Ten Most Asked Questions About Investment Casting
1. What is a “lost wax” “investment casting”? What advantages do I have switching to this method?
Wax is injected into an aluminum tooling to produce a pattern. This is a positive replica of actual part increased by a calculated shrinkage of the wax and steel. For every casting one wax pattern must be made. The patterns are then assembled on a common “sprue” The assembly is commonly referred to as a “tree” of parts. The tree is repeatedly dipped into a ceramic slurry followed immediately with a refractory material (sand or stucco with excellent thermal properties). After a proper shell thickness is built, the wax is removed (usually by autoclaving.) The hollow shells are then preheated up to 2000 deg. F and molten metal is immediately poured into the hot shells. After cooling, the spent shell material now cracked and fractured is removed. Cleaning operations (cut off, grind, heat treat, straightening, blasting) takes place.
The goal is to supply near net shape parts often requiring little or no further machining or subsequent operations. This process offers cost savings, design improvement, broad alloy selection including material interchangeability, close tolerances, weight reduction. Once the initial tool is made you can start producing consistent parts order after order.
2. What alloys do we pour?
Our company specialize in Stainless and Alloy steels including carbon, low alloy, stainless-(austentic, ferritic, martensitic, duplexes, age hardening), some nonferrous materials – nickel and cobalt base. Also including ultra high strength steels, heat resistant, corrosion resistant, wear resistant, and some tool steels.
Value add services include machining, precision grinding, assembling, spray coating, plating, painting etc. Occasionally we are asked to support custom alloy requirements for our customers.
3. What size range of parts can be produced by our process?
We can produce parts from ounces up to approx 150 lbs. Dimensionally we are able to handle parts approx. up to 20x15x5″.
4. What “as cast” dimensional tolerances I can expect?
Listed below are typical, general tolerances. This varies depending on the size and complexity of the part. In addition straightening or specially foundry operation and inspection often may improve tolerances on one of two critical dimensions. Therefore we recommend that every part and its application should be discussed and designed individually with our Engineering Department.
Premium tolerances should be applied only to those as cast features which are considered critical relating to function and reliability or a feature which cannot be machined. From a foundry point we suggest to assign as a critical dimension only one or at maximum two features. Anything else may require a costly inspection/upgrading operations.
A. Linear Tolerances.
|Up to 1/2″||+-.007″|
|up to 1″||+-.010″|
|every inch after||add another +- .005″|
B. Minimum Wall Thickness.
|300 Series SS||.050″||.80″|
|400 Series SS||.060″||.90″|
The following table shows the amount of eccentricity:
D. Flatness (dishing).
|Length||Measured by “filler gauges”|
|2″ or less||.020|
The method of measuring of straightness should be established between foundry and purchaser. Also deepening greatly on type of the material.
F. Circularity (Roundness).
|Dia.||T.I.R. (for solid sections)|
On larger diameters linear tolerances to apply.
For a larger tubular castings closer tolerances can be held thorough mechanical straightening.
As cast tolerances of angles depends on their location in casing. They range from +- ½ deg. For a well supported position to 2 deg. (or more) where a large distortion could be expected.
5. What type of surface finish can I expect from an investment casting?
Because of the type of process – fine ceramic shell is built around smooth pattern made by injecting of the wax into a polished aluminum die. Casting surface is excellent. See the table below.
Casting Surface Finish.
|RMS Range||60-170 (depending on material)|
6. Are not investment castings expensive? And if so, how can they save me money?
Investment castings are generally more expensive then forging or castings produced by other casting methods. They make up for the higher cost through the reduction of machining, through near net shape tight tolerances that can be held as cast. Many parts that require miscellaneous machining operations or fabrication can be investment cast to full customer satisfaction. This way you may also get a part with a “sounder” and more internally consistent part from a metallurgical point of view then parts fabricated, with preheating straightening, stress relieving etc. Again, it is important for our customers to meet us and discuss final finishing requirements and therefore potential savings.
7. How many pieces do I need to make buying an investment casting practical? What is the most efficient way to quote an investment casting?
Tooling amortization is a key factor to determine in whether or not an investment casting is feasible. From our experience we have often about 25-35 pcs. Per order with a few releases per year. Also do not forget this process is one of the most versatile casting processes known.
The best way how to quote a new investment cast part is which would produce a cost effective part is:
establish a good communication channels. Then next step is to supply a drawing (3 minimum) (and sample if possible), and request for formal quotation, with all pertinent informations such as actual weight, material required, condition required (kind of heat treatment, NDT, any special requirements such as surface quality, etc.). Also including critical dimensions or surfaces and service of the part. Our Eng. Department would contact you in writing with any suggested design changes or recommendations, tolerances, etc. All above have to be approved by you.
8. What type of tooling pattern equipment is necessary?
Usually, a split cavity aluminum die is manufactured which is a negative of the final part increased by a shrink factor of the wax and steel. Depending on the complexity of the casting, various combinations of the aluminum, ceramic or soluable cores may be employed to produce a desired configuration. Most tolling price range is withing $1,500 – 10,000. This tooling is a customer property.
9. What about integrity of an investment casting?
Investment castings are being used for many critical applications with a great success. Our pre-production castings are x-rayed to meet definite soundness criteria.
10. What are lead times I can expect when ordering an investment castings?
Generally 6-12 weeks is typical for tooling and sample castings and another 6-8 weeks for production. For more information please contact our sales department.