How to Solve Platinum Casting Problems

Bob Romanoff

The first thing for all of us to remember is that Platinum, just like other metals, has its own list of characteristic in all phases of manufacturing that must be professionally addressed. The following guide is designed as an easy to use list of Platinum manufacturing related problems. The guide is intended to get you looking at ways to improve your operation or reduce your costs to manufacture. Model and design limitations in this guide will not be addressed.

SECTION 1

WHAT CAUSES FINS ON MY CASTING? 

1. Poor quality investment
   Corrective Action:
   In general, a phosphoric acid investment will set up harder and better than non-phosphoric based investments. “Finning” is usually caused by internal fractures in your investment flask in to which the metal will flow. There are a couple of high grade investments on the market led by Romanoff’s “J” formula Platinum Investment. Plastic resins produced from 3D printers, expand before they “melt” out and can lead to cracks and rough surface on the castings.
2. Flask put into hot oven
   Corrective Action:
   Oven should be at room temperature when flasks are first put into the oven. (J Formula)
3. Wrong absorbent paper used.
   Corrective Action:
   Use only the absorbent paper from your supplier of investment. The engineered rate of absorbency varies greatly from brand to brand. Use at least 3 paper towels under your paper base to help absorb more moisture.
4. Rough handling of flasks prior to burnout.
   Corrective Action:
   Flask should be gently moved from location to location prior to burnout. It is best if the flask is moved directly from the vacuum table to the hearth plate of the room temperature burn-out oven.
5. Too much moisture still in investment when oven temperature reaches 212ºF, 100ºC (boiling point of water).
   Corrective Action:
   Reduce amount of investment from the top of your highest piece to the top of the flask to no more than 1” (25mm) (for J Formula). Also flask must be accurately held at 190º F to 200º F, for the first 60-90 minutes of burnout.
6. Centrifugal Speed on your casting machine is too high.
   Corrective Action:
   Reduce settings to lowest needed to fill your thinnest pieces. 
7. Improper powder to liquid ratio.
   Corrective Action:
   Instructions for proper mixing ratios must be followed accurately. Keep in mind that Platinum Investment as well as gold Investment is hydroscopic in nature and will be affected by humidity in your area. Be aware of vast seasonal changes and track humidity on a day to day basis to detect corresponding problems with finning. You must keep the bags tightly sealed when not in use and unused, investment should not be opened until needed. The shelf life is indefinite as long as you follow these basic procedures. Water to powder ratio change of up to 2% might be appropriate depending on your exact humidity conditions.
   General Statement
   Most “investment related problems” reported by customers using all types of investment are called in as the season’s change, winter (low humidity) to summer (high humidity). Do not underestimate this phenomena. Track humidity when you run in to problems.
8. Absorbent paper has absorbed atmospheric moisture and therefore has lost some of the absorbency it is capable of.
   Corrective Action:
   a. Always store the paper in plastic bags that are tightly sealed. Reseal after each use.
   b. If this is suspected you can “dry out” the paper by placing on the top of a warm oven for a few minutes prior to use.
9. Wax not eliminated completely before the boiling temperature of wax is reached. The wax boils and breaks down the adjacent investment surface. 
   Corrective Action:
   Cut a hole which is approximately ½” (12mm) diameter in the center of the paper base directly under the wax cone to which the pieces are sprued to allow the wax to drain out.

2. WHY DO I GET A ROUGH SHANK SURFACE ON MY CASTING?

1. Improper sprueing.
   Corrective Action:
   Increase diameter of your sprue and decrease the length of it.
   General Statement:
   For Platinum pieces, sprues are generally larger and shorter than other metals.
2. Improper sprue wax used.
   Corrective Action:
   Use only lower temperature sprue wax for your buttons and sprue to allow the wax to melt out easily. 
3. Poor quality wax used.
   Corrective Action:
   Use only waxes with very low ash content. Certain brand waxes are very high in ash content which will leave a deposit in the investment.

3. WHAT ARE THE BASIC CAUSES OF ROUGH SURFACES ON CASTING?

1. Generally from metal temperature too high.
   Corrective Action:
   a. Check with your metal supplier for proper melting and casting temperatures.
   b. If melting with temperature sensing, set your machines accordingly to avoid overshooting temperature.
   c. If melting with a torch, cast no more than three seconds after metal puddles.
   General Statement:
   If your pieces do not fill with the correct casting temperature, it is a sign of something else wrong.
   Raising temperatures to higher levels to fill will only lead to other issues such as porosity or roughness or investment breakdown etc. Use this guide to try and determine the root cause of the original problem.  
2. Burnout temperature too high. Overheating of investment will result in deterioration of the investment.
   Corrective Action:
   a. Follow burnout directions from the manufacturer.
   b. Do not exceed 1600ºF. 904ºC (maximum 1650ºF for very thin designs)
   c. Check your oven controller for proper calibration at both the high and low range of the cycle.
3. Poor original model and/or wax
   Corrective Action:
   a. Examine your casting carefully and compare it to your waxes and model for repeat roughness. Correct the defect as needed.
   b. Powder on the wax which is not cleaned off will show up on the surface of the castings.
4. Poor quality investment.
   Corrective Action:
   In general, non-phosphoric acid based investments will give a poor surface finish with much higher reject rate. Romanoff Brand “J” Formula will provide you with a smoother and higher luster surface. This much smoother surface will save you time and money in finishing labor and supplies.
5. Improper mixing of investment which may leave powder “lumps” in the mix.
   Corrective Action:
   Follow investment directions carefully.
6. Cross contamination from gold or silver investing process.
   Corrective Action:
   Ideally, Platinum should be processed 100% separately from gold/silver and other metals. If this is impossible due to budget or site constraints you must take extraordinary measures to clean equipment for mixing/investing. At the very least invest in a separate mixer and all of the basic accessories. Keep your investing areas and equipment spotless as a gem.