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Frequently encountered problems in a wax room.

Success in investment casting begins with the proper handling of wax during melting, conditioning and injection.

Pattern wax is a valuable raw material that is composed of relatively sophisticated, high-priced ingredients. It is the starting point for the production of investment castings.

The way you handle your wax during the melt-down, conditioning and injection stages will directly reflect on the quality of wax patterns produced by your wax room. Remember, your final castings can only be as good as your wax patterns.

A wax room's best friend is a good thermometer, either mercury or digital, that should be checked for accuracy periodically in boiling water (212F) and ice water (32F). Temperature control is very important during all three processes in the wax room: melting, conditioning and injection.

Avoid Overheating

Overheating can oxidize some of the raw materials used in pattern waxes, causing the wax to be brittle or rubbery, depending on the formulation. This will affect injection characteristics and dimensions. The appearance of the wax in its molten state in the melter can indicate when the wax is overheated. Some indicators include:

* color change;

* black specs within the molten wax, and on the bottom of the tank and walls;

* a translucent look to filled wax, showing signs of filler fallout;

* smoke or vapors emitted from the melting tank.

Was meltdown temperature should be kept between 180-210F.

When using conventional melting tanks, waxes are best melted in an oil-jacketed container. Control the temperature with the best thermostat you can afford. Agitation should be used for all pattern waxes, filled or unfilled. Using a jacketed melting unit in conjunction with agitation will help maintain uniform temperatures and prevent localized overheating and damage to the wax.

Do not use a rectangular melting tank with filled waxes because the filler will become trapped in the four corners of the tank and not remain in suspension. A round cylindrical melter should be placed with the lower blade of the stirrer as close as possible to the bottom of the tank to prevent any filler from settling out under the blade.

Today's melting units are built to melt flake or slab wax on a continual basis. These cylindrical or box design units sit above the conditioning tank and will provide liquid wax directly into the hold or conditioning tank on demand.

The solid wax is heated in a wax hopper either by a hot plate or hot air. The temperature control is set and monitored by a digital controller, which is usually mounted on a melter's electrical panel. A sloping melt surface prevents the filler from setting out and the melter usually has a high-temperature cutoff switch to prevent the wax from overheating or burning.

Wax temperature is the cause of most problems experienced in the wax room.

When checking the temperature of liquid or paste wax in your injection machine, the tendency is to place the thermometer in the wax and wait until the mercury or readout stabilizes. This can cause problems because a cold thermometer placed in warm wax will immediately cause wax to solidify around the thermometer, insulating the thermometer from the rest of the wax and giving an inaccurate readout.

Heat the thermometer higher than the estimated temperature of the wax, place it in the wax and slowly move it around. Occasionally, remove the thermometer or probe from the wax to remove solidified wax from the tip. This method will give you a more accurate reading when the temperature stabilizes.

Dimensional changes in wax patterns can be attributed more to differences in the temperature of the wax at injection than to any other cause.

Conditioning Wax

A number of problems can arise if the wax isn't properly conditioned prior to injection. Most waxes are injected below their melting point and sometimes well below the melting point of some components. To ensure uniformity, it is best to temper the wax by holding it at injection temperature several hours before use. If properly conditioned, wax viscosity, flow and surface finish will be optimized.

You can't expect to make a good quality wax pattern if you take wax directly out of a melt tank at 180-210F, and pour it into a liquid injection press with the thermostat set between 145-150F (depending on the type of wax and wax pattern being injected). A poor conductor of heat, wax takes a long time to melt and lose its heat.

Problems caused by injecting hot wax include:

* dimensional changes;

* excess cavitations or sink;

* excess flash on wax patterns;

* excess shrinkage;

* long dwell times;

* slow setup of wax patterns;

* trapped air due to turbulence.

A good habit for wax rooms to follow if they aren't using a central distribution system is to fill wax machines when they are about one-third empty. This will allow hotter wax to mix with cooler wax in the machine and come to equilibrium faster. Do not wait until the machine is completely empty before filling the reservoir because you not only will be injecting hot wax but also will have a lot of air entrapped in the reservoir.

Pattern waxes have both a melting point and a solidification point. The solidification point can be 30F below the melting point, with the injection temperature somewhere in between. Poor temperature control or carelessness can allow the wax injection temperature to drop too low.

Injecting cold wax can cause:

* wax non-fill;

* flow lines, ripples and knit lines;

* dimensional changes;

* surface finish defects such as graininess or orange peel.

If wax becomes too cold, raising the wax temperature to its proper injection temperature may not cure the problem. There are ingredients in a wax formula that have high melting points and, when the wax has dropped below their solidification temperature, these components crystallize.

By raising the temperature slightly, these hardened crystals of wax may not soften enough. The best solution is to raise the temperature above the melting point of pattern wax, when it will be a liquid with all components of the blend completely melted. Then, reduce the temperature to the correct injection temperature, allow wax to temper and proceed with injection.

Other Factors

Temperature is not the only factor that plays a part in poor pattern production. You now must look at the mechanics of how your wax patterns are injected and what established standards and procedures are followed with each job.

Variables affecting wax pattern production are dwell time, injection pressure, die temperature, sprue design, platten temperature, die lubrication, wax flow rate, post-injection cooling, die venting and die surface.

Pattern wax assembly is the final stage in the wax room before proceeding to the shell room. Problems that may arise in the assembly area include:

* water in your sprue wax;

* weak joints between the gate and the runner bar, caused by welding knives that are too cold, a low cold stick tight temperature or a delay in applying the part to the sprue;

* gate size design;

* incorrect spacing between the patterns on the runner bar;

* poor coating of dip seal wax on the runner bar;

* improper seal between the part and the runner bar.

Improved Castings

With proper temperature controls on all three processes--melting, conditioning, injection--you can feel reasonably assured that your wax room will be producing wax patterns with consistent dimensions and surface finishes.

If you can correct your problems and monitor your process in the wax room, the quality of your castings should improve.
COPYRIGHT 1993 American Foundry Society, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1993, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

Article Details
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Author:Argueso, John P.
Publication:Modern Casting
Date:Aug 1, 1993
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