Experiences in Problem Solving
VENTING - "THE LOST ART"
By George DiSylvestro, DiSylvestro Videography Service

ABSTRACT
With the enormous progress made in new production casting methods during recent years, the art of venting has been LOST. One of the main reasons is due to automation, mechanization procedures, design, and costs, to retain the successful venting practice of the past. Only when casting quality has suffered and casting losses increase, does the necessity of venting become important. (However the time to maintain training of personnel was challenged as foundries downsized with loss of technical people and increased tooling cost.)

The review of basic fundamentals and creativity of the tooling engineer is of the utmost importance in the competitive atmosphere of today. We must continue to have the desire to increase productivity and maintain a high degree of profitability. The what, why, how, types of examples, and cost of venting will be reviewed to assist the metalcaster in boosting and maintaining production.

BASICS OF METALCASTING
PROCESS
The casting process utilizes a sand mold to contain the molten metal. The aggregate is usually silica sand that is bonded with bentonite clay or a chemical binder to maintain its shape. The sand provides natural pore spaces between the grains that lend themselves ideally to the casting process that accomplishes a basic requirement to produce a quality casting. That is a permeable mold or core surface that allows air or gas to pass without allowing the molten metal to penetrate.

There are three major basic factors why natural permeability is necessary:

1. To allow the air in the mold cavity to escape, permitting the molten metal to take its place with a minimum of turbulence

2. To permit the gases that evolve from oxidation of molten metal, from moisture of the mold, and any gas produced by chemical reaction to escape during and after pouring is complete

3. To allow a closed mold to be filled at an acceptable rate of speed with the minimum restriction of air entrapment and back pressure

PERMEABILITY IS AFFECTED BY:

• size of sand grains
• shape of sand grains
• bonding mechanism
• compaction characteristics
• degree of compaction
• amount of water or other
• additives used
• expansion of silica during heating
• mold and core coatings

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WHAT IS VENTING?
Venting is a part of the mold or core forming process that is usually necessary to a variable degree, to produce a quality casting. In some extreme design configurations it is associated with safe pouring practice. It allows air and gases to escape from the mold.

As molds are made with a granular refractory aggregate such as silica sand, the pore spaces produce some natural venting. Unfortunately, to improve casting surface finish with increased compaction, these pore spaces are drastically reduced. This leads to the increase in gas pressure during and after pouring. To prevent gas entrapment, some means of venting becomes necessary.

Venting molds and cores is a simple means of reducing internal gas pressure during and after pouring.

VENTING PREVENTS:

• metal boiling in the mold

• mold explosions

• mold lifting

• mold runouts

• misruns

• blows and gas related porosity

VENTING IMPROVES:

• production of net shape castings

• casting soundness

• safety during pouring

Unfortunately, all these benefits cannot be guaranteed, and assurance be given completely to prevent the entrapment of gas because of high temperature pouring and solidification rates. Pouring temperature is extremely influential in the effectiveness of venting as well as gating geometry.

For these reasons, some auxiliary means of venting to enhance permeability must be incorporated to insure quality. The venting art now becomes a science.
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BASIC VENTING METHODS

1. Use of a coarse base molding or core aggregate and/or use of a center filler such as cinders, core, slag, Styrofoam pellets, or similar filler material and hollowing out core center if applicable.

2. Mechanically form vents such as can be drilled, round rod or flat plate poked vents made during the forming process.

3. Pattern and core box fixed vents designated in the tooling and casting geometry as part of the formed mold or core. These could be mounted at the parting or in conjunction with loose pieces.

4. Formed wax or flexible textile tubing implants inserted in the mold or core during production.

5. Selective gating geometry such as: bottom gating favoring directional metal flow to pre-align favorable gas escape pattern and reduced mold pressure. Or, using pop off, flow off, or open cope riser.

6. Selective or variable degrees of mold or core compaction, such as: making the cope less mold hardness than the drag.

WHEN TO VENT

• when the casting design presents compressive restrictions that delay or prevent air or gasses in the mold to escape normally without pressure build-up during or after pouring.
• when gasses generated from mold and core materials, molten metal oxidation or chemical reactions are not allowed to escape normally without the build-up of pressure during or after pouring.
• when internal cores are used that are surrounded by metal more than 50%.
• gas problems occur and increase when cores are located below the parting line and more severely in the drag.
• when internal cores are used that are not completely cured, of high compacted density, have a refractory coating with increased thickness and depth of penetration.
• cores with high organic binder content, and when cores have been subjected
  to high humidity and used.
• when extremely hard green sand molds are made (over 90 mold hardness) without openings other than the pouring sprue.
• when pouring extremely hard and at a high ferro-static head to facilitate running or metal fluidity by pressure to run light section or high surface area design castings.

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HOW TO VENT
To improve permeability and reduce core and mold pressures during and after pouring, consider these parameters:

1. Use coarser base sand aggregate.

2. Use a rounded sand grain shape.

3. Decrease core and mold compacted density.

4. Cure cores to optimum cycle.

5. Make judicious use of pre-mounted mold and core parting vents, especially at the core prints and take them out to the edge of the flask.

6. Hollow out cores whenever convenient and economical. Vent the cavity.

7. Venting vertically is usually more effective than horizontally if choice permits.

8. Vent at highest point of mold cavity if possible. Carry core vents to highest point of design when possible.

9. Reduce pouring speed and ferro-static height whenever possible to reduce mold pressures when gas problems occur.

10. Reducing organic content, drying humidity laden cores, drying coatings, etc., all reduce gas pressure and if venting procedures are followed, improved venting effectiveness occurs.

CAUSES OF GAS-RELATED PROBLEMS ASSOCIATED WITH VENTING PRACTICE

1. Failure to recognize the importance, and apply the basic metalcasting fundamentals.

2. Failure to document and retain successful basic venting technology for use in future similar casting designs.

3. Failure to automate, simplify, venting technology for easy communication and training when installing new production processes.

4. Tooling engineers practicing undue caution in their methods to prevent runouts and vent cavities being filled with molten metal.

5. Lack of, or unavailability of, technical information for operational supervision for use in training and available for quality assurance programs.

6. Failure to understand the basics of permeability and the importance to include in the decision-making process where necessary.

7. Management’s continual endorsement for any cost cutting measures to yield higher production without conforming to the laws of nature.

8. Lack of common sense, respect for the basics of metalcasting.

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PREVENTION OF GAS-RELATED PROBLEMS ASSOCIATED WITH VENTING PRACTICE

1. Incorporate venting where applicable in any future quality casting procedures and methods, the gas pressure potential in the mold that would cause gas problems.

2. Formulate means for the most effective, economical venting methods and options possible, to insure against gas problems.

3. If in doubt, over vent the venting methods. Do not add unfavorable core or molding cost burden.

4. Establish and incorporate mandatory training of new tooling engineers or production supervision, such as with in-house video training cassette programs specifically designed for this subject.

5. Encourage management to approve investing in venting practices that can improve quality, lower cleaning costs, and provide a measure of safety during pouring.

6. Apply common sense to any gas related casting problems.

CONCLUSIONS
As continual successful metal-casting processes have emerged, the "art of venting" has been lost. It has forced the serious profit-oriented metalcaster to develop venting into a science. Production experiences have brought to the surface the following general basic parameters.

1. Natural mold or core permeability of the molding aggregate may not be sufficient to maintain consistent casting quality, especially with the increase in design complexity or the restrictive production processes and equipment used today.

2. Natural permeability is drastically reduced as increased compacted density is developed to increase mold strength and improve surface finish.

3. Mold and core gas pressures are increased when permeability is restricted. Evacuation of air and gasses during and after pouring can create a host of casting problems if restricted, including mold explosions which impose on the safety of personnel.

4. Multiple means of venting have been developed and improved to take care of gas evolution and in many cases they are "cost free".

5. Knowing the basic parameters of venting has stimulated creativity and progress in this important process area.

6. Proper venting of complex molds and cored casting configurations should be mandatory at the initial pre-production layout engineering.

7. During pouring of the casting with the multiple of process variables such as speed of pouring, ferro-static head, etc., internal mold pressures can be great enough to create a reaction and explosion that could create scrap and could injure the pouring crew.

8. To benefit by cost effectiveness, the initial tooling engineering should include all the various types of venting that can be affixed or designed into the pattern or core equipment. This could provide insurance against pressure build-up in the mold, which could be difficult to determine later.

9. A continual training program with new personnel or apprentices should be included in any quality assurance and auditing program. This can be done effectively and economically by the use of videography with video recording and training tape cassettes. This allows personal and/or group training.

REFERENCES & BIBLIOGRAPHY

"Venting: A Continuing Need for an Old Art" Matlin, J. E. - Mobley, C. E. -Modern Casting, pages 54-56 (December, 1980)

"Venting of Molds and Cores" BCIRA Broadsheet #188 (1980)

"Venting Cores and Molds"  Bex, Tom - Modern Casting, page 42 (August, 1991)

"Venting - The Lost Art"  DiSylvestro, G. - Wisconsin Annual Foundry Conference, (February 14, 1985)