Author Topic: Lost Foam Casting: a Crankcase in Zinc Alloy (Read 65023 times)

mattinker · « **Reply #25 on:** August 10, 2014, 02:22:10 AM »

Al would be great for casting Zn and Zn alloys.

Regards, Matthew

vtsteam · « **Reply #26 on:** August 10, 2014, 07:14:31 AM »

Feral as in not being part of the social conventional but also not apart from it in a way that uses what that society discards to support something creative.

Yes I was thinking of aluminum as one possibility for a material. Though I am sure that industry has probably already tried it somewhere early along the line of die casting technology, and moved on. Still, they didn't leave notes. So, maybe I'll just go ahead and repeat their early mistakes....

I have about ten experiments I'd like to perform today. Too bad I'm not ten people!

My big problem is I can already smell winter coming, and I shouldn't be doing any of this. I should be figuring out how to make an accessible heated shop in winter, or all this will stop for 8 months as it did last year. Time is slipping by.

Joules · « **Reply #27 on:** August 10, 2014, 09:11:27 AM »

Figure a way to burn snow

vtsteam · « **Reply #28 on:** August 10, 2014, 05:26:10 PM »

Had another go at zinc today. Altered the pattern, and it went well. This time I've got good fidelity -- the texture of the foam is plainly visible. Even the little blob of hot glue attaching the sprue got cast as zinc.

mattinker · « **Reply #29 on:** August 10, 2014, 05:36:01 PM »

That looks satisfying!

Regards, Matthew

vtsteam · « **Reply #30 on:** August 10, 2014, 05:52:55 PM »

It is Matt. Here it is off the sprues:

Manxmodder · « **Reply #31 on:** August 10, 2014, 05:57:45 PM »

Quote from: mattinker on August 10, 2014, 02:22:10 AM

Al would be great for casting Zn and Zn alloys.

Regards, Matthew

Matt,I can confirm that aluminium works well for zinc casting molds.

About 20 years back I made some latch components for a customers double glazed windows to replace the original plastic items which were prone to fatigue failure.

The mold I made was a several piece jigsaw affair milled from aluminium,screwed together and preheated before each pour.

The detail and tolerance of the latch components was really good with the milling marks from inside the mold being clearly reproduced on each of the components.

Steve, that last cast you've done looks really good,looks like you've mastered that technique.

I'm looking forward to seeing more of this project.....OZ.

vtsteam · « **Reply #32 on:** August 10, 2014, 07:35:09 PM »

Thanks Oz!

In case it helps others, these are my notes re. zinc and lost foam on this scale casting:

Pour temp 450C

A sprue is needed every 35 mm or so on perimeter of a 6mm wall, 2 spue minmum before loss of detail.

Max depth of 6mm casting wall is about 25 -35 mm before too much heat is lost.

Horizontal 6mm plate doesn't work. Keep mold cavities vertical.

Deep pouring basin/riser needed -- there is a great deal of shrinkage and this kind of thin casting needs good head pressure to maintain detail.

Use plaster of Paris in preference to drywall (calcium carbonate). It speeds up the process (pattern can be cast in 3-4 hours including cure and open air drying instead of 24-36 hours open air drying. )

And, critical difference noted today -- young plaster of Paris absorbs molten polystyrene, which drywall does not, thus removing it from the casting. That enabled good fidelity in this margnal low temp lost foam process. While zinc pour temps can't flash the foam into gas as aluminum can, poP makes up for this by absorbing the liquid polystyrene away from the melt. Calcium carbonate traps the polystyrene inside the mold.

mattinker · « **Reply #33 on:** August 10, 2014, 09:12:30 PM »

Are you using pure Zinc or a ZA12 alloy?

Regards, Matthew

vtsteam · « **Reply #34 on:** August 10, 2014, 09:21:43 PM »

Zamak 2

Eugene · « **Reply #35 on:** August 11, 2014, 04:17:18 AM »

Steve,

May I ask a question?

How do you make such beautiful moulds, their accuracy and sharpness looks excellent. I'm on the beginner slopes with hot wire cutting and my efforts look distinctly agricultural when compared to yours. Could it be a CNC foam cutter?

Eug

vtsteam · « **Reply #36 on:** August 11, 2014, 07:09:22 AM »

Eugene, the pattern is the most important part of any mold. People often get caught up in the technology of sand, or furnace refractories, burners, metal composition, gating and risering, fuels, etc. in casting. Yet knock a couple pieces of wood together for a pattern.

Casting starts with the pattern and its finish.

I spend a lot of careful time with very fine sandpaper, even on the foam cutouts to get them as good as I can before using them. More time than they took to cut out by machine. I use little strips of sandpaper under a strong light. Oudtoors on the porch in summer is perfect!

I do the same with wooden patterns, and use filler and sanding sealer coats, each sanded down, and several coats of spray colored lacquer to finish. Most of my practice for this kind of thing comes from building model airplanes, which I've been doing as a hobby since I was 14.

Oh one other thing about cnc hot wire foam cutting. You need to go VERY slowly for a machine process. 1mm/sec to get good quality cuts. and you need thin wire and low temps to match. I use .011" stainless steel fishing leader as a hot wire.

If you don't have a good pattern, all the time and money in the world invested into the latest metal casting forum technology will just get a person a lump of scabby metal. Sand break outs happen because the pattern is rough. Inclusions ditto.

Conversely, I've found that the crudest casting equipment will turn out good faithful castings with good finish if the pattern is good and the technque is sensible. My iron foundry is made from ordinary firebrick, and clay and sand. The fuel I use to cast in aluminum is store bought charcoal briquets or home made charcoal. For the last few zinc castings I just used wood for a fuel. Ordinary plaster of Paris with no additives, no bake out, air dried for a few hours and ordinary scraps of foam were used for a pattern.

I like simplicity, and I am impatient, and I hate to waste things. I use what is available and try to do a good job with it. I study what happens and I gradually think through why it didn't go as I wanted. Eventually the problems get worked out by trial and error until I understand my own equipment and materials.

Anyway, it is great that you ask, and the answer is a lot simpler then I've ended with here. Just take a little sandpaper and do a very careful job, with a light touch. A few times through it, and you'll get good at it. It's nothing more than that.

vtsteam · « **Reply #37 on:** August 11, 2014, 07:52:18 AM »

Eugene, re-reading your question, I see you probably don't have a cnc foam cutter, and that isn't a problem for getting good patterns at all -- you must be using a hand bow, or vertical fixed wire. I did that for many years. The main thing again here is to have good templates --smooth ones, just like the need for smooth patterns in casting.

The way I made foam cutting templates was to cut them out of sheet aluminum with tin snips (after pasting a paper printed pattern on top) and then filing to the line with a fine file. The fine file is essential.

For small patterns (rather than wings) a vertical wire and table is best. After that comes sanding the foam pattern, as it does with CNC, as well.

Low temperatures, slow smooth movement, thin wire are all helpful, but the blank is only a blank, and hand work refines a pattern to what it should be.

awemawson · « **Reply #38 on:** August 11, 2014, 08:55:05 AM »

Steve,

I don't think that we've had the pleasure of pictures of your CNC hot wire cutter. It sounds interesting, any chance that you could oblige?

vtsteam · « **Reply #39 on:** August 11, 2014, 09:55:33 AM »

Hi Andrew -- my wire cutter is just a plans built wooden drawer slide style wing cutter -- quite crude, and looks like the original here:

http://www.drayconstruction.com/foamstuffs/

except that I used 3/4" birch plywood instead of the plans shown MDF.

It's not a particularly good or accurate "machine" as you can imagine but it does cut wing cores, and an occasional pattern. I bet practically anybody here could design and build a better one. One big problem with the drawer slides as designed is they protrude off the ends as they slide to the far end, so you need a far bigger area than the cutting area.

In general, I don't like cnc hot wire cutting, and for all tapered wings I now use a pivot wire (manual wire attached to a fixed point, used with a single airfoil pattern). It's much faster and not prone to CNC programming errors -- which can quickly eat up a LOT of good foam. If I had to cut the same wing in production, maybe CNC would make sense. But for occasional one off, it seems I waste 3 panels getting one good one, and it takes hours of first programming, generating G-code, cutting a blank and positioning it for wire cutting specing the taper and finally cutting at 1mm/sec, meanwhile watching it in case something goes wrong.

I wish I could say that it's magic, and easy but frankly, I'd rather cut a one off part with a hot wire by hand.

I'll show you my crude hot wire jig saw instead, which, with a simple foam pattern to do, can cut far more quickly than the CNC would. Back with pics later.....

awemawson · « **Reply #40 on:** August 11, 2014, 10:10:16 AM »

Thanks Steve.

I suppose this stems from your RC modelling activities, an arena I've not been involved with. I'd have thought with careful construction that wouldn't do too badly, after all you're not looking for +/- 1 thou are you ?

Eugene · « **Reply #41 on:** August 11, 2014, 10:58:04 AM »

Steve,

Thank you, that's been a great help. I've already dropped the power (and hence temp of the wire) considerably and got a much better result just working freehand; time for some sheet ally now.

Your set up isn't "quite crude", this is crude ... !

The only excuse is that I made it yesterday from leftovers from the roller shutter cupboards which were themselves made from left overs; like you I hate waste. The only difference between it and a thousand others might be in the power supply which is from an old PC.

I just made a contact block to take the various power leads.Switching them and moving the position of the crocodile clip on the wire gives pretty fair control; something I did after reading your reply. The thing that looks like the end of a safety pin between the green and black ground wire is actually the end of a safety pin between the green and black ground wire; it just provides a load to switch the unit on.

Today I scrounged an old (Polish!) propane bottle which is just the right size for a charcoal smelter, got the brass tap out and filled it with water. It'll stay that way till Friday when I'll be back int t'hut.

Again thanks,

Eug

vtsteam · « **Reply #42 on:** August 11, 2014, 11:31:16 AM »

Andrew, sorry, you're right it is quite accurate enough for wings since movements are relative and we tend to sand things, cover them etc. I just wanted to warn exacting machinists that the accuracy isn't what they would expect from ball slides and a rigid gantry, etc for a CNC mill. and metal work. And for pattern work, we usually expect surface finish to be at least foam texture, and likely having a machining tolerance as well.

A hot wire has a variable kerf width also depending on the direction of cut. Because the wire is cutting by radiant heat (properly) rather than contact, and the melted foam gasses travel upwards an upward vertical cut is wider kerfed than downward, and horizontal varies with angle from horizontal, etc. Also depending on software, speed of cut tends to vary around curves, somewhat -- naturally the wire is in an area of foam longer when doing a tight radius and can actually melt out a section even though the linear speed stays the same. etc.

Some software compensates for this (somewhat) or some people play the heat control manually. Still others accept the slight variation (yours truly!).

All this tends to make for even more wasted material when you suddenly lose a section that on paper (in CAD, I mean) it looked good.

When you work manually, the knob between your shoulders does some amazing compensation at times after a little practice, seemingly without any calculation, just by the appearance of the cut. And quickly adjusts placement of a part to best utilize remaining scrap after a botched piece -- something that's difficult to re-program into a CNC machine as quickly.

vtsteam · « **Reply #43 on:** August 11, 2014, 11:42:36 AM »

Eugene, that looks great!!! Much better than my vertical wire. Now I'm ashamed and no need to post mine.

Also, if you want variable heat control with your computer supply DC output -- assuming you aren't "attached" to your alligator clip method (we have alligators here, crocs very rare and confined to the Keys) try one of these:

http://www.ebay.com/itm/DC-12V-24V-Single-Color-Adjust-For-5050-3528-LED-Light-Strips-Dimmer-Controller-/171104731465?pt=US_Lighting_Parts_and_Accessories&hash=item27d6a30149

awemawson · « **Reply #44 on:** August 11, 2014, 11:58:58 AM »

A comment from someone who knows nowt about hot wire cutters:

Why are you bothering with DC power supplies - I'd have thought a suitable low voltage transformer (transformer not wall wart) powered by one of the very cheap phase angle 'power controllers' available on ebay would have given you full control as needed and mains isolation.

http://www.ebay.co.uk/itm/High-torque-50V-220V-10A-AC-Motor-Speed-SCR-Controller-Power-2000W-UK-Seller-/281387453688?pt=UK_BOI_Industrial_Automation_Control_ET&hash=item4183ffccf8

Eugene · « **Reply #45 on:** August 11, 2014, 12:54:05 PM »

Andrew,

Two reasons ...... first I never thought of it

and second I had the PC power supply to hand anyway. Steve, at the bottom of my "thingies box" I've got a 220V dimmer switch which I can press into service if I need it. When I've done the immediate casting that I need, I have a cunning plan for the ATX unit, so again I'll be recycling the already recycled! I've got a DC motor with a 2 rpm reduction gear not unlike a windscreen wiper motor that the unit drives a treat ..... it might very well do for a fine feed on my wee Myford.

Eug

vtsteam · « **Reply #46 on:** August 11, 2014, 01:40:51 PM »

Eugene you can't use an AC light dimmer on the input of a regulated computer power supply!!! Don't do it.

It can be used on an older transformer type AC or bridge rectified DC supply, like a "dumb" unregulated automotive battery charger. Or even on a transformer alone for AC output (as long as it's output is safely under say 25 volts or so.

However, The DC LED Dimmer can be used with your supply -- it is five dollars and change postpaid --or equivalent more or less in your money. It goes on the Output of your PS, not the input.

Andrew -- hot cutting wires can be run on either DC or AC as you probably have guessed -- just a resistance wire. A computer power supply is fine as long as it has enough current capability, and you do DC power adjustment on its output (not its input)

awemawson · « **Reply #47 on:** August 11, 2014, 01:53:39 PM »

Steve, I don't think Eugene, and certainly I am not, suggesting that you use one of these devices on a DC power supply.

The suggestion is to use them on the mains feeding a low voltage transformer (this is how the current is controlled on many welders of a certain vintage)

vtsteam · « **Reply #48 on:** August 11, 2014, 02:12:30 PM »

No I didn't think you were, Andrew, but I thought Eugene was considering it. Apologies if I misunderstood.

I've run all three for cutting wires:

A 25V transformer and dimmer on the AC side with 24V AC output to the wire
An old style auto battery charger with a router speed controller on the AC side with 12V DC output
A computer power supply with the 12V DC side feeding a DC LED light dimmer like the one I've shown.

vtsteam · « **Reply #49 on:** August 11, 2014, 08:38:08 PM »

This morning with the sun coming in low through a window it was possible to highlight the difference in surface finish and detail fidelity of the second casting and the third (last) one.

On the left we can see the zinc separated from the mold walls by layers of molten polystyrene. The drywall compound was not absorptive and the plastic stayed inside the mold.

On the right the you can see that the actual texture of the foam has been recorded and transferred to the metal. Polystyrene was pulled out of the mold by absorptive plaster of Paris allowing the metal to cool against the mold sides and record the very fine detail of the foam texture. And a blob of hot glue which was used to attach the sprue to the pattern is also preserved in perfect detail on the left

News:

Author Topic: Lost Foam Casting: a Crankcase in Zinc Alloy (Read 65023 times)