Can software calculate casting shrinkage? - Page 2
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  1. #21
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    Quote Originally Posted by Fadriver View Post
    it was that time no cad software was popular.
    was expensive as hell, like 100k including computer
    only huge companies could afford, all drafting table a cassio
    calculator, I would apply in thicknesses as previous expierences
    because,we were allowed only 0.050 inch, for machining stock, molds
    were 17x10 round, and aluminum was expensive so lots of pounds can be saved
    or wasted,i always sometimes add shrink or minus from dimensions, and got so
    good to maybe 0.01 error,also gates can play a role how it shrinks,so I would
    say foundry is an art,to make castings with limited leaks, they had to be air tested 100%, also mold paint, paint is insulator from heat, think can remember
    like 1200 deg, melt was lot of fun working that kind enviroment, making 60K
    parts a month that size, machining side was big, also new workholding tooling
    had to be made per part#, I was for auto oe product.
    So no std practice, all goes how thick is section
    required in part, most other companies were using std 7% shrink
    i glance their dwgs at pattern makers shop, so all by expiernce
    if need to be right most of time, some times scaling does not work as
    intended, I own a machine shop and do scaling same style with 5" in dia.-wise
    so if scale by ratio wont work, so scale some areas and merge others.
    at least in line of work,where function and asthetics is the sell.

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    To the OP.
    Yes, You are going about it wrong.

    Do You intend to make 3-4 parts / year ??
    "Mold sets" is not a quantity I can identify.

    Forget canada/halifax as such.
    That is irrelevant to your final goal.

    Ask ;
    1. is there someone, who for 500$ can make my 3d part better for casting ?
    My desired uses, and expected accuracies, are a,b,c, ... but I am willing to adjust.

    This will save You huge grief, hours, stuff and very much lower costs overall.

    2.
    Ask;
    How much will it cost if You machine for me n quantities of this part ?

    E.
    "I desire low price, thus the alloy is changeable, and You can do it slow, off-peak, and I need very low accuracy."
    4-8 weeks turnaround is fine.

    --
    Do not try to learn casting, machining, and production of parts.
    You won´t be able to learn any of them to any relevant degree, vs your costs and expectations.

    Just ask the professionals here what they cost, done, and how to get them done cheaply or for what cost you are aiming for.

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    Quote Originally Posted by nihilistic View Post
    I see, that makes sense then.

    What about an alternative wood-like material like MDF? It would maintain dimensional accuracy better I would suspect.

    Also you could just ask them what they recommend material wise?
    The problem with MDF is the moisture in the sand mixture. The recommended material was laminated layers of white pine glued with type III.

    I see online that there are many options for software that can calculate the rate of shrinkage, I was just hoping someone on here might have access. Im going to try asking the software makers for a one-off.

    kai.

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    Got it about the fillet, and also about the distractions. The pattern already adds 3/16" to the machined surfaces as recommended by the foundry. The foundry recommended 3% but they make rough hardware and Im trying to acchieve something with more accuracy.

    thanks.

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    The first thing I did was to send out for quotes for the work. What I found was that with my current experience all I needed was to have access to software to modify the shape of the pattern to adapt to the shrinkage of the aluminum.

    Thanks for taking the time to think about the problem.

    kai.

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    Quote Originally Posted by kaiwinthrop View Post
    Well, the question is, can I scale it by 2% and call it good or will I be wasting a lot of effort and material.

    k.

    No, you should not do that. Shrink rates are very specific.

    1) Find out what the shrink factor is of the material which would be cast. For example (in imperial) if the material shrinks at .006"/inch then you would scale the cad model by 1.006". Use the same formula for metric...find out what the shrink factor is in mm/mm and go from there.

    2) The material for making a pattern is not a big deal as long as it does the job. For your purposes it sounds like wood or a synthetic material will work. Be sure to inspect your patterns once in a while to make sure they are holding shape and not warping.


    Post back here if you need someone to scale a cad model for you. I just programmed three molds for customer who supplies us only with STL models. They are easy to work with if the software can handle it.


    By the way, here is a link on sand casting basics and principles...
    Sand Casting Process, Defects, Design

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    Doing sand cast molds, you actually have two shrink rates to contend with: the aluminum shrink rate during casting (I use 3%), and then the shrink rate of the actual material being molded. By and large, calculating non-uniform shrink rates has not been useful for me, unless the casting was exceptionally shaped (long and narrow). Generally the pattern maker and foundry are able to adjust the pattern to achieve ideal fit. That’s one of the reasons they use wood for patterns.

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    Quote Originally Posted by G00 Proto View Post
    Doing sand cast molds, you actually have two shrink rates to contend with: the aluminum shrink rate during casting (I use 3%), and then the shrink rate of the actual material being molded. By and large, calculating non-uniform shrink rates has not been useful for me, unless the casting was exceptionally shaped (long and narrow). Generally the pattern maker and foundry are able to adjust the pattern to achieve ideal fit. That’s one of the reasons they use wood for patterns.
    I learn something today, when i was involved in foundry molds
    they used wood patterns, but i was thinking with no computers to
    model with,wood was easy to grind and glue and make pattern shape before
    going to edm sinker,i was doing alu permanent mold , cast iron shells, cores
    4140 steel,but mold shrinkage was based on thickness sections,but being sand
    casting, and as he stated 3/16 machine stock, i think he would be just fine,
    maybe solidworks can help on this subject,but needs to make the model and then
    analyze, but for big parts and 1.5 mm stock i doubt it , but not being solidworks expert who knows.

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    I used to make a lot of patterns when I was an apprentice, it's been 10 years since I last used it but we used to use a scale factor of 1:1.01298.

    or 1 in 77

    IE for 100mm we would make the pattern 101.298mm

    right or wrong this is embedded in my brain forever!

    our machines could scale so we would program in the size we wanted but put a scale command at the start of the program so the parts came out bigger.

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    The biggest problem with scaling a model for a pattern is the tendency to pick a factor that is too big to be sure that there is material for machining on all the exterior surfaces that are to be machined. The problem is this spreads the interior features apart, and if the casting does not shrink as much as anticipated, there may not be any machining stock on the interior features.

    All materials exhibit a range of shrinkage (and designing and building injection molds rather than foundry patterns, I'm not going to suggest factors for aluminum.) In this discussion alone we have had suggestions from 1.2% to 3% and more. The way I would approach this is to scale the solid model multiple times. I would increase it by the minimum factor for the interior features, and the maximum factor for the exterior features, then pick some mid-range value to build the actual pattern, after adding extra machining stock so the exterior features exceed the dimensions obtained from the maximum scaling, and the interior features have stock added so they meet the minimum scaling.That should keep you safe. In reality, if wall thickness is not an issue, you could scale the exterior by the largest factor and the interior volume by the smallest, and just let the difference yield slightly thicker walls; you will have to decide if this is acceptable.

    Dennis

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    Notes I copied from a post on this forum in the last couple of days:

    Shrink Rules

    Molten metal will shrink as it cools. To allow for this, patterns are made slightly oversized. A pattern that is 12 1/8" long will produce a 12" long part when cast in iron. Shrink rules are elongated rulers that take shrinkage into account. (A one foot shrink rule with 1/8" shrink per foot looks like an ordinary ruler. It is actually 12 1/8" long.) By using a shrink rule to measure parts for a pattern, the pattern maker does not have to scale up the dimensions of the desired casting.

    Each metal has its own shrinkage rate:

    Metal Shrinkage
    Britannia* 1/32" per foot
    Tin 1/12" per foot
    Iron 1/8" per foot
    Bismuth 5/32" per foot
    Brass 3/16" per foot
    Aluminum 3/16" per foot
    Copper 3/16" per foot
    Steel 1/4" per foot
    Lead 5/16" per foot
    Zinc 5/16" per foot
    * Lead free form of pewter

    From Stanley Folding Rules A History and Descriptive Inventory by Alvin Sellen's

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    Shrinkage for Aluminum is typically 1/8"/for to 3/16/foot. Which translates to roughly 1%-1.5% I know a lot of the old manuals say 3/16 but the times I have compared castings to the patterns we have made or have dealt with foundries who do more precise QC it has usually been closer to 1/8"-5/32". Solidworks will easily compensate for shrinkage using the scale command. However with those castings I would probably make the inside to standard rule and the outside to 1/8 especially if they are using airset sand. You won't be able to do that with scale in Solidworks You will have to do the math as you are making the model unless you copy the body scale the outer one and combine the models.

    The reason for doing this is often on a casting like that the sand will hold the casting out preventing it from shrinking leaving the inside of the pattern the same size as the pattern. You cannot rely on this happening so you use the shrinkage on the outside of the part. This way you will be sure to have metal there for your machining.

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    STL files suck for anything but 3d printing. Do you have a drawing? PM me and I will make you a model for the pattern. We can talk about a making you a pattern if you have a little time. How many castings are you looking at making at a time?


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