RTV Silicone Molding with MJP Master Patterns and Direct Printed MJP Molds

RTV Silicone Molding with MJP Master Patterns and Direct Printed MJP Molds

Best Practice

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It is often advantageous to use rapid prototyping in the fabrication process of another part. The 3D printed part is typically destroyed or discarded after being used and can make one or more of the final desired part. This type of process is termed “indirect manufacturing” or “indirect prototyping” and the 3D printed parts are commonly referred to as “indirect tooling.” Many traditional manufacturing processes can leverage rapid prototyping in this way, with the most common uses being plastic injection molding, thermoforming, sheet metal forming, sand/investment/flask casting, indirect and direct printed silicone/urethane molding and wax molding for investment casting.

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Injection Molding Thermoforming Sheet Metal Forming

 

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Investment Casting Direct Printed Eggshell Mold 2-Half Direct Printed Mold

 

Direct Printed 3D MJP Molding Capability
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Indirect Manufactured MJP Mold (Using MJP as a Master Pattern)

Wax Molding(for casting)

Rapid Tooling / Indirect Manufacturing Examples using MultiJet Printing Technology

RTV molding, sometimes referred to as “resin casting” or “urethane casting” or “silicone molding” or simply called “soft tooling” by engineers is a molding workflow method used to create prototype parts out of Polyurethane and Silicone resins for product demonstrations, functional testing, and even short-run production. In the traditional “indirect manufacturing” mold making process, a master pattern is either hand made by a model maker through traditional methods or is 3D printed. A mold is made of the part by pouring liquid Room-Temperature-Vulcanizing (RTV) silicone rubber over the pattern. The RTV molds are firm yet flexible and are able to reproduce the master pattern accurately even for fairly complex geometry and intricate details. There are many different mold materials that can be used for different needs with tin- and platinum-cured silicone, polyurethane rubber, and even latex being very common.

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Numerous RTV silicone materials and polyurethane rubbers are available as well as latex which serve specific application molding needs

 

After the master pattern is created, there are a number of methods used to create the mold and that depend on the molding material of choice, part complexity, part size and capability/experience and available equipment of the mold maker. For example, a simple part with one flat side can be attached to the bottom of a flat container and silicone poured directly over the part(s) to make the mold.

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Top: Flat side of 3D parts stuck to the bottom of a pan with clay and covered with RTV silicone

Bottom left: Parts and RTV removed from pan,

Bottom center: Center 2-part urethane resin poured into a RTV mold

Bottom right: Cast resin part and 3D printed part

 

More complex parts can be placed upright or hung by wire in a container allowing the pour to cover the entire part. Special methods and tools are used to cut through the mold down to the surface of the master pattern until the master pattern part is able to be removed.

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RTV Silicone Mold of Complex Part

Left:Mold is cut down to master pattern surface, Right: Master pattern is removed

 

Yet another method involves applying the molding material directly to the part in numerous coats until a thick enough coat is formed. The material is then cut through and the master pattern removed. Yet another process involves embedding half a part into clay (at its parting line) with one side exposed and pouring RTV silicone over both the clay and the master pattern. After the silicone cures, the clay is removed and the other side of the master pattern is poured over to create the final mold. Special release agents allow the two halves of the mold and master pattern to release leaving a two part mold. After the mold is made, often one must hand cut channels to allow the part material to be poured into the mold (a spout) and channels to allow the air to escape as the material is added (air vents). The remaining 3D cavity can then be used to make copies of the master pattern part with the mold being poured and separated numerous times along the cut lines. The casting process can be broken down into about 8 steps.
1. Print the master pattern. This is most commonly done directly from the CAD file. A black marker is often used to mark the parting line making it easier to see where to cut the mold after the silicone is hardened.
2. Build a frame around the master pattern to hold the silicone molding material. Mount the casting in a frame and cover with mold-release agent. Air vents and material flow gates can be added at this point by inserting/adding additional material. The additional material can be 1) 3D printed as part of the master pattern, 2) can be individually 3D printed and inserted or glued into place, and 3) can be made from small sticks or formed clay materials, etc..
3. Pour the mixed (and degassed) silicone into the prepared frame. Pour slowly to avoid bubbles and at the corners of the frame to allow smooth flow around the master pattern.
4. After the silicone hardens, remove the frame and cut the mold along the parting line. Special knives and tweezers and other hardware is used by professionals in this process.

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5. If the air vents and gates were not added previously, they need to be hand-cut out of the mold.
6. Mix the desired material that will form the part. Two-part polyurethane is most common, but RTV silicone rubber and many other curable resin materials or wax (for investment casting) can also be used.

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7. Allow the material to cure. This can take seconds for wax to solidify and minutes, hours or days for other materials depending on their formulation. Faster cure is sometimes desirable facilitate faster cycle time, but can waste more material and can make filling the mold more difficult. Slower cure materials allow more time for small spaces to fill and for entrapped air to be removed. It is not uncommon at this step to apply agitation to the mold to further facilitate complete fill of the mold.
8. Remove and finish the part. Gates and vents may need cut away and the surface sanded/polished at those locations. Any flashing along the parting line is removed. Mold release is removed and any final surface finishing is done.
As there are numerous castable molding materials, there are numerous castable part materials that can be used in the molds. RTV silicone rubber is commonly used as the part material as well as the mold material. Polyurethane resins are also very common as they often have sufficient mechanical properties for prototyping plastic injection molded parts prior to the creation of a hard tool capable of making the actual parts. There are even high temperature silicone rubbers that are able to cast low temperature alloys such as solder and pewter. Also, any material that is able to cure into a mold can then typically be used to pour back into the mold to make a final part. For example, RTV silicone molds can make RTV silicone parts as well as polyurethane parts. Polyurethane molds can make RTV silicone parts, etc. For engineering purposes, parts that require molding are often either rigid polyurethane thermoset materials to simulate injection molded plastic properties or silicone rubber which is needed for many product designs. Even within those two different part materials there are numerous vendors available offering materials in a vast array of mechanical, thermal and electrical properties. Most vendors offer extensive training aids and videos to facilitate use of
their products by hobbyists and engineers. Tap Plastics and Smooth-On are two such good companies for such training. Thousands of videos are available online as well.
Indirect RTV Silicone Molding using MJP Master Patterns
The traditional approach of making a master pattern for RTV silicone molding is to machine the pattern. However, this is time consuming, expensive, and greatly reduces the complexity of the molded parts. Also, design changes require substantial lead times. Important factors to 3D printed master patterns include:
1. The pattern is complex or detailed
2. Challenging geometry (thin walls or internal geometry)
3. Multiple duplicate RTV molds are required
4. Design changes are likely (thus needing repeated molds to be made from new master patterns)

3D printed master patterns for engineering purposes are almost always the best method for molding purposes. 3D printing of the master pattern removes the time, labor and cost of RTV molding and open the design space for part complexity. There are also a number of important printer and material requirements that enable a safe and functional use of rapid tooling with the RTV molding process. Of course, the printer needs to reproduce geometry sufficiently to create the basic features and the build size is important to allow the most ease-of-use and flexibility. There are numerous other important capabilities of MJP technology that are difficult to achieve and bring substantial value to RTV silicone molding master patterns.
1. The material’s sufficient stiffness and strength to properly represent the geometry
2. The material’s elongation for successful part and for removal needs
3. The technology’s smooth surface quality
4. The overall bulk geometric accuracy of the parts
5. The ability to form extremely small features for things like text marking and/or to create high fidelity and complex surface texture

One of the great strengths of 3D Systems MultiJet Printing (MJP) technology is in its broad and individually optimized material set focused on different customer applications. Each of the materials have special capabilities for different applications, however, any of the Rigid, Engineering or Specialty materials can be used to make master patterns. The choice will be dependent on the user’s specific geometry and applications. For example, all the Rigid and Engineering materials can be used for drilling/taping/machining/pressing and achieve the good surface finish, sharp corners, fine features, and create high fidelity true-to-CAD parts that is well known for MJP technology. The Rigid white, clear, and gray materials (M2R-CL/WT/GRY) are optimized for general-purpose properties. They are fairly rigid with good heat deflection temperatures, and yet maintain adequate elongation. All Rigid materials are good for concept modeling and light end-use prototype part needs and all would be a good choice for RTV molding master patterns.
The Special materials have the highest heat deflection temperature and are very strong and stiff. M2S-HT90 also has 6-7% elongation which allow some distortion of the part during molding without breaking and could be used as a master pattern material. M2S-HT90 is stiffer at temperature during post processing and could be a better molding material for certain parts with thin features. The Engineering materials M2G-CL (Armor) and M2G-DUR (ProFlex) were designed for the most aggressive Engineering and Rapid Tooling applications. ProFlex is not as stiff as Armor, but has the highest toughness and elongation capability. ProFlex behaves similar to polypropylene and can be substantially twisted, flexed and deformed without cracking or breaking. Armor also has very high toughness and elongation, along with a moderate stiffness making it the typical of choice for sheet metal forming applications. While possible, ProFlex is probably too soft to recreate many small features accurately for molding master patterns. However, Armor is likely the best choice if available for RTV silicone molds. It has the stiffness and strength needed, but also high elongation that will help in pattern removal. It is also the material with the best geometric accuracy over a wide range of geometries. The elastic property of the material also allows sufficient flexibility that is required to remove final formed parts without damaging the tool or destroying small features. This elastic property of the material also allows small features to be formed at elevated temperatures without cracking or breaking.
The MJP technology itself address the remaining surface, feature, and fidelity needs. 3D Systems MultiJet Printing (MJP) uses a high-resolution inkjet technology combined with a melt-away support material system. The MJP printing process generates a part with over 1.1 billion drops per cubic inch of part volume allowing the creation of even the smallest details. The support wax technology instantly solidifies in place during part formation and greatly contributes to sharp features and smooth surfaces. Injection molded plastics are able to reproduce even the smallest of surface imperfections. MJP technology creates surfaces that are smooth enough that it creates an aesthetically pleasing and functionally smooth surface desirable to designers and engineers. Also, because of its inherent smooth surface quality and its high fidelity capability, MJP is able to create very small and highly detailed surface textures which are often a desirable attribute of an molded part.

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M2R-GRY part showing detailed surface textures

 

Additionally, the wax support also allows for simple and hands free post processing removal of support from even the most complex geometry and entrapped cavity without scarring of the part surfaces. There is nothing that typically needs to be done to accurately match your part design dimensions with an MJP printer. For most geometries, the part should come out true-to-CAD the very first time printed and every time printed. If needed, the printer was designed with a scale factors for each material that is built into the 3D Sprint software. Also, part accuracy is independent of orientation for even the smallest and sharpest of features so you don’t have to worry about numerous special part design rules, part orientation and complex placement issues or special printer setups. The high-resolution capability of MJP also allows for things like the formation of printed threads for assembly and fixture needs.
A few examples will be presented next to facilitate such knowledge; however, ultimately each design is unique and one must iteratively test to determine the appropriate load/pressure requirements for their specific needs (material, thickness, and geometry).

Direct Printed Rigid and Elastomeric MJP Molds for Silicone or Polyurethane Parts
It is also possible to avoid the mold making process altogether and simply print the mold on the MJP printer. This avoids all the materials and know-how involved in making a mold and reduces the problem to that of only having to pour the final part material into the 3D printed mold. One such common “Direct Printed” mold method is called an “eggshell mold” and is used to make RTV silicone parts. In this process, a thin “skin” or “shell” of the part geometry is created. Spouts and/or connectors are added to allow filling of the hollow shell structure. Numerous air vents are added for air bubble removal. The entire shell of the part along with the filling connectors and air vents are printed together. The material is poured or injected into the cavity and allowed to cure. The 3D printed shell structure is thin and fragile enough such that it can be broken and removed without damaging the part. While this process typically involves special tools and training, is a very capable process and well known in certain industries such as medical modeling.

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Direct printed eggshell mold of a flexible camera cover

 

The field of molding is extremely broad. One can work in this field for a long time and continue to learn for years. There are even many other special methods and tools available to a skilled artisan, but are typically beyond the capability of individual designers/users. For these needs, it is often better to utilize 3D systems Service Bureau as they possess the necessary skills and equipment. However, there are many things an individual can accomplish timely and for low cost. For example, a designer and/or engineer is an expert at CAD and will have their geometry in CAD enabling them to relatively easily make a two-part or multi-part mold of their geometry. Adding basic material fill and air venting geometry is also pretty simple for a talented designer. Therefore, a more common type of direct printed mold commonly used by engineers is a two part mold that can form a part within the cavity of 3D printed pieces. The same concept can be used to make a more complex part by creating a mold out of 3D elastomeric materials or a mold with more than two pieces.

 

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Left: Two-part 3D printed rigid mold forming a large flexible RTV silicone rubber gasket
Center: Two-part 3D printed elastomeric mold forming a rigid polyurethane part
Right: Multiple part rigid mold that splits in four directions to create a more complex RTV silicone part

 

Both of these allow the formation of 3D geometry and is too complex to be formed by pulling apart a simple two part mold. Also, the elastomeric material can flex somewhat during the part removal process which can serve the same purpose as adding more mold parts. Each of these molding methods require slightly different processes and leverage MJP materials and capability differently as will be discussed.

 

Indirect and Direct Printed Molds for Wax Patterns

Indirect molds using MJP master patterns and direct printed molds can also be used to make wax patterns for investment casting. Investment casting is a common manufacturing method for complex metal parts requiring good surface finish. A wax pattern is required as part of the manufacturing process. In this process, investment slurry is formed around a wax part and is burned later burned with heat, leaving a cavity in the shape of the wax part. This cavity is filled with metal that forms the final part.

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Investment Casting Process

 

The traditional method of forming the wax part is to make a metal tool and inject the wax into the tool. However, this takes substantial time and is expensive. 3D Systems MJP 2500W and 2500IC 3D printers are designed to print wax for jewelry and industrial casting markets. These printers are the method of choice for prototyping needs and parts can be ordered from 3D Systems On Demand service bureau. However, for certain casting applications, it is also common to use an MJP acrylate printer to make a plastic master pattern and RTV mold and form numerous wax patterns for subsequent investment casting. This can be much less expensive and faster because numerous wax patterns can be produced from only one 3D printed MJP part.

 

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(1) Numerous MJP acrylate jewelry master patterns printed
(2) RTV Silicone mold made from MJP part
(3) Numerous wax patterns made from mold
(4) Investment, burnout and casting into metal part

It is possible and can be advantageous for some needs to direct print the wax mold (and again skip the mold making process altogether). 3D printed molds are much cheaper and much, much faster to produce compared to steel tooling. 3D printed molds are also cheaper and faster to produce compared to RTV silicone molds which are also common in the investment casting industry. For example, RTV Silicone molds produced by a professional can cost up to $300 US even for a relatively small size mold. The same size mold could be printed on an MJP printer for <$100. A 3D printed mold is able to make tens to hundreds of wax parts making it an extremely fast and extremely cost effective option for some casting needs during development and/or even for some product needs. Both elastomer and rigid wax molds are possible to create and use with the ProJet MJP 2500. Special mold release is required and all casting rules will apply with respect to things like draft angles and feature size, rounds, etc… Iterative testing is likely required to create a successful direct printed wax mold. Careful control of the draft angles, mold release and cooling is often required for proper wax part release.

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Elastomeric wax mold made from M2-EBK

 

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Flexibility of elastomer mold assists wax removal from mold

 

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Rigid wax mold of car cab made from M2G-CL (Armor)

 

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Rigid wax mold of clamp made from M2G-CL (Armor)

 

Example 1: Simple 1-D RTV Silicone Mold Using Multiple 3D Printed MJP Master Patterns

One of the simplest uses of RTV Silicone molding are for parts with one side flat and the other side with features that are simple enough that they can be easily removed in a single direction after casting. Geometries like this are very common because they obey plastic injection molding design rules and there are many predominately flat surfaces in products. Many geometries with these constraints are found in both the hobby and engineering worlds. Things such as automotive naming and logos, product logos, as well as items like badges, awards, pendants, and furniture/home decor molds are all examples. The part design in CAD can easily be saved to .stl format and printed.

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Simple 3D Printed Master Patterns

 

If larger or smaller parts of identical geometry are desired, this is often better done in the 3D Sprint slicing software using the scale tools which are very good at scaling 3D geometry. The parts were printed and post processed normally using an oven and mineral oil. To create the mold, the parts were adhered to the bottom of a flat metal container using small pieces of clay. It is important to keep the clay thickness to a minimum so that the silicone mold material does not seep under the part. Two part platinum-cured silicone rubber was mixed in a 50/50 ratio and poured over the top of the parts to a thickness about ¼” past the tallest part. Platinum silicone typically does not require degassing to remove air bubbles from the mix. Entrained air will come out of solution during the cure process and will float to the top (tapping the side of the mold with a rigid object is often used to promote bubble removal). Once the bubbles float to the top, they can easily be
removed from the surface to create a glassy smooth surface with the use of a propane or butane flame lightly directed over the mold before the cure has taken place.

 

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3D printed parts attached to the bottom of a metal container and covered with RTV silicone rubber
Both the parts and the silicone rubber are translucent allowing visualization of the clay adhesive

 

No mold release was required, as silicone rubber releases well from most surfaces. After cure, the entire part was removed as a single slab and the individual parts were removed from the silicone forming the individual part cavities for each master pattern.

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Left: MJP 3D printed master patterns, Right: Master patterns after molding, but before being removed
If there is any thin flash around the part, it can be removed by hand before removing the master patterns by simply rubbing the surface with a soft cloth or with your finger. Thicker flashing should be avoided but can be removed with a sharp knife.

 

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Flashing of silicone rubber that can be manually removed, but should be avoided if possible

 

Mix the appropriate molding material based on your need. For this example, three materials were mixed and poured into the same RTV silicone mold, 1) Rigid Polyurethane, 2) Soft Polyurethane, 3) Silicone rubber.

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2-part, 50/50 mixture of rigid polyurethane resin

 

Different resins cure at specific rates depending on the formulation. This resin cured in about 4 minutes and so the pour had to be done quickly. The material is translucent amber and turns opaque yellow/white when cured.

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Polyurethane resin poured into RTV silicone rubber mold. Curing process turns resin opaque

 

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Fully cured parts

 

The surface quality and feature fidelity of MJP technology is synergetic with the creation of highly accurate and highly detailed parts with good surface properties, sharp features and very accurate and aesthetically pleasing parts from RTV Silicone molds. Most competitive technologies do not have the surface or feature quality for many such casting needs.

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MJP master patters and cast polyurethane parts from RTV Silicone mold

 

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Most competitive technologies have poor surface and feature quality

 

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Parts cast in RTV silicone mold, Left: soft polyurethane parts, Center: RTV silicone parts, Right: rigid polyurethane parts

 

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Deformable solid, clay-like consistency RTV silicone molding material is available for small jobs
that cure after mixing similar to the liquid RTV.

 

Example 2: Indirect Printed Simple RTV Silicone Mold with Integrated Casting Box

It is possible to print both the master pattern and the casting container that will hold the RTV silicone rubber and avoid the need for other supplies and processes. In this example, the master pattern formed a two part mold with the center piece containing six 3D Systems logos and the edges forming the walls of the mold to contain the silicone molding material. Examples of black, white and yellow polyurethane rigid resins formed with the silicone mold as well as a high temperature molding material forming Pewter parts.

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3D Printed RTV silicone mold made from M2R-GRY material

 

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High heat resistant silicone rubber molds made with MJP master patterns that are to cast low temperature alloys such as soft solder/pewter up to 560F/294C, 24 hr cure time.

 

Example 3: Direct Printed, Large 2-Part Mold Using M2G-CL (Armor) and 3D Sprint Split Tool

RTV Silicone rubber is not only used as a molding material for casting, but also has many unique material properties that are leveraged by engineers and designers for specific product capabilities. For example, it is soft and flexible and yet has good tear strength. It also has good heat resistance and chemical resistance and is often compatible with foods or medical equipment needs or for thermal component design like heaters. These properties are often desirable to engineers, but difficult or impossible to create by a 3D printer. The actual properties may be needed in the design phase or for small volume manufacturing phase of a product development making indirect manufacturing with rapid prototyping the best option. Many engineering applications for silicone rubber are for gaskets and spacers and can be formed with a simple 2-half mold. For this example, simple gasket was needed that could be created with a 2-part, split mold, but was larger than the build size of an MJP printer. This was easily accomplished with the ProJet 2500 using the M2G-CL Armor Engineering material and the splitting tools in 3D Sprint.

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Split tool in 3d Sprint software

 

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3D Sprint “Split Tool” is easy to use, semi-automatic, and parametrically drives the creation of features that allow easy reassembly using glue

 

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Seam line after printing and gluing the two parts of each mold halve back together

 

The two mold halves were created using the “cavity tool” in SolidWorks. In this workflow, the two halves of the mold are imported into an assembly file and the part is assembled between the molds on the parting line between the two mold halves. The cavity tool is used to subtract the part geometry from the mold geometry. For this case, the material could be filled in one cavity and the closing of the mold filled the other mold half. Numerous air vents and drain holes were added at the edge to facilitate a complete fill. The two halves were held together with clamps during cure of the silicone material.

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Clamps holding mold halves during silicone cure

 

After the material was cured, the mold halves were separated with flathead screwdriver and the silicone rubber was removed and finished.

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Mold halves being split open using a screwdriver

 

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RTV Silicone rubber part being removed from a mold printed in M2G-CL material on a ProJet 2500.
Note: Numerous flow paths on edges of mold allowing bubble and material flow for complete part formation

 

Example 4: Direct Printed Four-Part, Multi-Directional Pull Mold Using M2G-CL (Armor)

Often a part needs to be more complex that that possible to form with a two-part mold. However, most all high volume parts will ultimately be manufactured with traditional manufacturing technologies (which inherently limit the complexity of part geometry). This is true for traditional high volume injection and molding processes. Therefore, most part geometries that cannot be made from a two-part mold, can often be made from a 3- or 4-part split mold for prototyping purposes. One such example shown here is a cap that is used to seal a printhead in a printer design. Such a seal is often needed for thermal insulation requirements and to reduce environmental and printer born contamination that both lead to reduced reliability due to missing jets. The following is a print head cap that will be made of RTV silicone in the product design. It needed to be molded during product development prior to the tooling being complete.

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Silicone rubber cap used to seal a printhead

 

The top of the cap was formed to allow fit and seal to the print head. Two cylinders extending from the bottom of the seal were used for assembly of the cap to the printer. The cylinders fit through two holes on the printer and a small feature at the end of the cylinders locked the part into place. The very end of the cylinders were hollowed out to facilitate easier installation. To make the mold for this part, four individual mold pieces were created and assembled together in SolidWorks. Two side mold pieces came together and formed a parting line on the outside edge of the print head cap (a mostly nonfunctional surface). A top piece mated to the top of the side pieces along this parting line and formed the top part of the cap which functionally mated with the printhead faceplate in the design.
 

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Two mold side pieces mated together creating a parting line along the edge of the cap

 

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Left: Top molding piece removed, Right: Top molding piece installed

 

The two side mold pieces assembled together along the center line of the print head cap allowing the formation of the two long assembly cylinders.

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Side molding part opened up showing creation of lower cylinders used for installation

 

Finally, a bottom mold piece was assembled to the bottom of the two side molds and was used to seal off the part and also formed the hollow feature at the bottom of the cylinders.

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Bottom mold piece with larger cylinders that seal the bottom of the mold and two smaller pins that hollow out the part for assembly needs

 

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Installation cylinders hollowed out at the bottom

 

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Four mold pieces and RTV silicone molded part

 

Three pieces were assembled and the silicone material was poured into the holes at the bottom of the part. After filling the mold, the bottom piece is added.

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Bottom mold part removed allowing fill of the RTV silicone. Bottom is reinstalled before final cure

 

Numerous venting holes were used around the parting line at the intersection of the top mold piece of the two side mold pieces to allow bubble release and material flow for complete filling and part formation.

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Vent holes around the edge of the part at the parting line

 

Example 5: Small, 2-Part Direct Printed Mold Using M2-EBK/ENT Elastomeric Materials to Make both Silicone and Polyurethane Parts

Rigid MJP molds made from the MJP Rigid materials or M2G-CL Armor material work well for RTV Silicone parts because the silicone has excellent release properties and is flexible enough to facilitate part release after molding. However, often an engineer or designer requires a harder, stiffer, and/or stronger part be molded. For example, prototyping of injection molded parts is primarily the need due to the propensity of that manufacturing process in product design. For this case, it can be advantageous to make the direct printed mold in the elastomeric materials such as the M2-EBK or M2-ENT instead of the rigid or engineering materials. M2-EBK and M2-ENT are 30A materials that are rigid enough to form many required geometries, but soft and flexible enough to facilitate part release after cure. For this example, a simple 2-piece mold was created that made simple plastic pipe clamp. The same CAD workflow (cavity/Boolean tool) can be used for elastomeric molds as has been described previously. The two mold halves are held together with clamps and the material is poured into the mold.

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Filling features and air bubble features can be designed in CAD and printed, but for elastomeric materials can also be cut into the material after printing.

 

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Left: Opening a direct printed mold made from M2-EBK and filled with RTV silicone
Right: Same mold, but filled with rigid polyurethane resin showing small flashing and air venting

 

The key process difference with this method is to ensure adequate release performance between the elastomeric material and the polyurethane resin. Release can be more difficult with polyurethane resins and mold release is typically required on all surfaces.

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Polyurethathane resin releases from MJP elastomeric material with the use of a mold release

 

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RTV Silicone rubber can release without mold release form both Rigid and Elastomeric MJP materials

 

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Bubble formation due to improper venting, Left: RTV Silicone rubber, Right: polyurethane resin.

 

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Two-part mold and successful polyurethane resin part

 

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Car body example using M2-EBK elastomeric material and polyurethane casting resin

 

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Example using large clamps and steel/wood clamping plates to seal the mold without mold deformation

 

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Fill hole diameter dependence on casting resin fill performance. I.e., larger and shorter fill holes are more robust to bubble release and complete cavity fill.

 

Example 6: Direct Printed Eggshell Style Mold

Eggshell molding is a two-step, sacrificial mold-making process for casting silicone rubber parts. In this process, a thin “skin” or “shell” of the part geometry is created. Special software workflows are typically used to create the shell and also add the needed gates and vents to allow filling of the hollow shell structure and bubble removal. Besides vents and injection ports, often structural bracing is required in eggshell molding to facilitate fluid flow for complete cavity filling and for structural needs of the printed mold. Once the eggshell is designed based on the part geometry, the entire shell of the part along with the filling connectors and air vents are printed together. The material is poured or injected into the cavity and allowed to cure. The 3D printed shell structure is thin and fragile enough such that it can be broken and removed without damaging the part. While this process typically involves special tools and training, it is a very capable process and well-known in certain industries such as medical modeling.

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Example TRV silicone rubber part made from an eggshell mold

 

As long as the part will fit in the build volume, MJP is a great technology to make eggshell molds. The value of MJP technology to eggshell molding includes

1. Geometric Freedom
2. High Resolution
3. No Contact Support Structures that Cause Surface Defects
4. Simple Post Process

A special post processing method is often used for eggshell molds whereby the fine wax removal step is skipped allowing a very thin film of support to coat the internal surfaces of the mold. This improves part removal and surface cure and surface quality. The MJP M2R-TN material has been found to be the best material for this need. It has good strength and stiffness allowing creation of the thin structures, vents and ports. It also has sufficient heat deflection temperature allowing good dimensional accuracy during post processing. Finally, it has good enough toughness, strength and elongation allowing successful post processing and handling during the mold fill process, yet is brittle enough to allow successful removal of the shell after pouring the mold.

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MJP eggshell mold printed in M2R-TN material along with the resulting silicone rubber camera housing

 

Eggshell molding technique can add tremendous value in certain RTV casting applications. For anyone interested in this type of molding, contact 3D Systems Applications Engineering.