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Concept: The EZ-bake RepStrap

Posted by Labrat 
Concept: The EZ-bake RepStrap
November 24, 2009 07:10AM
I have an idea for reproducing reprap parts without a reprap.

I plan to use air-cure silicone (available at Home Depot and other hardware stores in caulking gun tubes) to make a reusable mold of already existing pieces to cast duplicates out of plastic (probably using polypropylene or polyethylene... whatever I can pull out of the plastics recycling bin).

First I'm going to try a proof of concept to determine just how accurate a copy this can make. I'll make my first attempt duplicating different sizes of Lego bricks.

I have two main concerns right now:

1. Chemical Compatibility- ABS is known to swell slightly in pure Glacial Acetic Acid. Air-cure silicone makes use of acetic acid derivatives (smells like vinegar until fully cured). Will the mold be of a true lego brick, or a deformed one?

2. Hidden geometry- This technique should work very well duplicating shallow legos, but the deeper ones may not come out of the mold without tearing it.

Once the molds are made, duplication should be as easy as melting the plastic, cooling, and demolding.

If this concept works its my hope that I could find someone who could LEND me a few of the more geometrically complex Mendel pieces. Given the current state of the Reprap community (far more people who desire printed pieces than those who can produce them), a method of making reprap pieces without a reprap could be all thats needed to allow the less innovative among us to make our repstraps.

The pieces most likely won't me as good as original printed pieces... but its my hope that they will be good enough to make repstrap machine that functions well enough to be able to replace all of its "baked" pieces with printed ones.

Thoughts? Comments?
Re: Concept: The EZ-bake RepStrap
November 24, 2009 07:34AM
Use an platenum based RTV silicone and urethane trying to inject molten plastic into a silicone tool will destroy it. They make urethanes and silicones that were engineered for casting. These urethanes have been engineered to have the same or similar properties as ABS or Polyethelyne.
Also heating your mold before using such urethanes will compensate for shrink of the material. I use all of the porfessional equipment to do this and can cast parts that are +/- .001 of an inch. This is a huge undertaking and getting a good bubbe free mold with a nice clean parting line is difficult if not impossible with out a prober vacuum chamber or a static mixer for the RTV.

I have wondered about using regular air cure silicone for molds, but have never tried it. I think it might be too soft and not have the tear strength needed for use in molds. However, if you can get it to work more power to you, here is a link to one fo the better urethane companies out there


They sell silicone and all of the urethane you could ever want.
Re: Concept: The EZ-bake RepStrap
November 24, 2009 08:10AM
... i tried with 2K-silicone (capable of 350 centigrades continuous and 450 centigrades short exposure) with moulding chocolate and roses metall (melts at 98 centigrades) - image atached. As i had only a small amount of Roses metall, it wasn't enough to fill the complete cavity, so the backplate isn't complete.

When no forces are present, the form is stable ... but even with the different load/weight of the metal poured into the form you can see some elastic deforming, so the dimensions of the chocolate and metal parts are something different because of the different weights ..

open | download - Han_Solo-Metallguss.jpg (138.3 KB)
Re: Concept: The EZ-bake RepStrap
November 24, 2009 08:58AM
used to have a kit made from parts made in molds. (Or moulds.)

We've got a write-up on it,
although I recommend "The Mmouldmaker's Handbook" by Jean-Pierre Delpech et. al.
and the intro PDF here:

I would use a polyurethane mold, cast concrete or similar in it, and I'd suggest making the mold at a community college or art school. Polyurethane is expensive to make mistakes in.
Re: Concept: The EZ-bake RepStrap
November 25, 2009 02:20AM
has the RepMan and that is laser-cut Acryl.

* homeprototype free 3d design repository
* Blog
* Google+
Re: Concept: The EZ-bake RepStrap
November 26, 2009 09:44AM
Thanks for the feedback and all the tips. The writeup on moldmaking that Sebastien pointed to looks quite useful. Although there was one other part to my project that I didn't mention. I'm trying to do this with limited resources (i.e. funding).

It would be awesome to have an unlimited supply of RTV silicone. But that stuff goes for around $20 per pound (approx 12 fluid ounces). I'm starting my project here with stuff that I already have lying around or I can quickly buy at the local hardware store.

That is not to say once I've exhausted my desire to innovate I won't go for specialty materials. However, it looks like it would take around $200 worth of silicone to make one complete set of molds for the Mendel. This may be one of the reasons why there doesn't appear to be a community yet for repstrappers mass producing molded parts.

Since I don't have the resouces to go into a full scale production of "reprap parts not borne of reprap" (damn I've been waiting for an opportunity to say that), I'm hoping to come up with an economical way to produce reprap parts for this moment that we don't have a community permeated with functioning reprap machines.

Also, for the moment I'm not looking at polyurethane as the casting material- for the same reason as above... specialty material, not entirely economical.

I am determined to see if I can make functional parts out of stuff that I can dig out of the garbage.. erp... recycling.

I have some lego bricks half coated in Silicone Sealant I (labeled 100% silicone - Window & Door) that I already happened to have on hand (I think that costs around $2-$3 per 10 fl.oz caulking tube). Granted, it comes out as a highly viscous goo, so I had to spend some time working the material into the nooks and crannies by hand. I'll work one side at a time and then after full cure split the mold down the center by razor blade. I'll keep everyone up to date on the success of the mold.

I plan to make my first casting using polyethylene (Melting Point 120-130C) using an old grocery bag as my source material. If that works I'm going to try casting parts out of PET and polypropylene. Wish me luck!
Re: Concept: The EZ-bake RepStrap
December 04, 2009 02:22AM
Just for reference, this reprap was built completely from cast parts cast by Bits from Bytes last year.


This was back when the original printed parts cost $2000 to $3000 (printed on a commercial RP system), so hand casting 110 different parts seemed like a good idea. Not many of these were made before BfB switched to the laser cut acrylic design; I'm assuming they switched due to the amount of labour involved in producing the parts.

It has worked pretty well for me; I've printed out 3 Darwins and a Mendel on it so far. The entire extruder assembly has been replaced with printed parts several times now, but other than that it's held up well.

I suspect at this point if you can get your hands on a set of parts to make the molds, you're probably better off just using the parts to build a reprap and print the copied parts out yourself.
Re: Concept: The EZ-bake RepStrap
December 06, 2009 08:44AM
I suspect at this point if you can get your hands on a set of parts to make the molds, you're probably better off just using the parts to build a reprap and print the copied parts out yourself.

And right there you've identified the conundrum. At the moment, putting together a reprap is as much of a social engineering challenge as it is an ENGINEERING engineering challenge.

There is currently a very small user base with fully functioning machines... with a comparatively enormous number of people who want to try putting together one. So right now it comes down to either know someone who has one personally (I do not), or show the community that you have something that you can offer for it.

My hope here is that if I can prove I have a reliable method where one can economically produce Mendel pieces for the masses, I can get a sponsor who is willing to loan me a set of Mendel pieces. So without the concept, its unlikely I can get someone to lend me the pieces...
Re: Concept: The EZ-bake RepStrap
December 06, 2009 09:26AM
Project update:

I've had some limited successes with my lego duplication proof of concept project... but nothing quite so spectacular that its worthy of a blog. Though at the moment I do have a less than perfect duplicate of a 2x1 lego piece (deep) cast from a polypropylene Olivio container and 3 bumps worth of shallow 1 bump wide lego cast from a polypropylene Rx bottle (clear orange).


On making a mold out of off the shelf 1 part Silicone sealant:

Minute details are easy... the "Lego" logo is clearly visible in each copy... unfortunately the major details (sharp corners, 90 degree angles) were a little more difficult to remove from the mold without tearing. My biggest success so far has been using a generous dollop of Petroleum Jelly on the original piece prior to enveloping in Silicone. Then after demolding, examine the mold to see where the jelly obscured some of the detail, add some more silicone and stuff the piece back in.

On casting materials:
Starting with recycleable plastics

Polyethylene (MP: 120-130 degrees C):

Grocery bags:
Melts very easily, but single sheets shrivel into almost nothing. Tried folding up a full bag and melting. Unfortunately the melted plastic is of a VERY high viscosity and can barely be shaped with prodding instruments, let alone flow into a mold.

Milk Jugs:

Less shrivel on melt.. .makes nice clear sheet. Once again difficult to shape, but not as bad as the grocery bag PE. After melting was able to force into a shallow lego mold by adding pressure. There may be a shrinkage problem on cooling. Worth further research.

PET (MP: 260 degrees C):
Source: Soda bottle (Mountain Dew bottle - green)
My oven just cannot get hot enough to melt this well enough to flow. Tried using a heat gun on highest setting. Could get plastic bits to melt, but without an oven to surround the mold with the high temperature, it is unlikely that this plastic will be suitable for silicone molds. Also... heat gun had tendency to blow away my mold and plastic pieces, and whatever I used to hold it down wound up getting stuck in the plastic. Its a shame too... as PET green melts into a pretty semi-translucent frosted green colored plastic.

Polypropylene (MP: 150-175 degrees C depending on length of polymer chains)

Currently found two sources from the recycling:

Tub margarine container (technically Olivio olive oil spread):
Nice flow upon melt. Best success to date. Best with a two part mold where one half contains the majority of the volume of the piece. Melt PP in high volume side then squish other half of mold on top.

Result: Well formed piece. Very ugly (the printing on the container mushed into the plastic). Deformity caused by a mismatch of the top and bottom mold piece (I'll concentrate on accurately lining up the molds later, right now I'm working on materials). Other deformity caused by trapped air bubbles. can probably be fixed by cutting up source plastic into smaller bits to allow air to escape during melt (I just stuffed sheets into mold this time).

Perscription Pill bottle (orange):
Results are similar to the PP margerine container. Nice flow properties.. much more attractive piece. Will try more with this one.
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