short and understandable way. Thanks for the write up.

Incredible and invaluable ressource for all my efforts in designing the right sprue, runner and gate system.

Easy to understand with all necessary dimensions and including perfect tablets and diagramms (I build an XLS file based on this knowledge in order to make designing the right dimensions easier).

Thanks to Bob Hatch for this brilliant work which made my mold possible whereas everybody said it cannot be done without a sophisticated scientific approach.

Please think about the fact the ABS tends to shrink about 0,5-0,7% in the cavities while cooling and hardening. So you need to inflate all your dimensions by this factor to achieve a product with the required dimensional accuracy.

Designing the mold cavities

I found out that the mold design requires some good computing power in Sketchup as objects need to be intersected/fused together.

As I do not have this power at hand I almost accidently came across the ebook „3D Modelling and Printing with Tinkercad“ from James Floyd Kelly which explains the process of creating a mold with Tinkercads cloud based computing – this makes it really fast and easy to create all negative spaces in the mold like the runner system and the cavities.

Please see all details in the chapter „Another Tinkercad Special Project“, pages 138 – 155 – the ebook can be easily obtained via the Amazon Kindle app.

With this inspiration it was easy to get to the final mold cavities in literally no time. You just align all necessary parts in the needed way, link all parts with the runner/gate system, add some very small venting channels (only on one side of the mold), define the parts that will be the cavities as holes and then group everything together – don´t forget to build in some alignment pins. These will hold the mold halves together in the correct way.

Now repeat this procedure with the other half of the mold.















Export an STL or OBJ file out of Tinkercad for printing at home or send it for professional printing to one of the 3D printing services that are integrated in Tinkercad.

Stay tuned for part 2 next week.


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