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.