Over the past year, the wood and resin craft market has hit its stride. A quick search on Etsy or YouTube yields thousands of results. We’ve fielded numerous calls from artists, furniture makers, and entrepreneurs trying to find a good epoxy to use for casting river tables, wood/resin jewelry, lamps, and various other artistic endeavors.
Examples of high end goods that marry wood and epoxy with incredible results.
We were prepared to formulate a new material to meet the demands of the market, but first we wanted to gauge how our current formulations performed. We decided to test some of our high-end epoxies to get an idea of the handling properties that would be important, and also to gain insight into some of the challenges facing end-users.
One of the many tests. This was made using Ultraclear 480N-10 with a drop of TD-23 blue tint.
After months of testing, we found that our Sympoxy™ 1010-CA810 yielded the best results. It has a 1:1 ratio, great viscosity for coating, a 45 minute gel time, 24 hour cure time, and it has a beautiful, glossy finish. To really put this material to the test and to gain more first-hand experience, I decided to use the Sympoxy™ for a personal project that I had been planning for a few months: A ‘Game of Thrones’-inspired, epoxy river cribbage board.
Ultraclear™ 480N-40 is weighed out and poured into plastic containers. The B side is then added to the A side container.
The Ultraclear™ 480N-40 is mixed thoroughly for 2-3 minutes. It is a good idea to periodically scrape the sides and bottom of the container. Pouring into a second container and re-mixing is also recommended.
The mixed resin is placed into the X-Vac™ Chamber and degassed, removing air and moisture from the mixture.
The mold is placed upright in the X-Vac™ Chamber and the Ultraclear™ is slowly poured into the mold, leaving about 1/2” from the top of the mold.
The vacuum is turned on and the mold is watched carefully to avoid any material rising over the edge of the mold. One hand is kept on the valve to avoid any mishaps.
The mold box is placed on the X-80 Molding Chamber shelf and topped off with Ultraclear™.
Close the chamber door and tighten the clamps in a crisscross pattern. Slowly open the valve until the tank reaches the desired pressure. We recommend between 70-80 PSI.
The following day, the chamber is depressurized. Once relieved of pressure, the C clamps are loosened and the mold is removed.
The wooden frame is disassembled and the silicone mold is removed.
The silicone mold is laid flat on the bench and the two halves are carefully pulled apart.
The flash and vents are trimmed off. There will be a slight visible line where the parting line was. This can be buffed and polished after the piece is post cured.
The piece is placed in an oven at 80˚C for 8 hours to speed up the cure time and to strengthen it.
If you have any questions regarding this tutorial, or any of Hapco’s products or equipment, please feel free to call us toll free at: (877) 729-4272
In this article, we show you step by step, how to duplicate a complex pattern using Hapco’s high performance materials and equipment.
After taking measurements of the pattern and creating a drawing to outline our plan, we constructed a mold box using medium density overlay.
Orient the pattern inside the mold frame in a manner that will maximize the flow of material and minimize the amount of air that could get trapped. The paper represents cutouts that will reduce waste and save on material costs.
Pieces of cardboard were cut and layered to follow the shape and contours of the unicorn. This creates a foundation for a layer of clay that will represent the parting line for the two mold halves.
The clay is carefully smoothed out up to the halfway point to raise the part from the board and create a parting line along the middle.
The term “viscosity” refers to the thickness or flowability of a liquid. Viscosity numbers range from 1 (water) to millions of centipoise (cP) or pascal seconds (Pa.s), 1cP = 0.001 Pa.s. Refer to our viscosity comparison chart here.
Urethane and epoxy resins with viscosities ranging from <100cP to 1,000cP are ideal for most generic casting applications. They de-air very well on their own and flow easily into closed molds, whether mixed and poured by hand or dispensed using meter-mixing equipment. However, there are many specialty materials, such as, Hapco’s Steralloy™, Filterbond™ and Hapflex™ resins that are formulated for highly-engineered applications, and because of their unique chemistries, they have a thicker viscosity than other products, making them a bit trickier to process.
When mixing and pouring by hand, Hapco always recommends vacuum degassing the mixed resin prior to pouring. With viscous materials, it can be helpful to add a few drops of a surfactant, such as Hapco’s
Anti-Air™ product, which reduces surface tension and allows the resin to degas more easily. However, vacuum degassing alone does not always alleviate air bubbles due to cavitation of the material as it flows through the mold. It may also be necessary to cure your parts under pressure using a pressure-pot or molding chamber, like Hapco’s unique X-Series Molding Chambers.
When using meter-mix dispensing, Hapco recommends designing a mold that fills from the bottom up. A general rule in this case is to design the mold so that the output opening(s) equals 2-4 times that of the input. In simple terms, if you have a 0.50” diameter input, your out-put should equal 1”-2” in diameter. This enables a “pressure drop,” which minimizes any back-pressure build-up caused by shooting a viscous material into a closed mold.
When dealing with complex mold geometry, it may be beneficial to use a two-step degassing process. After initially degassing the resin mix, fill the molds and place them under vacuum again for an additional few minutes. This not only helps to release trapped air caused by material cavitation, but it will also “pull” the viscous material into the cavity to ensure a complete fill, especially if your mold has thin walls or complex geometry. While degassing the molds, the material inside will not swell up as it did during the initial degassing step, however, it may continue to “boil” somewhat. Therefore, it is advisable to fabricate a small “chimney” around the top of your mold to prevent material from spilling out. You can do this easily with wax, putty, or a simple strip of packaging/duct tape wrapped around the top of the mold. After secondary degassing you may find the need to top off the molds to ensure they are filled to proper height, in which case you should be able to do so without the need for further degassing
Other suggestions for thinning higher viscosity materials are as follows: Pre-heat the resin to 80° – 110°F. It is really only necessary to pre-heat the thicker component which is typically the Part A for most materials. As a general rule, for every 10° you heat the material above room temperature, the material viscosity is cut in half. Bear in mind though, that heat will also cause the material to gel faster, thereby reducing your overall work time. In lieu of pre-heating the resin, you can pre-heat the molds instead. This will maintain work time for mixing, and still thin the resin viscosity as it flows into the warm molds. Another suggestion would be to add a small amount of solvent, such as, isopropyl alcohol or acetone into the resin mix. Solvents will cut the viscosity without impacting curing or material properties in most cases, as they will flash off quickly once the material starts its exothermic reaction.
The bottom line is that you will need to incorporate the proper equipment and techniques into your process in order accommodate using viscous materials. Water-thin materials require very little in the way of specialized equipment and they certainly make things easier. However, limiting your material offerings can also limit your opportunities for getting more of those “high-dollar” projects. My advice for expanding your business opportunities is to think “outside of the mold-box,” and have enough flexibility in your process to take on those jobs that nobody else wants!
Achieving clear, bubble-free parts using clear epoxy or urethane is not impossible, but as anyone with experience will tell you, it’s not without its challenges either. The task can be even more daunting if the part possesses complex detail or undercuts, however with the right combination of materials, equipment, and expertise, attaining water-clear castings without excessive rejects is possible.
Ultraclear 480. The sample on the right was cast at 70 PSI while the sample on the left was cast at ambient pressure.
Vacuum degassing and/or pressure casting are perhaps the most popular if not the most efficient methods employed to create clear, void-free and bubble-free castings. Additional time and energy are required, and rejects are still possible.
Pulling a vacuum on liquid resin will remove air. Pressurizing will squeeze it down to invisible sizes while the part cures.
Heating resin and vibrating the mold is another method of choice for casting clear or any thermoset resin. This procedure helps relieve surface tension and allows air bubbles to more easily escape while filling the mold. Ultimately, the combination of heat and vibration can yield better results, but it is not a failsafe.
A mold is secured to a vibrating table to help air move to the surface while filling.
Regardless of the process, cast and mold material must be compatible for water-clear casts. For instance, some mold materials and release agents are not compatible with aliphatic urethanes(clears).
A medical component cast in an silicone mold that wasn’t post cured. This was left with a tacky surface and many tiny bubbles which were exacerbated due to not being pressure cast.
Choosing the correct release agent and applying it correctly is important to avoid tackiness and surface defects. Silicone-based release agents tend to react poorly with clear resins, causing cure-inhibition and other defects. This is why many molders will opt for a silicone mold to avoid the releasing process altogether, but it brings its own set of challenges to casting clear resins.
Grease-IT 2 is an example of a PVA release agent.
The only release agent that can be considered a fail-safe is Polyvinyl Alcohol(PVA). This one part liquid, which can be sprayed or brushed on, dries to form a non-reactive film over the part.
RTV Silicone rubber, be it tin or platinum-based, are most often the choice of liquid molders because of their self-releasing properties and flexibility. The major issue with casting clear resins in silicone molds is the fact that the surface of the part can be tacky or uncured upon de-molding. This phenomenon, typically referred to as cure inhibition, is a major challenge with very limited solutions. Post-curing the silicone mold before use is essential in flashing off some of the natural oils and acids on the surface. Those substances are the major reason why many clear resins have trouble fully curing. Unfortunately, post-curing is not always possible when molds are exceptionally large.
Polishing the finished piece is almost always necessary, especially when considering that upon de-molding most parts have parting lines, gates, and vents that require removal. This can be achieved with a benchtop buffing machine or done by hand. Either method will require a polishing compound. This can add a considerable amount of time and energy depending on the size and complexity of the piece.
Ultimately, success when casting crystal clear resins is best achieved when the process (pressurizing, vibrating, etc.) and materials come together to provide the best outcome.
Ultraclear™ is Hapco’s series of water clear casting resins. They are a 1:1 ratio by weight and volume and very low viscosity to make mixing and pouring easy. They are also 100% mercury free unlike most clear resins on the market.
Hapsil™ 360 is Hapco’s RTV silicone rubber that was designed to be compatible with aliphatic casting resins and not inhibit the cure.
Q. What is the difference between vacuum degassing and pressurizing?
A. Vacuum de-gassing expands the air trapped during mixing or pouring, causing the bubbles to grow, rise to the surface, and in most cases, release. After a period of time the amount of trapped air decreases. The material’s viscosity and surface tension will determine how easily the air will escape. Certain materials appear to bubble indefinitely until the vacuum pump is turned off. In order to maximize the vacuum’s potential for air removal, the pump must be capable of pulling 29.6 inHg.
When placed under pressure, any air bubbles entrapped from the mixing and pouring process shrink to the point where they are no longer visible. Pressure ranging from 60-80 psi significantly reduces the chances of visible air bubbles. For pressure to be effective, the liquid thermoset material must remain under pressure until it has reached its gel time, otherwise the bubbles may expand once the pressure is relieved.
A. One method really isn’t preferred over the other. Whether you choose to use vacuum or pressure depends on the application, your capabilities, and budget. In fact, we often recommend vacuum degassing your product after hand mixing, pouring into the mold, and then using pressure to make sure all parts of the mold are filled. When using dispensing equipment, there is no air introduced during mixing. That doesn’t mean air cannot be introduced while shooting into your mold, especially one with many thin walls, sharp corners, or intricate details.
If you find yourself in a situation where you absolutely need a bubble free part (e.g., using a plaster mold or a large quantity of expensive material) it’s best to play it safe and employ both methods. If neither option is in your budget and you need a void free surface, we recommend using low viscosity materials, mixing slowly and thoroughly, and brushing a thin coat onto the mold or pattern’s surface. Using a hair dryer while brushing the thin coat will help to ease surface tension and reduce bubbles.
Potting consists of immersing the part or assembly in a liquid resin, and then curing it. Although often confused for each other, potting is different from encapsulation in that it retains the shell that is used to contain the thermoset resin while it’s curing.
Encapsulation involves building a mold or frame around an object, e.g., wires, filling the space between the frame and the object with a thermosetting material such as Di-Pak™, waiting for the resin to cure, and then removing the frame.
These processes are commonly employed to protect semiconductor components from moisture and mechanical damage. They are also used in high voltage products to allow live parts to be placed much closer together, so that the product can be smaller. They keep dirt and conductive contaminants such as impure water out of sensitive areas and serve as structural reinforcement, protecting sonar transducers and other deep submergence items from collapsing under extreme pressure. Potting with black or opaque epoxies and polyurethanes can be used to discourage reverse engineering of proprietary products such as printed circuit modules.
In the past, we’ve discussed the importance of temperature control when casting with thermoset resins. In this article, we will focus on the post cure process for molds or parts made with thermoset resins.
What is post-curing?
Post curing is the process of exposing a part or mold to elevated temperatures to speed up the curing process and to maximize some of the material’s physical properties. This is usually done after the material has cured at room temperature for at least 12 hours. In general, thermoset materials will achieve full cure at room temperature over a period of 7-10 days. After a full cure is achieved at room temperature, post curing will have no effect on the material’s properties.
Why is post curing necessary?
Post curing will expedite the cross-linking process and properly align the polymer’s molecules. Much like tempering steel, post curing thermosets can increase physical properties (e.g., tensile strength, flexural strength, and heat distortion temperature) above what the material would normally achieve at room temperature.
Post curing is extremely important when an application requires secondary machining.
Fred DeSimone of Hapco says, “Post curing your parts prior to machining is critical to ensure dimensional stability, particularly when trying to maintain tight tolerances. Elevated temperature acts as a catalyst to complete the cross-linking process and stabilizes the cured plastic so that it does not continue to creep over time. Although most thermosets will appear to be cured after several hours at ambient room temperature, the reality is that it can take up to two weeks for the material to fully cure. If secondary machining is completed during this time on non-post cured parts, they can either shrink or grow out of specification while the polymerization process is still occurring.”
How should I post cure my parts?
An oven is best for applying uniform heating, but we don’t recommend using the one in your kitchen. Applying too much heat to some materials may result in dangerous fumes being emitted or a material may melt, ruining your oven. Digital, vented lab ovens are ideal for post curing parts and molds; however, this can also be done in an X-Series Molding Chamber.
During elevated temperatures, thin-walled parts may bubble or deform. Keeping them in the mold or using a fixture during post cure is recommended. Make sure that anything you place in an oven can take the heat.
Is there an ideal post cure temperature?
Generally speaking, rigid materials are post cured at 175F for 8-24 hours and flexible materials at 140F for 8-24 hrs. Be aware that post cure temperatures vary for different materials. Please review the Material Handling & Safety Notes supplied with your specific Hapco products, or contact Hapco’s Technical Support for more information.
If I had to come up with a list of the most common issues our customers call us about, along with air bubbles, a sticky surface on their clear castings would be at the top. My first question to them is always: Are you using silicone?
In 95% of tacky surface issues, I can only remember a few instances when silicone wasn’t used either as the mold material or release agent. The problem seemed large enough to dig deeper and I found this issue to be more complicated than any one factor.
Why is this phenomenon more common with clear resins?
The properties of a polyurethane are greatly influenced by the types of isocyanates and polyols used to make it. Of the two types of isocyanates, aromatic and aliphatic, aromatics are the most common. In general, they are less costly and produce shorter gel times, while aliphatics are used when longer gel times or UV stability is necessary. If you are using a water clear resin, chances are it is an aliphatic system.
The chemistry of aliphatic urethanes is not necessarily incompatible with the chemistry of silicone; however, the more time it takes for a thermosetting material to crosslink and cure, the more chance it has to react with by-products of the silicone, particularly on the surface.
Is there a difference between tin or platinum cured silicone?
The type of silicone used, tin or platinum cured, is an important factor when looking at this problem. Isopropyl alcohol is a by-product of the chemical reaction in tin cured systems. The presence of alcohol on the surface of a mold reacts negatively with aliphatic urethanes, resulting in a semi-cured part with a sticky surface.
In the early days of my career at Hapco, we would generally recommend using platinum silicone vs. tin with our clear resins, but this rarely, if ever, solved the issue. After researching this problem in depth, the causes are not so straightforward. Like Hapco does with its urethanes and epoxies, manufacturers of silicones use a variety of additives to produce different physical properties. The quality or chemistry of raw components can, and does, have an effect on how well they work with aliphatic resins.
Ultraclear Part cast in an RTV silicone mold.
While it’s true that some platinum silicones worked better than others, post curing any silicone with heat can be the difference between a perfect part or a reject. Many of the platinum silicone users who called in regards to this issue didn’t know they had to post-cure their molds. Even tin based silicones designed to work with aliphatic resins, like our Hapsil™ 360 for example, must be post cured to flash off any alcohol. In addition to flashing off negative by-products, preheating a mold to around 90F prior to casting is a good way to avoid shrink marks and suck backs, especially in larger parts.
Grease-IT 2 is an example of a PVA release agent.
Even though silicone molds are self-releasing, many customers choose to use a mold release to extend their useable life. Using silicone-based mold releases with aliphatic urethanes can exacerbate the problem even further. A non-silicone based release, like Hapco’s Grease-It™ Two is recommended.
What can be done to avoid this issue?
Some users have found that rubbing Vitamin C on the mold can help neutralize some of the negative by-products, although it hasn’t been researched sufficiently yet. The best advice I can give is:
1.) Always post cure your platinum or tin catalyzed silicone molds even if it will cure at room temperature.
2.) Always test a small amount of your desired casting resin with whatever silicone you plan on using.
There may not be a simple answer to every problem that casting clear resins in silicones presents, but I hope this article can at least give you a better understanding of some of the root causes. As they say, “knowing is half the battle.”
The liquid molding, or the resin casting process as it is typically referred to, lets designers produce high-quality plastic parts economically in low volumes. Any shape that can be machined or injection molded can be cast, however, liquid molding becomes most attractive when part geometry poses design restrictions for traditional processes.
Varying wall thicknesses, internal curved passageways, and zero draft are routinely designed in liquid molded parts. The use of flexible rubber molds allow internal and external threads, O-ring grooves, or other modest undercuts to be easily cast in place without the need for costly secondary operations.
Combining multiple pieces in the same mold can save time and money.
Combining several parts into one is usually an easy way for a designer to get added value with this process. Custom colors, textures and graphics can also be designed as cast-in details. And Insert-molding items or encapsulating delicate components and electronics can be achieved without damage because of the relatively mild processing conditions associate with liquid molding. Many thermoset resin formulations will fully cure at ambient room temperature over the course of several days, and if post-curing is required or recommended, these temperatures typically do not exceed 80°C (175°F).
Standard tolerances on part geometry defined by rubber molds (soft tooling), are approximately ±.004-.006 in./in., and are referred to as “as-cast tolerances.” With soft tooling, one needs to consider the harness of the flexible mold material which can range from putty-like to semi-rigid. In general, a harder durometer material will allow for tighter tolerance control. Using metal (hard tooling), rather than rubber to define critical part features will typically yield tolerances of ±.002 – .004 in./in.. A careful assessment of tolerance and dimensional requirements is essential at the onset of a project, and material shrink factors should be taken into consideration prior to building your mold or pattern. However, bear in mind that many things affect shrinkage, such as part geometry, mass, and material gel times and cure temperatures.
A few months back, we discussed the differences between Liquid Molding and Injection Molding and described when each method is appropriate. As a follow up, we thought it would be important to focus on Liquid Molding and to discuss some of the tooling options as well as the advantages and disadvantages to using each.
Aluminum is the most commonly used metal when it comes to mold making. Extremely tight tolerances are possible with today’s CNC milling machines. Close to mirror finishes are possible straight from the machine. Mold designers will ideally design in draft to the mold for easy part removal; however, zero draft angles can be accommodated.
Production rates for aluminum molds are limited to one part per cavity per day. Releasing and preheating the mold is often necessary and should be factored in when considering turnover time. Once part removal and mold cleaning are factored in, yield rates of 15-20 parts per month can be expected.
Plastic or composite molds are usually made backwards. A pattern is created first using wax, clay, foam, etc. and then, via liquid molding or fiberglass layup, a mold is formed around it. This method is done using thermoset materials; however some thermoplastic materials can be milled like aluminum. A good example of this is High Density Polyethelene(HDPE) which, because of it’s self-releasing properties, can decrease cycle times.
Liquid Molding and fiberglass layup should be considered for larger parts with 2 or more dimensions. Some of the primary benefits are a lower cost and design change flexibility. Surface finishes are typically as good as the original patterns. Even a fingerprint on the on the original pattern will show up on the cast mold.
Much like plastic or composite molds, silicone and urethane rubber molds rely on a pattern as the primary tooling element. A flexible rubber, such as RTV silicone, is poured over the pattern and allowed to cure. The pattern is then removed and a liquid plastic is then poured into the cavity, replicating the part.
Three-Part Silicone Mold
Silicone rubbers, while more expensive than urethane rubbers, do not require release agents on the pattern or finished mold. They are also rated for much higher temperature environments which should be considered when a post cure is necessary.
This video will teach you how to create a mold with a high performance surface material and a machinable, low cost backup, which in this case is Hapco’s Fill-Its/Haprez combo. The reason for creating a mold like this is to cut down on the thickness of the Hapflex 668, a high performance material, which can be expensive. This method also reduces the amount of shrinkage that would normally occur as a result of casting large quantities of liquid plastic around an odd shaped pattern.
If you would like to download a printable version of this tutorial, click here.
Use some clay to raise the part from the parting board and orient the part to maximize the flow of material and minimize the amount of air that could get trapped.
Build up a mound of clay underneath the part, leaving room for the gate and vent. The angle from the clay to the board should be greater than 90º to make de-molding easier.
Make sure there is a complete seal between the edge of the part and the clay to avoid undercuts.
Using Greast-It™ Wax P or another wax release agent, wax and buff the parting board and registration buttons. This will help seal it and act as a buffer between the board and the Grease-It™ Two.(PVA spray release)
Using a Spray Gun, evenly coat the part and board with Grease-It™ Two. This is a Polyvinyl Alcohol which forms a thin film that polyurethanes or epoxies won’t stick to. It is water soluble and can be easily washed off with soapy water.
Using a household hair dryer, dry the release agent in-between coats to speed up the process and ensure a smooth, even coating. Generally, 2-3 coats should be applied.
After releasing the part, create a frame around it and mount it to the parting board. This should be at least .5” from the part on all sides but no more than 2” to minimize cost and shrinkage. Make sure to seal the edges where material may leak from using hot glue, clay, or wax.
We are filling this particular mold through a backplate that will be permanently attached to the rubber once it’s cured. Drill vent holes roughly 1 1/2” apart all over the top and add one larger hole that will be used as the fill port.
Secure the mold frame and parting board to a flat surface or a vibrating cart. This will allow the air to rise to the top more easily.
For this straight casting application, we are using Hapco’s
MINIFIL™ dispenser. This meter mix machine is necessary to save time and money and eliminate the measuring, manual mixing, and mess associated with casting urethane rubbers and plastics. This particular mold will be cast using Hapco’s Steralloy™ 2036-5.
Noon – 5PM
Using a slightly rounded filleting tool, remove the clay from the pattern. Be careful not to gouge or stab the mold because it is still very soft.
When all of the clay is removed and the part is clean, add another piece of half round wax to the top of the other half. Use clay or wax to connect the gate and vent to the part.
Using a spray gun, evenly coat this half of the mold with Grease-It™ Two
. Use a hair dryer in-between coats to speed up the process and ensure a smooth, even coating. Repeat this process 2-3 more times.
TIP: Apply tape to the top of the frame and then remove it after the second half is cured. This will create a tighter seal between the two halves of the finished mold.
Cut and drill a second backplate and screw it onto the frame. Seal around the seam line of the two halves using tape, hot glue, clay or wax.
Secure the mold box to a vibrating table using clamps or straps to make sure the mold doesn’t vibrate off of it during the filling process. Again, we are using Hapco’s MINIFIL™ dispenser. This half of the mold will also be cast using Hapco’s Steralloy™ 2036-5.
In just one day we have created a two part mold using Hapco’s Steralloy™ 2036-5. This material is considered food grade and very soft with a
durometer of 35A. It’s 5 minute gel time allowed us to get the job done fast.
“It ain’t rocket science” as they say, however, the resin-casting/liquid molding process does require “the right stuff” in order to ensure positive results. Whether you’re producing one or two prototypes, or manufacturing a low-volume production run of a few hundred, your goal should always be to cast perfect parts and eliminate rejects.
One way to achieve this is to document the equipment, supplies and procedures necessary for each project ahead of time; prior to hand-mixing and pouring or meter-mix dispensing your material. A pre-casting checklist will help eliminate the need to scramble at the last minute, while your resin gels, in order to incorporate something into the process that you overlooked. Every project is different; however, there are some basic essentials required for just about every application.
Many materials have a different mix ratio by weight and by volume, and one of the key factors to ensure success is ratio accuracy, especially when hand-mixing versus meter-mixing. In this case, an accurate gram scale is an essential tool. Try to avoid using measuring cups or beakers to portion out your components by volume. The volume ratio should only be considered when using accurate meter-mix dispensing equipment, and even with this type of equipment, ratio checks should always be calculated by weight.
In most cases, you will be pouring and mixing materials in containers other than those they were packaged in. Plastic and/or metal buckets and mixers are recommended over paper or wood products since these can retain moisture that will contaminate your mix.
It is good practice to calculate your formula and document it prior to actually weighing out the components, and be sure to tare your scale before pouring off the material (this involves “zeroing-out” the scale with the empty mixing container on it). You should try to achieve ratio accuracy within ±2% of the manufacturer’s specification.
Be sure to wear a protective smock or apron, eye wear and gloves, not only for safety, but to avoid the difficulties of cleaning resin and pigments off of skin and clothing. Also, make sure to keep plenty of lint-free towels at your disposal to clean off any excess or spilled material from your containers and scale.
If you are using vacuum degassing or pressure casting equipment, there are some general guidelines to follow. In order to effectively degas liquid materials, your vacuum chamber must be capable of pulling a minimum of 29.6 inHg. If curing your materials inside a pressure chamber is required, 60-80psi is recommended in order to compress air bubbles down to microscopic size. Periodic maintenance of pumps and seals is critical to keeping this equipment in good working order, and a “preventative maintenance log” should be formally documented and kept on file. Thermoset resins can be quite costly and it would be a shame to go through all the previous steps just to find you have a leak in your tank.
Here is a basic checklist to keep on hand before embarking on your next casting project:
Temperature and humidity in your work environment is controlled and kept constant throughout the process; 20-25°C (68-77°F) with relative humidity of 43-47%.
All equipment and tools are clean, calibrated and fully operational.
Liquid materials, molds, inserts and encapsulates have been properly stored, pre-heated and coated with mold release if necessary.
Plenty of containers, mixers, and towels are within easy access.
Casting work benches are level and free of extraneous materials.
Work instructions and formulations are close at hand for quick review and verification if needed (always double-check your formula/mix ratio).
Clock or timer is within easy viewing distance. A general rule-of thumb is that your material should be mixed, degassed and poured within ½ – ¾ of the material’s gel time.
Trays, flat-plates and carts are readily available for easy transport of poured molds around your work environment or into vacuum and pressure vessels.
Protective eye wear, latex gloves are on and any other safety precautions in place.
Although this topic is pertinent year-round, it is especially relevant at this time when most of the country is being plagued with unusually hot, humid weather that can wreak havoc on the liquid molding process. Thermoset polyurethane resins, which are by far the most widely used materials for this process, are particularly sensitive to moisture.
Isocyanate compounds, used as the reactive ingredients in these formulations, will actually begin to cure prematurely and “foam-up” if they become contaminated. This can result in the cured material being laden with tiny air bubbles throughout which, in most cases, will leave you with a cosmetically unacceptable part. A work environment temperature of between 20 – 25°C (68-77°F) with relative humidity between 45-50% is ideal.
Moisture contamination on the mold or in the part can result in hundreds of tiny air bubbles.
Other precautionary steps to safeguard against moisture contamination include; purging opened material containers with dry nitrogen gas or warm air (hairdryer) prior to resealing; using metal or plastic containers and stirrers for mixing and pouring; and pre-heating masters, models and molds before processing to make certain they are moisture-free. Also, material containers should be stored off of cement floors and on shelving if possible.
Heat alone does not adversely affect thermoset materials. As a matter of fact, in many cases it can be used to a molder’s advantage. Be aware that it has two primary effects; it will thin the material’s viscosity and also speed up the reaction times (gelation & cure). A general rule-of-thumb is that for every 10°C you increase the temperature of the reactants, the reaction-rate will double. In other words, the gel time will be cut in half. Heat can be used effectively to allow a viscous material to flow more easily and also to increase throughput, however, if not properly controlled it can also cause problems, such as, sink marks and excessive shrinkage on your parts. Conversely, cold temperatures have the opposite effect; material viscosity thickens and gel/cure times will be lengthened. Again, this can be used to a molder’s advantage if the proper controls are put in place. It is a common practice in mold-making to incorporate heating or cooling elements within the structure of a tool to control a material’s exothermic reaction while it cures.
The bottom line is that temperature/humidity control is extremely important in order to produce high-quality parts. Ideally, your materials, molds, and any ancillary components used in the process should all be maintained at an even temperature. Learning how to tweak these elements to your advantage is part of the “magic and art” of Liquid Molding.
Outside of its design, precisely how your product or part is molded has the greatest bearing on its cost and capabilities. Great ideas can founder taking the wrong course. So it is prudent to consider your options early in design. How do you know when liquid molding ends and injection molding begins?
Thermoplastic and Thermoset Differences
One of the first—but not always the most important—questions to ask is: How many pieces do I need to make? High volume generally requires high production speed, which is the primary benefit of injection molding. However, thermoplastic molds are expensive to make and production runs in the thousands or millions are typically required to offset that initial mold investment. Conversely, several tooling techniques can be used for liquid molding; such as, plastic or composite molds or soft rubber molds that can significantly minimize up-front expenses, yet still produce high-quality plastic parts for both prototyping and low volume production. (1-5,000 parts)
The three main families of thermoset resins are polyurethanes, epoxies and silicones. Of these, polyurethanes are by far the most widely used and are available in both elastomeric and rigid formulations. Epoxies are usually the materials of choice for high temperature and corrosion resistance, but their cross-link density also results in a tendency towards brittleness. Silicones should be considered where continuous flexibility over a broad temperature range is required. Their natural self-releasing(non-stick) attributes can be used to a molder’s advantage.
Benefits of Liquid Molding
Thermoplastics are available in pellet or sheet form and must be melted and formed utilizing specialized molding equipment. A major benefit to using these materials is that they can be re-melted and re-formed over and over, unlike thermosets which are chemically inert once cured.
Another major advantage of liquid molding is that the process imparts much more design freedom over conventional injection molding. Since the casting does not involve the use of high heat and pressures to initially melt the material so that it can flow evenly into the mold, the designer is not limited to maintaining uniform part geometry.
Liquid molding lends itself well to producing highly complex parts at lower volumes and may always win in the early stages of production. It can also have advantages over injection molding in the manufacturing of medical and surgical equipment, scientific instrumentation, and electronics just to name a few. When part volumes exceed a few thousand; however, injection molding is usually the best option.
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