What Fillament, wall thickness and Fill Percentage for Patterns

[GreenTwin]

Maury use to cast hard plastic production patterns to send to the foundry, so as not to risk damaging the original patterns.

The problem with all the castable resins as I see it (two problems: expense, and shelf life).

I think I can cast a permanent aluminum pattern much cheaper than a cast plastic one, and my aluimum ingots don't go bad over time.

Much boils down to how many times you plan on using a pattern; ie: 1-5, 5-50, 50-100, more than 100 times, etc.

As the numbers go up, I think a cast aluminum permanent pattern is the most cost effective.

Bound sand stresses the pattern alot more, since the pull has to be straight out of the sand, and you can't wiggle the pattern around in the sand like you do with greensand or petrobond.
And the pattern sticks to bound sand.
The more complex the pattern, the more sticking surfaces.

Much to consider if you start thinking of production, and not just a few one-off hobby castings.

.

 

[Jasonb]

The advantage of resin over aluminium is you don't have to polish out the sand cast surface that you would if pouring your aluminium master is sand. The mould and therefore the resin can be near mirror smooth. So actual pour of ali may be faster but the post pour prep of the surface is longer.

Couple of Nick's
Master pattern in PU Board

Mould taken from master and working pattern cast

Boards being made, mould can be used multiple times which sand can't

 

[abby]

Exactly the same principle for lost plastic v lost wax , if you only need a couple then lost plastic is ok I guess , but if you want dozens then make a silicone rubber mould and get wax replicas.
The most complex parts can be produced in wax , it burns out cleaner too and is re-useable.

All of the hundred or more pieces in this mould started as wax patterns , if each had been printed it would have taken several hours rather than the few seconds for each wax.
Just to answer an elsewhere question this printed pattern for a Boxford/Southbend top slide handle is totally hollow and has only 1.0mm thick walls.

This was printed by Shapeways and as they charge by weight as much as 50% or more saving can be made if care is taken with the design.
It was my intention to cast these in nickel silver by lost wax process so I would take a silicone rubber mould off the printed pattern , but for sand casting I would fill the pattern with resin for extra strength.
Industrially hard plasters such as Herculite are also used for both patterns and odd sides used during sand moulding.
Dan

 

[GreenTwin]

Jason-

The poured plastic material is definitely the deluxe method, with excellent results.
If I had infinite money, I would use the poured plastic method.
And if I ever got into a situation where I could pour a lot of patterns all at the same time, it may be worth it, so that the material could be used up, and not go bad on the shelf.

A cast aluminum permanent pattern can be buffed out pretty easily with a fine sanding sponge in a tool and die grinder, but any defects must be filled.
The key would be to use a ceramic mold coat to get the very best finish on the aluminum as possible.

For me, it is more a matter of having a permanent pattern that will not go bad over time.
I don't have plans for any significant production of castings; generally it would be in the 1-5 range.
But a good set of permanent aluminum patterns, such as for the Ball Hopper Monitor, would have significant worth in and of themselves, since they could be used for mass production.

Lots of pipe dreams at this point, but perhaps some will become reality one day.

Dan-

The lost wax process produces some stunning casting results, and lends itself to mass production.
It is a multi-step, time consuming process though compared to bound sand castings.
And a resin-bound mold that is coated with ceramic mold coat can meet or exceed the surface finish that can be had with the lost wax process.
The beauty of the lost-wax process is the complexity of shapes that can be cast, with no consideration given to draft angles.

The lost-PLA process looks very promising, but again, it is a multi-step involved process compared to sand mold casting.

I see it as levels in casting work.
One has to decide which level one wants to work with.

I see level 1 as aluminum castings in petrobond or greensand molds.

Level 2 would be castings in some sort of bound sand mold, using either resin-bound or sodium-silicate bound molds.

Level 3 would be brass/bronze castings.
I consider brass/bronze castings more difficult to make than gray iron castings.

Level 4 would be iron castings in any type of sand.
Level 4A would be iron castings in a bound sand mold with ceramic mold coat sprayed on.

Level 5 I would consider a lost-PLA type process, which involves a coating process, and a burnout.

Level 6 I would be a lost-wax process, with accurate permanent metal molds created to make the wax patterns.

Others may see the levels differently, but this is the way I see it.

So I work to the lowest level possible that will give me consistent results with excellent surface finish, for very low production numbers (1-5), and that is resin-bound sand with a ceramic mold coat.

For the green twin parts, I could go out to the shop, ram up perhaps six multi-part molds in about 2 hours, coat them, melt the iron in an hour, and have castings a few minutes later. So final castings are available in a single session measured in hours.

And for me, time required is a large factor, and thus a faster process can justify more expensive molding materials.

.

 

[abby]

Green twin , I agree entirely , I started casting at 9 years old by melting lead ( sawn off the next door neighbours waste pipe) in a tin can on the domestic coal fire when mum had gone out , and poured into plaster moulds to make toy soldiers.
Wasn't very successful but I learned quite a bit.
After a couple of years green sand casting brass as a small part of a larger business I moved to lost wax following an approach by a well known (in the UK) animal sculptor , I reckon I had just about got my process perfected by the time retirement came round.
Wish I could start again knowing what I now know....lol........bet that's what Da Vinci said!
Dan.