Category Archive: Liquid Rubbers
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.
Part 1 – Making the Mold
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.
Hapsil™ 360 is mixed thoroughly and degassed in an X-Vac™ Chamberto remove any trapped air before pouring.
Hapsil™ 360 is slowly poured over the pattern until it reaches the top of the mold box.
The silicone is left to cure overnight at room temperature.
Once the silicone has cured, the cardboard and clay are removed. The cured silicone mold half is temporarily removed from the frame for easier cleaning.
The pattern is temporarily removed allowing the mold to be cleaned thoroughly with isopropyl alcohol.Grease-It™ 5 release agent is also sprayed on the mold.
Once the unicorn pattern and mold are placed back in the frame, a final coat of Grease-It™ 5 is evenly applied over the surface.
Step 5 is repeated and the Hapsil™ 360 is poured evenly over the part in a thin, steady stream. It is best to pour from one side to allow the air to escape as it’s filling.
The uncured silicone is allowed to cure at room temperature overnight.
The silicone is removed from the frame and separated. Once the pattern is demolded, the silicone is cleaned thoroughly with isopropyl alcohol.
It is important to understand how the mold will be oriented and to consider where air may get trapped. Vents are carved out of one mold half to give air bubbles a path to escape.
The two mold halves are separated and placed in an oven at 125°F for 8 hrs. It is important to separate the mold halves to allow any oils or
residue to flash off.
A hole is cut where the material will be poured into. This represents the top of the mold.
The silicone rubber is completely supported by the wooden box. The two side pieces are screwed into place and the mold is rotated so that the hole is on the top. It is now ready for casting.
To learn how we cast a clear part using Ultraclear™ 480N-40, view Part 2- Casting the Unicorn.
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!
Silicones are commonly used in the liquid molding process to make molds and parts. Understanding the differences between the different types of silicone can be helpful before deciding what to buy for your application.
There are three basic types of what are called RTV (Room Temperature Vulcanizing) silicones. The simplest are called RTV-1 silicones which are commonly used for sealing or calking. All materials in the RTV-1 group are one component, condensation curing materials. This means that they only need to be exposed to the moisture in the air to cure. This type of silicone is not used to make molds or parts but can be useful if sealing a mold box or assembling a prototype.
Tin and Platinum based systems are both RTV-2(Two component) silicones. Tin based systems are condensation-cure and Platinum based systems are addition-cure. They are both composed of two components, designated A and B.
Condensation(Tin) cure silicone rubbers are excellent for mold making and prototype applications. They are generally easier to process and they will cure at room temperature over almost any surface with minimal shrinkage.
Platinum based RTV rubbers are more expensive than tin based materials. They provide two major advantages for mold-makers:
1.They give a longer mold life for production items.
2. They have superior heat resistance.
Whatever the application, it is always a good idea to talk with a customer service rep from the silicone manufacturer before you make a purchase. There are also a host of forums online that focus on casting and mold-making, where discussions with other members can help you find the right silicone. Finally, no matter what the circumstances, always test a small amount of your casting resin with a cured sample of the silicone to make sure they are compatible.
Fun Fact: Vulcanization is named after Vulcan, Roman god of fire.
Clear part with an uncured surface.
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.”
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.
8:00AM – 10:30AM
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.