Category Archive: Rapid Prototyping
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!
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.
Vacuum Degassing using the X-Vac™ Chamber
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.
Pressure Casting using the X-11 Molding Chamber
Q. Is one method preferred over the other?
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.
Hapco’s General Manager, Fred DeSimone, discusses the similarities and differences between the RAPIDFIL™ and MINIFIL™ meter-mix dispensing machines.
“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.
The world’s first 3D printed gun was successfully fired a few weeks ago and besides sparking numerous debates about gun laws and regulation of this technology, it also introduced 3D printing itself to mainstream America. In this article, I will discuss where this technology started, some of the current trends and the future possibilities for 3D printing.
Printing objects? What is this sorcery?
The media made it seem as though printing parts was a brand new technology and something out of Star Wars, when in fact, several 3D printing processes have been around since the 1980s, although much larger and more expensive.
3D printing generally involves creating a digital three dimensional model and then printing that model using thermoplastics in ultra-thin layers to make detailed plastic parts. There are multiple additive processes available that differ in the way layers are deposited and in the materials that can be used to create parts.
Selective laser sintering (SLS) and fused deposition modeling (FDM) melt or soften material to produce the layers, while others cure liquid materials using different sophisticated technologies, e.g., stereolithography (SLA). Each method has its own advantages and drawbacks, and some companies consequently offer a choice between powder and polymer for the material from which the object is built. The main factors that dictate the appropriate process are generally speed, cost and choice of materials.
What are the current trends in this growing industry?
Fixed and variable costs for 3D printing have dropped substantially over the last few years, contributing to a trend known as “distributed manufacturing.” Consumers can download digital designs from a file sharing website and produce a product at home or at a local manufacturing facility with 3D printing equipment.
Building on this concept even further is a 3D printing, online marketplace called
shapeways.com, where users can have their models printed and shipped anywhere in the world. They also provide the option to open an online store and for selling parts without needing to stock items or pay any initial investments for tooling. Shapeways.com has integrated every step in the chain except creation of the 3D file. Carine Carmy of Shapeways says, “It’s going to force us to change the way we think about not only buying products, but how they’re made.”
Where is 3D printing headed in the future?
It’s becoming apparent that additive manufacturing has indeed reached the mainstream. President Obama even mentioned 3D printing in his State of the Union Address. This means that new business opportunities in many industries will start to develop, leading to even more interest, investment, research and development, new products, and wider consumer adoption.
One emerging use for this technology is the printing of food. Sugar and chocolate, which, like plastics, can be melted and re-hardened, have already been used to make edible art.
Even NASA has shown an interest in the possible applications of 3D printing in outer space. Not only are they exploring the possibility of printing tools and other materials, they recently gave a company in Texas a grant to develop a 3D food printer for use in space.
NASA tests 3D printing capabilities in zero g.
There may even be a time when slaughtering cows and chickens is no longer necessary. “Bioprinting” is a combination of tissue engineering and 3D printing, that researchers have been using to generate live tissue. The ability to print a burger patty or chicken breast that tastes normal is admittedly far off, but mostly because of current economies of scale and lack of enabling technologies.
The future of regenerative medicine also looks bright thanks to these technologies. In 2010, a company successfully bioprinted functional blood vessels made from the cells of an individual person. Researchers at Princeton and Cornell have already made progress 3D printing ears, and it seems likely that there will come a day when the wait time for an organ is only as long as the printer takes to create it. The possibilities for this technology seem to be limited only by our imaginations.
