For over 4,000 years, the evolution of the filter has been directly linked to the improvement of human health and life expectancy. The first great civilizations, like the ancient Egyptians, used sand and gravel as filter media to improve the taste and appearance of water. Today, filters have become an essential component to our entire way of life. They are found in countless industries, manufacturing facilities, processes, and in many cases, the end products themselves. More importantly, filters are enabling the tools and devices that are essential to defeating this invisible enemy and returning the world to some semblance of normalcy.
Since the onset of this pandemic, our society has gained a new appreciation for respirators, ventilators, and vaccines, as well as the vital role they play in saving lives and preventing future outbreaks. Like everything else in our modern industrial society, these life-saving tools all rely on specialized filter media and advanced filtration technology to function. It is obvious how filters are utilized in equipment like respirators and ventilators, but when it comes to vaccines the use of filter technology is not immediately apparent.
How are filters used for making vaccines?
A successful vaccine is the result of complex scientific processes that include the concentration of proteins and enzymes, blood plasma purification, virus and bacteria concentration and removal, as well as cell harvesting, clarification and washing. These procedures are all enabled by specialized filters and equipment.
Some common methods used in bioprocessing include membrane filtration, tangential flow filtration, centrifugation, and depth filtration. Implementing the proper filtration technology can have a positive effect on yield, product consistency, and overall efficiency of the entire operation.
What types of filters are used?
Hollow fiber filters possess excellent filtration performance and are commonly used in dialysis, water purification, reverse osmosis, separation of components from biological fluids, and cell culture devices to name a few.
Tangential flow filtration (TFF) systems are used extensively in the production of vaccines and other pharmaceutical drugs. They can be used to remove virus particles from solutions, clarify cell lysates, harvest and retain cells, and they can concentrate and desalt sample solutions ranging in volume from a few milliliters up to thousands of liters.
A HEPA (High Efficiency Particulate Air) filter works by forcing air through a fine mesh that traps harmful particles such as dust mites, pollen, pet dander, smoke, and even airborne viruses. HEPA filters are used in applications where contamination control is required, such as the manufacturing of semiconductors, disk drives, medical devices, food and pharmaceutical products, as well as in homes, vehicles, and hospitals.
How is Hapco involved in the filtration and ultrafiltration industry?
Hapco has been custom formulating adhesives, sealants, and potting compounds for some of the world’s largest filter manufacturers for over 40 years. Our materials and processing equipment are a key component to manufacturing a wide variety of specialized filters. As a preferred supplier to corporations like MilliporeSigma, Pall Life Sciences, and Koch Membranes, we take pride in our ability to provide customers with the highest quality polymers and the most reliable processing equipment available.
As we look to a post-pandemic future, our chemists are developing new formulations and processing methods to meet the needs of filter manufacturers around the world. We are currently conducting in-house testing on Filter-bond™ R-3590: a new epoxy formulation for the filtration market that is both Bisphenol-A (BPA) and nonylphenol-free.
What other Hapco products are used to manufacture filters?
The Filter-bond™ series was first developed in the 1980’s for various filtration and ultrafiltration applications. It includes formulations that do not contain aromatic amines or carcinogenic or mutagenic materials, systems that can be used to pot moist membrane material in place without foaming, and systems that are easily trimmed when used for pre-potting filters. Filter-bond™ includes a line of flexible and rigid materials to meet a wide variety of filtration applications. All Filter-bond™ products are compatible with Hapco’s MiniFIL™ and RapidFIL™ dispensing machines, which are used for potting or encapsulating various filter media.
Filters are one of mankind’s greatest achievements and a major reason our life expectancy has increased dramatically over the past 200 years. They clean the air we breathe, the water we drink, the fuel that moves us forward, and the medicine that keeps us healthy. Without them, there is simply no way to manufacture the life-saving and preventative drugs that offer us a light at the end of this tunnel.
Fun Fact: Hippocrates (460-370BC) was the first major proponent of water filtration in recorded history. He advised people to first boil, then filter water through two sewn together pieces of cloth which eventually came to be known as a Hippocrates’ Sleeve.
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.
In our previous post, we discussed the application of potting and encapsulating using urethanes and epoxies. When choosing the proper urethane or epoxy for an electrical application, there are some important considerations to keep in mind. In this article, we will discuss those considerations and how they apply to the world of urethanes and epoxies.
Here are some examples of electrical applications using urethanes and epoxies:
The 3 most commonly sought after resins for electrical applications can be classified as electrically conductive, electrically insulative, and statically dissipative.
Electrically conductive materials have a low electric resistance and electrons flow easily across the surface or bulk of the material. Charges go to ground or to another conductive object that the material contacts. These materials have a surface resistivity less than 1 x10^5 Ohm/sq or a volume resistivity less than 1 x 10^4 Ohm-cm. Electrically conductive resins are typically filled with metallic or conductive particles.
Electrically insulative materials prevent or limit the flow of electrons across their surface or through their volume. Insulative materials are difficult to ground and have a high electrical resistance. Static charges remain in place on these materials for a very long time. These materials are defined as having a surface resistivity of at least 1 x 10^12 Ohm/sq or a volume resistivity of at least 1 x 10^11 Ohm-cm.
Statically dissipative materials have a surface resistivity equal to or greater than 1 x 10^5 Ohm/sq but less than 1 x 10^12 W/sq. They have a volume resistivity equal to or greater than 1 x 10^4 Ohm-cm but less than 1 x 10^11 Ohm-cm. For these materials, the charges flow to ground more slowly and in a somewhat more controlled manner than with conductive materials.
The European Union set forth the RoHS (Restriction of Hazardous Substances) Directive to establish environmental guidelines and legislation to reduce the presence of six (6) materials deemed hazardous to the environment. To comply, products entering the EU must not have a homogeneous presence of these materials above the following levels by weight percentage:
Lead (Pb) < 0.1%
Mercury (Hg) < 0.1%
Cadmium (Cd) < 0.01%
Hexavalent Chromium (CrVI) < 0.1%
Polybrominated Biphenyls (PBB) < 0.1%
Polybrominated Diphenyl Esters (PBDE) < 0.1%
As of July 2019, certain raw ingredients have been added to the list of restricted chemicals. Some Hapco products contain these restricted ingredients and no longer meet RoHS requirements; however, the majority of our product line still complies. If you require a material certification for a specific Hapco product please contact us.
Enacted in 1989 and amended most recently in 2006, The Toxics Use Reduction Act(TURA) requires Massachusetts companies that use large quantities of specific toxic chemicals to evaluate and plan for pollution prevention opportunities, implement them if practical, and annually measure and report the results. Learn more.
TURA Reporting & Fees
Each company considered a Large Quantity Toxics User is required to file an annual toxics use report for every listed chemical it manufactures, processes, or otherwise uses above applicable thresholds.
TURA Planning Requirements
A Toxics Use Reduction (TUR) Plan is a document that provides both economic and technical evaluations of the toxics use reduction opportunities available to a company, and identifies those methods if any, that the company intends to implement.
Under the requirements of the Massachusetts Toxics Use Reduction Act (TURA), Hapco has been submitting annual chemical use reporting forms for di-isocyanates. As part of this TURA compliance program, Hapco expects to prepare a Toxics Use Reduction Plan update by July 1, 2014 aimed at reducing the use of our reportable chemicals. This TURA Plan must address the location and performance of our process equipment, and the plan must be approved by a certified Massachusetts Toxics Use Reduction Planner to assure that it demonstrates a good faith effort to identify toxic use reduction options and it meets the requirements of the Massachusetts Department of Environmental Protection.
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.
Polypropylene has a tensile strength of 4,500-6,000 psi, an elongation of 1-600% and an izod impact of 2.2-no break at all. For the best simulator of these properties, Hapco would recommend using Hapflex 666 or Hapflex 671. Both products come in slow and fast gel times and are available in a flame retardant version.
ABS (Impact resistance)
ABS has a tensile strength of 5,500-7,500 psi, an elongation of 5-25% with a flexural strength of 11,000 – 13,000 psi. Hapco’s Tuffalloy™ Series is a great candidate for simulation of ABS. You might also consider Ultralloy 109 or Hapflex 671, both of which have exceptional mechanical properties.
High-Temperature “Rubber” (heat resistance)
Hapflex™ 666 has a heat deflection temperature of 110°C and a service temperature of 135°C and Hapflex 671 has an HDT of 130°C and service temperature as high as 150°C (300°F). These would be our best recommendations for a semi-flexible product with high heat resistance.
Nylon 6-6 (Bearing and roller-type parts/gears)
Nylon comes in a variety of types, with each different type having a variety of grades. On average, it has a tensile strength of 7,500-11,000 psi, and an elongation of 30-100% with an izod impact of .6-2.2. Ultralloy 200 Series or Hapflex 671 would compare favorably to Nylon. By adding 20-25% milled glass to the products, you can make them very strong.
Polycarbonate (High impact with some flexibility)
Polycarbonate has a tensile strength of 9,000-10,500 psi with an elongation of 110-120% and a flexural strength of 12,500-13,500 psi. For simulating this product we would recommend using Hapco’s Ultralloy 200 Series, Ultralloy 900 Series or the Tuffalloy Series. Ultralloy 912 has exception impact strength, high HDT and is rate UL 94V0 @ .125” thick.
Polystyrene (Like polycarbonate but cheaper in production)
Polystyrene has a tensile strength of 5,200-7,500 psi, an elongation of 1.2-2.5% and flexural strength of 10,000 to 14,000 psi. For simulating this material we would recommend using Hapco’s Ultralloy 108 or 109.
Santoprene (rubber parts)
Santoprene is a thermoplastic rubber that comes in hardnesses ranging from 35 Shore A to 50 Shore D. IT exhibits good resistance to fatigue and has high tear strength. It also has good chemical resistance to many acids and aqueous solutions and performs well in thermo-cycling applications. For simulating these characteristics we would recommend the entire Hapflex™ Series which ranges from a 25A to 70D in hardness.
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.
“There are so many different static mixers, which one is right for my application?”, is a common question asked by our customers. The answer, as with many aspects to liquid molding, is complicated and should ultimately involve testing by the user within their application; however, there are some basic differences in the static mixer, which can help to at least narrow the choices down. This article will help to explain some of those differences.
What is a static mixer?
A static mixer, which is sometimes called a motionless mixer, is a disposable device with no moving parts. It consists of internal baffles or elements inside a plastic tube. However, this seemingly elementary device can effectively mix two liquids. As adhesive components are forced through the mixer, they are repeatedly divided and recombined, creating a uniform mixture.
What are the benefits to using a static mixer?
Consistency and reduced processing time are both good reasons for using a static mixer over traditional, hand-mixing and processing. The most beneficial aspect to static mixers however, is the fact that no air is introduced into the material during mixing. Air bubbles and voids can cause reject parts or bonding failure, and while air can still be introduced prior to mixing or via poor mold design, the mixing process is the most common cause of air bubbles.
What equipment can static mixers be used with?
In general, static mixer applications are used for 2 types of equipment: For handheld cartridge dispensing guns, where the components are separated in a molded plastic cartridge;
Or for use with meter, mix and dispense equipment, where both components are stored in separate steel cylinders before being introduced into a static mixer.
The diagram above shows a small fraction of the static mixers available. The visible differences include the diameter of the mixer, length, number of internal elements, shape and color, which denotes the material that the elements are made of.
Comparing the costs associated with continuously running the machine and purging the static mixer vs. frequently stopping the process and throwing away the mixer, can be extremely valuable financial information.
Here are some questions to ask before testing various static mixers:
What inside diameter and number of mixing elements create the right flow rate?
Is the static mixer the right length for the application?
Is the adhesive being dispensed in locations that are difficult to reach?
A custom mixer with an extension would be an effective solution in this case.
Does the application require specialized attachments?
How much “content volume” waste can be afforded?
Are the pressures high enough to require stronger elements?
If there is too great a pressure drop, a static mixer may not keep its shape, and the components could pass along the mixer walls instead of being correctly mixed. Likewise, if the elements break, the system could pass along broken plastic fragments.
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.”
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.
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