PRO-SET Epoxy products have been successfully used for many years to build composite tooling, and several products have been specifically formulated for this application. Molds for boats and yachts, wind blades, aircraft and all manner of composite parts have been built with these products. Review this document for descriptions of  proper fabrication techniques that will help ensure success with tool building projects. 

Refer to the data sheets for more specific information on each of the resin and hardener combinations by clicking the links in the left hand column. 

Tooling is described as the equipment necessary to build composite parts. This can include plugs and molds, but also assembly fixtures, drilling and cutting jigs, and masking or grinding fixtures. Often the words mold and tool are used interchangeably, and for clarity in this document, we will use the word mold to describe the structure to build a part and use the word plug to describe the structure required to build a mold. Molds vary widely in shape, size, and method of construction. Generally they are designed to perform two functions. They must hold the wet-out laminate in a specific shape until the resin system has cured. They may also be required to form half of an airtight envelope that contains the laminate to allow vacuum consolidation of the laminate. Some small molds are designed to fit completely inside an envelope and only need to be rigid enough to hold the laminate’s shape. 

The mold surface must be airtight and smooth enough to prevent bonding to the laminate. Porous surfaces such as wood or foam should be coated with epoxy, receive a laminate skin or covered with a material such as plastic laminate to provide the necessary airtight surface. Each part produced in the mold will have a rough (back) side and a smooth (mold) side. In most cases, the smooth mold side of the laminated part will be its outer finished surface. Greater care in finishing a mold’s surface will result in a part with a smoother finish. A mold release will allow the laminate to release cleanly from the surface. Plastic film can be used as a mold release in situations where surface quality is not critical and contamination of the part's surface could be a problem. 

Vacuum bagging molds take advantage of the fact that atmospheric pressure is equal everywhere on the outside of the envelope. Atmospheric pressure on the back of the mold will counteract all of the clamping pressure on the face of the mold. The quantity and stiffness of the laminate, the degree of compounding of the mold shape, the size of the mold and the precision of the finished laminate are factors that increase the amount of reinforcing required to stiffen the mold. 

Molds should have a flange at least 6" larger than the laminate on all sides to allow excess laminate for trimming and to provide a clean area around the perimeter to seal the bag to the mold. 

Figure 1 A flat, smooth surfaced table is a versatile mold for a wide variety of projects. Several lay-ups can be completed at the same time.

One of the simplest and most useful molds is a flat, rigid table faced with a smooth plastic laminate, glass or metal (Figure 1). This mold is useful for producing flat laminates or panels for bulkheads, doors, beams, and a wide range of custom structural components. Any portion of the table may be used, and multiple lay-ups of different sizes can be molded or vacuum bagged at one time.

Curved parts can be laminated over male or female molds. A female mold's surface is generally concave, producing a laminated part with the smooth finish on the convex or outside–a boat hull for example. A male mold generally has a convex mold surface, producing a part with a smooth surface on the concave side–a bathtub or cockpit well. A male mold may also be used to produce a boat hull. An existing hull, for example, can be used as a mold to reproduce a slightly larger version of itself. However, when a part is laminated over a male mold, the rougher back side of laminate will be the outside of the laminated part (the hull in this case) and will require additional fairing and finishing. 

A curved mold can be lofted and built in wood or other low density material, with a layer of fiberglass cloth and several coats of epoxy to provide a smooth, airtight molding surface. 

Some parts, because of their shape or size, must be laminated in two separate molds. An open or bowl shaped part, such as a small open boat hull, can be easily pulled from a one piece mold if the opening of the mold is wider than any point on the inside. A closed object, such as an enclosed boat, requires at least two molds. The part is divided at its widest point so that both molds will be wider at the opening than any point inside the mold. A typical small boat is widest at the shear which is generally dividing line between the hull and deck molds. (The catamaran plug in Figure 2 is widest about a foot above the waterline, which is where the deck mold and hull mold are separated). The part will then be laminated in two halves and bonded together after the halves are pulled from the mold and trimmed. 

Curved molds are often built in a two stage process. In the first stage, a plug or form is built to the exact dimensions and finish of the final object. In some cases an existing object, a hull for example, can be used as the plug. In the second stage, a mold is cast from the plug. In the case of a boat hull, a male plug (essentially a male mold) produces a female mold. To simplify construction, the female mold may be built upside down over the top of the plug, then flipped over after it is completed. For all but the simplest of forms, it's much easier to build, fair and finish a male plug than it is to build, fair and finish a female mold from scratch. 

The plug is an exact, full sized model or pattern of the finished part. A hull plug, for example, may be lofted and built in much the same way as a one-off hull, with frames, stringers and a skin. It may also be carved free form, using templates or calipers if necessary to transfer profiles, establish critical dimensions or keep the plug symmetrical. A common method today is to build a "buck" or "mandrel " and apply a spray urethane foam. Then this structure is shaped with a multi-axis router to an accurate shape approximately ¼" to ³/8" low of the final shape. This surface is then sheathed with either a composite skin or a tooling paste. This hard outer shell is then machined again to the final shape and finished to the required degree of smoothness for the mold surface. The PRO-SET 195/295 Tooling Paste is used in this application. View a video   of the 195/295 being applied with a metering and dispensing system. View a video   of the 195/295 being machined with a multi-axis router. For sheathing the plug with glass laminate and epoxy, use PRO-SET 125 or 135 resin with a PRO-SET hardener that will provide the pot life and working time necessary for the plug you are building. For high temperature applications, refer to the High Temperature section toward the end of this manual for specific resin and hardener recommendations.

Figure 2 A plug can be built of any combination of easy to shape materials. This catamaran plug’s cabin area was shaped in Styrofoam™ and then faired with epoxy/low density filler.	Figure 3 The catamaran plug was faired and finished to the same degree as the finished product. The shelf was applied to the plug where the mold halves divide the form at its widest point.

The strength and durability of the plug should be determined by the number of molds that will be made from it and how long it will have to last. A plug may be used to build many molds for production manufacturing or from time to time replace a damaged or worn out mold. The plug may be altered after molds are made from it to create variations or revisions of a design. 

Although any number of molds may be cast from a plug, a plug is often used only once. Any material or method of construction is acceptable, as long as the plug is fair, smooth and strong enough to accurately cast the required number of molds from it. Plywood frames and easy to shape planking materials like cedar or foam will help to reduce the costs and time to build the plug (Figure 2). The plug (and mold) should be extended at least 1" past the finished laminate edge to allow for trimming of the laminate. A 6" wide shelf, attached to the plug at the edge of the plug extension, will provide a ledge around the top of the mold when the mold is right side up. The ledge will reinforce the mold and provide a clean area outside of the laminate to seal the bag to the mold. 

Whether a plug is built for heavy use or to be used only once, no effort should be spared when fairing and finishing the plug. Every flaw in the surface of the plug will be transferred to the mold and to the finished product. This is true whether using more traditional methods with final hand fairing or using a multi-axis router and tooling paste to complete the job. When using the traditional method, the plug may be built to within ¼" of the finished dimension, using any combination of materials. An outer layer of fairing compound or tooling paste can then be shaped to the exact dimension of the finished product. The final faired surface should be sanded to an 80-grit finish. 

Two or three coats of epoxy applied to the faired plug will seal the surface. Wet sanding the cured epoxy to a 400-600 grit finish will make the surface smooth enough to prevent adhesion when the mold is cast. The plug's surface should appear as smooth and as fair as you wish the final product’s surface to appear (Figure 3).  If using the PRO-SET 195/295 Tooling Paste, the surface can be sanded and polished to a gloss appropriate for many applications (Figure 4). If higher gloss is necessary, a surfacing primer can be sprayed onto the surface and polished to the final finish. 

Figure 4 195/295 Tooling paste plug; machined, sanded, polished, and ready to build a mold	Figure 5 A plug for a rudder, with the shelf positioned at the rudder centerline, is waxed and ready for the application of the mold half.

After final sanding and polishing, several coats of mold wax or mold release should be applied to the surface of the plug and the shelf, with the last coat buffed to a high gloss. The mold release will fill pores in the surface and prevent bonding to the mold (Figure 5). Always follow the mold release manufacturers instructions. 

If the plug is a closed shape that requires a two piece mold, the break line or widest point around the plug should be determined. The plug should taper in from all points on this line. An epoxy coated, plywood shelf is temporarily attached to the plug at the break line (Figure 6). The shelf should be 6" wide and parallel with the floor. Small cleats fastened temporarily with drywall screws will hold the shelf to the plug until the mold is made. Beeswax (toilet bowl wax) or modeling wax can be used to seal the gap between the plug and shelf, and, if desired, make a small fillet in the mold/shelf corner. The completed mold should include a level 6" wide lip around the opening of the mold at the break (laminate trim) line, and the fillet  will leave the edge of the mold rounded. During the lay up, the laminates are extended past the lip onto the shelf. When trimmed, the laminate extension provides a flange around the edge of each laminate half that may be used to bond the two halves together. After the top half mold is completed, the plug and mold are turned upside down. The shelf is removed, and the holes from the drywall screws are filled and faired. The casting process is repeated for the bottom half mold, before the plug and top mold are separated. The top mold's 6" lip takes the place of the temporary shelf for casting the bottom mold's lip. 

Figure 6 A closed shape like a sphere or a hull with a molded deck requires two molds separated at the widest point. A 6" wide shelf at the edge of the mold allows the laminate to run beyond its trim line and provides a clean area to seal the bag to the mold.

Figure 7 Framing is being added to the bottom mold after lay-up is complete.

Building a mold over a plug is very similar to laminating a part in a mold. After the plug has been completed, the mold shell is built up in layers, or laminated, over the plug. Hull molds are generally built upside down. A framework is bonded to the completed mold shell to help keep it rigid (Figure 7) and to provide legs for level support when it is turned right side up. 

The schedule of materials for a mold shell varies depending on the size of the mold. A typical schedule begins with an epoxy gelcoat or surface coat to provide a resin rich, high density surface (Figure 8). The PRO-SET M1019 Surface Coat resin with 224 hardener provides a low porosity mold surface.  Refer to the data sheet for proper mixing and application information. A couple of layers of light reinforcing fabric  or surfacing veil followed by multiple layers of heavier fabric will make an adequate skin for small molds. Larger molds may require additional layers of laminate, or a core material and additional layers of laminate. If the part will be post cured in this mold, a solid laminate without a low density core will transfer heat more effectively. 

Figure 8 M1019/224 applied to a plug will form the inner surface of the second half of a mold.	Figure 9 Legs and wheels are added to mold.

When the final laminate skin has cured thoroughly, install the mold framework. The framework should support the mold shell at a convenient height and keep the mold from flexing when it is removed from the plug and placed into use. The mold framework may be fixed to the floor or mounted on wheels, in which case a strongback may be needed to keep the mold rigid (Figure 9). The framework should be built over the mold shell before removing the mold from the plug. The support structure should be spaced away from the mold shell and held in place with laminated tabbing. This will also minimize distortion caused by the framing and allow more efficient heat transfer through the mold shell during elevated temperature cures. If the mold will be heated during an elevated temperature cure cycle, the supporting structure should be built with the same materials as the mold shell to minimize distortion caused by different coefficient of expansion (CTE) for unlike materials. 

After the mold has cured thoroughly, remove it from the plug by carefully forcing wooden or plastic wedges between the edge of the mold and the plug. Then prepare the mold for use. Inspect the mold surface for pinholes or flaws which may be repaired with the same epoxy surface coat used on the mold surface. 

The plug/mold construction and laminating procedures described in this manual are based on the use of room temperature cure epoxies and materials. Plugs, molds and laminates that will be post-cured or subjected to temperatures greater than 140°F (60°C) will require an alternate epoxy system, cure profile and building method. 

High performance epoxies are often used in advanced composite manufacturing. These epoxies may require curing or post-curing at elevated temperatures. If the finished laminate is to be cured or post cured at elevated temperature in the mold, the mold must be built of materials and with techniques that enable the mold to withstand the elevated temperatures without distorting. And, if the mold must be post-cured on the plug, the same precautions must be taken when building the plug. 

When building molds that will be used with high temperature curing applications, first establish the target post-cure temperature of the part. Consider the highest and lowest temperatures at which the resin system will cure. Then consider the size of the structure to be cured and the type of mold construction you would like to use. All of these factors affect the cure schedule (the rate of temperature increase and length of cure time). 

The cure temperature of the mold and plug are based on the established target temperature of the part. The mold should be post-cured at a higher temperature than the part and there are situations when the plug should be post-cured at a higher temperature than the mold. If, for example, the part will be cured in the mold at 140°F (60°C), the mold should be cured at 150°F (66°C), and the plug should be post-cured at 160°F (71°C). The objective is to use the mold below the temperature at which it was cured. This way, the mold or plug can be used without exceeding the thermal properties of their structure's resin system. 

When choosing materials for the mold, consider the fact that a cored mold will not transfer heat as well as a solid laminate. The core in a composite sandwich mold will act as an insulator. If a core is also used in the part being laminated, the skin between the mold surface and the part core will not warm up as well as the skin on the other side of the core. If there is a large temperature difference between the inner skin and the outer skin, the part could pre-release or distort during the elevated temperature cure or post-cure cycle. Verify the dimensional stability of the core material you intend to use for the intended post-cure temperature. 

A mold built with PRO-SET 125 or 135 resin systems can be used at cure temperatures up to about 150°F (65°C) when properly cured. If cure temperatures are in the 140°F (60°C) to 210°F (100°C) range, use 145/237. For higher temperature applications, use the M1012 resin with M2010 hardener for use temperatures up to 270°F (132°C). Refer to the product data sheets or the web site www.prosetepoxy.com for recommended cure schedules and mechanical and thermal properties of the cured resin/hardener mixtures. 

Contact the PRO-SET Epoxy technical staff if you have questions about mold building or post-curing at elevated temperatures.