Casting Defects

Timothy L. Donohue and Helmut F. Frye have posted an excellent paper "Characterization and Correction of Casting Defects" (http://www.platinumguild.com/files/pdf/V5N2W_characterization.pdf).

They show cartoon images of the defects and discuss various causes. Highly recommend reading through the entire document before fixing on one defect as there are many and sometimes the differences are subtle.

Investment Flask Facts

The wax (art piece + sprue system + pour cup) determines the size of the investment flask. Since the flask is a cylinder (technically a right circular cylinder), the total volume is determined using the height (H) and diameter (D) or radius (R) (D = 2 * R) in the equation Volume = pi * R^2 * H = pi * (D/2)^2 * H, where pi = 3.14159265... is half the circumference of a circle with radius 1.

Here are some simple rules relating the height (h) and diameter (d) of the wax and the flask height (H) and diameter (D):

1. Because the flask will have a 2-3 inch cap to seal the bottom before filling, H = h + 2. Large flasks (diameter > 20 inches may need larger caps depending on the shape of the wax).
2. 2 inches between the widest points on the wax and the flask wall are ideal, so D = d + 4.

The flask's total volume (measured in cubic inches or in^3) is given in the first row of each cell in the table below.

Since the flask is filled using 5 US gallon buckets of wet investment material, in order to estimate the number of buckets needed to fill a flask, one needs to divide the total volume by 231, the number of cubic inches in 5 US gallons. The second row of each cell in the table below gives these values. Note that these are full buckets and it will take more "partially filled" buckets, so treat these values as lower bounds (minimum).

Finally, the third row of each cell provides estimates of the flask's total wet weight. These are based on the weights of water, sand, and plaster mixed in the proportions of 1/3, 4/9, and 2/9 (resp.). For a 5 US gallon bucket these weights are roughly 14, 22.3, and 7.9 lbs (resp), putting a 5 US gallon bucket of investment at 52 lbs. Note that actual full buckets have been weighed and come very close to this value.

Diameter and Height combinations shaded in green are "doable" with a few assistants to move the finished flask. Those in red are "challenging", and white is "beyond reason".

Volume (in^3) # Buckets Weight (lbs)		Flask Height (inches)
		10	12	14	16	18	20	22	24	26	28	30	32	34	36
Flask Diameter (inches)	10	785 0.6 35	942 0.8 42	1099 0.9 49	1256 1 56	1413 1.2 63	1570 1.3 70	1727 1.4 77	1884 1.6 84	2041 1.7 91	2199 1.9 99	2356 2 106	2513 2.1 113	2670 2.3 120	2827 2.4 127
	12	1130 0.9 50	1357 1.1 61	1583 1.30 71	1809 1.5 81	2035 1.7 91	2261 1.9 101	2488 2.1 112	2714 2.3 122	2940 2.5 132	3166 2.7 142	3392 2.9 152	3619 3.1 162	3845 3.3 173	4071 3.5 183
	14	1539 1.3 69	1847 1.5 83	2155 1.8 97	2462 2.1 110	2770 2.3 124	3078 2.6 138	3386 2.9 152	3694 3.1 166	4002 3.4 180	4310 3.7 194	4618 3.9 207	4925 4.2 221	5233 4.5 235	5541 4.7 249
	16	2010 1.7 90	2412 2 108	2814 2.4 126	3216 2.7 144	3619 3.1 162	4021 3.4 181	4423 3.8 199	4825 4.1 217	5227 4.5 235	5629 4.8 253	6031 5.2 271	6433 5.5 289	6835 5.9 307	7238 6.2 325
	18	2544 2.2 114	3053 2.6 137	3562 3 160	4071 3.5 183	4580 3.9 206	5089 4.4 229	5598 4.8 252	6107 5.2 274	6616 5.7 297	7124 6.1 320	7633 6.6 343	8142 7 366	8651 7.4 389	9160 7.9 412
	20	3141 2.7 141	3769 3.2 169	4398 3.8 198	5026 4.3 226	5654 4.8 254	6283 5.4 282	6911 5.9 311	7539 6.5 339	8167 7 367	8796 7.6 396	9424 8.1 424	10052 8.7 452	10681 9.2 480	11309 9.7 509
	22	3801 3.2 171	4561 3.9 205	5321 4.6 239	6081 5.2 273	6842 5.9 308	7602 6.5 342	8362 7.2 376	9122 7.8 410	9883 8.5 444	10643 9.2 479	11403 9.8 513	12163 10.5 547	12924 11.1 581	13684 11.8 616
	24	4523 3.9 203	5428 4.7 244	6333 5.4 285	7238 6.2 325	8142 7 366	9047 7.8 407	9952 8.6 448	10857 9.4 488	11761 10.1 529	12666 10.9 570	13571 11.7 611	14476 12.5 651	15380 13.3 692	16285 14.1 733
	26	5309 4.5 239	6370 5.5 286	7432 6.4 334	8494 7.3 382	9556 8.2 430	10618 9.1 478	11680 10.1 525	12741 11 573	13803 11.9 621	14865 12.8 669	15927 13.7 717	16989 14.7 764	18051 15.6 812	19112 16.5 860
	28	6157 5.3 277	7388 6.3 332	8620 7.4 388	9851 8.5 443	11083 9.5 498	12314 10.6 554	13546 11.7 609	14777 12.7 665	16009 13.8 720	17240 14.9 776	18472 15.9 831	19703 17 887	20935 18.1 942	22166 19.1 997
	30	7068 6.1 318	8482 7.3 381	9895 8.5 445	11309 9.7 509	12723 11 572	14136 12.2 636	15550 13.4 700	16964 14.6 763	18377 15.9 827	19791 17.1 891	21205 18.3 954	22618 19.5 1018	24032 20.8 1081	25446 22 1145
	32	8042 6.9 362	9650 8.3 434	11259 9.7 506	12867 11.1 579	14476 12.5 651	16084 13.9 724	17693 15.3 796	19301 16.7 868	20909 18.1 941	22518 19.4 1013	24126 20.8 1086	25735 22.2 1158	27343 23.6 1231	28952 25 1303

Spruing a Wax

Objective

Review a basic direct feed sprue configuration for a small bust (roughly 8 inches tall) shown in the bottom figure.

Description

Sprue systems are generally characterized as direct or indirect depending on whether the flow of bronze moves down or up (resp.) into the wax positive. It is common to use a direct feed for bust or bowl shaped piece and indirect for a flat piece with reliefs on one or more sides (generally with sprues feeding into the backside of the wax).

The top figure shows a tree like (branched) gate system oriented with the pour cup upwards. A 1" main gate, to which a cup will be attached, is shown branching into three 1/2" sprues that tapper into a triangular flat section of micro-crystalline wax.

So as to minimize the total height of the investment, the pour cup will be placed roughly 1" above the highest branch.

The gas vent is a 1/4" sprue attached in a similar manner as the secondary gates and worked to be rounded for roughly 1/2" distance from the top. A flexible straw will be attached to this gas vent and secured into the pour cup. After investment, the top bend of this straw will be cut off, leaving a gas vent hole roughly 2-3" from the pour cup.

A 1/4" sprue rod was threaded and attached inside the bust. Once cast, the threads can be cleaned by re-threading the bronze, and the piece will be mounted using this post.

Since the wax positive will be suspended by the sprue system and filled with wet investment material during the investment process, a reasonable test is to fill the wax piece with water and slowly and in a secure manner, lift the wax and water by the main gate. Better the sprues break off long before the investment day while time remains to strengthen them.

Art by Tom Patras (c) 1980, cast in 2000

Making a Fiberglass Retaining Shell

Overview
This procedure summarizes how to create a fiberglass mother-mold over a two-part polyurethane mold. Gelcoat is used as an undercoat next to the polyurethane mold to protect the mold from fiberglass particles. Fiberglass/resin is applied next, creating the durable, hard mother-mold that is lighter than a plaster mold. All of these procedures should be performed in a well-ventilaetd space using an appropriate solvent fume mask (protecting against styrene). Care should be taken when handling fiberglass as the material is highly irritating to exposed skin, eyes, and the respiratory tract.

Required Materials

• Mold releasing agent: Have successfully used SynAir 531 on polyurethane rubber molds.
• Fiberglass mat: Prefer the loosest weave possible and there are differences between brands. Napa AutoParts mat version works, but is not as good as others. Fiberglass cloth may have special uses, but is generally not preferred. Have successfully used the following brands: Napa AutoParts Mat.
• Gel coat: Have successfully used the following brands: TBD
• Laminating Resin: Have suceessfully used the following brands: Napa AutoParts.
• Acetone: Used to clean brushes and cans.
• Clean metal cans: 1qt size works well.
• Cheap 1 inch paint brushes: Have extras around in case they are needed.
• Gloves: You'll use more than you realize, so stock up before starting.
• Fume mask: Spend the money and protect your brain.

Procedure

1. Spray the polyurethane mold with mold release agent (531) prior to application of the gelcoat. Fill in any gaps or undercuts with clay.
2. Create 1-2” aluminum wedges (L-brackets). Align two brackets so the vertical faces are joined and drill a hole to allow a ¼” machine screw to pass through. These are a matched pair and should be kept together by joining them with a screw and nut. You’ll need enough for 2-4 pairs for each part of the mother mold (say 6-8 pairs for a two-part mold). Alternatively, you could create clay dams along the edge of each piece.
3. Add the hardner to the gelcoat and mix per instructions. Coat the surface of the mother-mold. Embed the aligned wedges along the border of each part of the mother-mold as the gelcoat hardens. Use a heat lamp if you want to speed the hardening time. Scrape away gelcoat from the border line between two parts of the mother mold to make it easy to separate the two edges later on. If you don’t apply the wedges now, you can add them later with bondo (polyester glazing putty) or 5-minute two-part epoxy. If working with clay borders, apply the gelcoat to the surface of the clay damto create a perpendicular wall along the mold surface.
4. When the gelcoat is relatively hard, can apply the laminating resin to the surface as a relatively thin coat. Apply fiberglass mesh that has been loosened by hand to allow greater surface area contact with the resin. Don’t brush the fiberglass, but use a patting motion to place the fiberglass where you desire it. Apply mesh with cut edges along the border of the two pieces to simplify piece separation later. If using wedges, apply the fiberglass material over the wedges and fill in any gaps under the wedges. If using clay dams, apply the fiberglass up the perpendicular surface of the dam on top of the gelcoat.
5. Apply the fiberglass in layers to gain the desired thickness (1/4” to 3/8” should typically suffice for a medium-sized piece (say less than 1.5 cu. ft). Can apply a second layer of gelcoat to protect the outer surface of the mold to protect against future handling.

Contributors: Rafael P, Mark W.

Bronze Pour Instructions at Pratt

Introduction

This blog contains information regarding the bronze pour activity at Pratt Fine Arts Center. It is intended to serve as a reminder of the pour process. See Pour Day Essentials if you're not responsible for pouring bronze, but have art being cast.

What follows is a mixture of process steps, cautions, and suggestions. And there may be a flow chart in here some where.

Task: Start furnace

The furnace has two intake lines supplying gas and air. At Pratt (picture right), the air is controlled by the Start/Stop buttons that also control the fans blowing into the furnace, and the gas is controlled by a valve (red handled) above the air control panel. The general logic for starting or stopping the furnace is to not have pure gas in the furnace, so air is the first on and last off. When starting the furnace:

1. Turn the Air on - or open air to full. Note that the mix of gas/air is controlled by valves below the on/off panel.
2. Proclaim in loud voice, "Fire in the hole."
3. Then slowly open the gas valve.

Adding too much gas all at once can "explode" out of the furnace opening.

Task: Crucible lift from furnace

1. Tiltman opens furnace door - slides toward deadman
2. Deadman continues opening - do not remove handle from door
3. Deadman and tiltman lift LiftTongs over crucible with lift arms rotated counter clockwise as far as possible so as to clear the furnace door.
4. To lift crucible, tong catch must be off notch.
5. Bring crucible to bricks with winch and align pour spout at center of pour field.
6. Tongs are lift off crucible by hand while still on winch chain - tong catch must be in notch to come free.
7. Hand tongs to WinchMan - who detaches them from chain
8. Close furnace door
9. Get winch hook & fasten to crucible tilt bar - tilt bar hook has an orientation that must be set down correctly before pour.
10. Raise winch until the crucible locks are above lip.
11. Keep tilt bar level at all times.
12. Lock both sides (Deadman & Tiltman)
13. Establish eye contact between all three pourers before lifting crucible from bricks. This is to serve as a safety double check for the team.

Task: Pour

1. "Dead" or "Tilt" commands from the WinchMan indicate which side of the crucible should be moved toward the center of the pour field.
2. Deadman is to stay even in x-axis with TiltMan at all times unless obstacles created by investments or ingot cups require adjustments

Task: Post pour crucible cleanup

1. TiltMan holds crucible at 30-40 degree tilt from horizontal while,
2. Deadman crapes crucible walls.
3. After scraping completed, Tiltman continues tilt to just below horizontal watching that the tongs remain engaged, while
4. Deadman pulls scrappings from crucible.

Task: Return crucible to furnace

1. Return crucible to bricks
2. Remove tiltbar (leave in place)
3. Open furnace door
4. Attach tongs by lifting over crucible, making certain catch is not in notch
5. Place wet cardboard in furnace to create carbon layer between crucible and pedistal
6. Lower crucible with tong arms rotated counter clockwise as far as possible - lower quickly as cardboard may burn before crucible is in place
7. Crucible needn't be centered
8. Lift tongs by hand from furnace - once locked in notch.
9. Close furnace door.