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Custom 181 hood KDF emblem, CAD to 3D Printed in ABS plastic
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Welt
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Joined: December 16, 2011
Posts: 107
Location: Finland
Welt is offline 

PostPosted: Mon Apr 17, 2017 1:27 pm    Post subject: Re: Custom 181 hood KDF emblem, CAD to 3D Printed in ABS plastic Reply with quote

Dropbox dropping hotlinking for all users has screwed me over royally on the stuff that is not 100% Samba gallery worthy, combined with the fact that one can't edit old posts. Guess I can do it this way, quoting a huge wall and editing those for the meantime! Nothing new to post this time.

Welt wrote:
Here's something about mould making. I'd highly suggest the OP or anyone who has a printed part to do something similar (using silicone as the mould material and casting other resins into it).

I'm making a wax injection mould here, so one print can be replicated ad infinitum. Doesn't need much. A piece of plastic pipe, locating pins or some modeling compound (something like play-doh or similar) and some tape pictured here.
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Tooling material in this case is 1:1 mix ratio fast cast 2K polyurethane filled with aluminium powder. Polyurethane is a bad thermal conductor, so the aluminium is added to increase conductivity, reduce shrinkage etc.
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Mixing powders into the resin (part A in this case) will introduce lots of air into it, so one should give the filler mix some time to bubble air out or put it into a vacuum chamber if applicable. As this is will be a waxing mould, I only waited a couple of minutes and went ahead. Some air bubbles will float to the top in most cases but in this case it does not matter in any way. The final mixture here has a potlife of about 2-3 minutes-> better double check that everything is in order before adding hardener.
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Polyurethanes are exothermic and the mixture will reach a temperature way over the wax patterns melting point in a couple of minutes. Does not make a difference, in the end. Mixture is so viscous at that point and the molten wax has nowhere to go.
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First mould half is finished and flipped around after approx. 30min (demoulding time).
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Some mould release (silicone) is sprayed on the first half and all is ready to cast the other half. New mixture doesn't really stick on old, hardened material, but it will be a PITA to separate the halves if nothing is used.
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Here's the final product after removing the pattern. The mix fully cures in a few days and is machinable, so I'll drill injection ports etc. in a few days. Some trapped air bubbles in the first half, but those were filled in the other.
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The surface is already quite nice due to the accuracy of the particular 3D printer I used, but I think I'll use some fine sand paper to finish surfaces and remove any flash.
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If the same kind of process pictured above would be used with flexible silicone mould material, that mould could be used to cast polyurethane or other resins. Finishing one ABS print with acetone or sanding etc. and doing a mould would give one easy way to reproduce these. Painted resin casts will be also much more durable than a somewhat porous 3D print. But in the end, this is just something to think about Very Happy Everyone has freedom to do what they wish with their equipment and time.


Welt wrote:
Solomon Grundy wrote:
Welt, that is so cool. Glad you are having fun and sharing your experience with the group. I think that looks great! If time allows I may try that myself. Can't wait to see some of the first molds. Very Happy


IMO casting in general is often forgotten or even somewhat looked down upon as a process, so I think it's fun to open peoples' eyes on what they are easily able to do if they just want to. Writing about it all is half the fun too. The following is not really possible for most people though, but the plaster mold part should be simple enough.

Surfaces lightly sanded, smooth Very Happy
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This geometry works quite nice, but no rounds or drafts makes the wax stick to vertical surfaces. Not such a big deal in this case, but if one would design a part just for this process these features would aid a lot.
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Multiple pieces are asembled, a complete wax pattern.
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In this process, the wax pattern is dipped into ceramic slurries (fine ceramic powder+colloidal silica) and stuccos. A face coat of zirconia can be seen here. Multiple coats of coarser molochite after that.
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After each coating step, the molds are left to dry. After enough wall thickness is achieved, molds are dried for a longer period. Wax removal is done by flash firing in this case. Furnace temp. set at around 650C and molds are put in. The temperature is so high that the wax has no time to expand and break the molds in their green state. After all wax has dripped out, furnace temperature is increased and the molds are sintered for a certain period of time.
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Wax will either flow out or burn, so only a clean hollow cavity is left.
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Sintered, high temp. molds would be good to cast straight up, but these will wait a while for something more important to come along to justify melting a full crucible of aluminium. Placed in a preheating furnace here.
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This is technique is easily available to all, so why not make one for show. Simple castign channel, part and a small plastic bucket.
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Personally, I'm not too fond of plaster casting as it requires a much more careful heating control. This mold plaster was mixed in 1.4:1 ratio to water and just poured inside the bucket. Plaster is somewhat similar to the polyurethane I used in the earlier steps as the mixed potlife is quite short. Also, old plaster in mixing container will start the hardening reaction super quick, so clean equipment is a must.
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These will be cast one day, so I'll post pics of the results when that happens.


Welt wrote:
Casting day came a little early this time.

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Most of these had one cog tooth missing. Looks like one of the "knobs" block wax injection somewhat and air is trapped inside. Thus, slurry penetrates into the air filled cavities. That combined with the fact that those little holes are hard to fill with slurry and stucco -> parts of the mold might break and pieces will rattle inside where there should be metal.
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That's why I filed the knobs down and started to do small air channels. Will experiment some more when I'll have spare time. Next ones will certainly be better Very Happy
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Welt wrote:
Sweet, seems that the shipment arrived in one piece!

While cooking another mold, tried detailing one from the first. Looks quite nice when you don't look too closely on it Laughing

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