Category Archive: Silicone Rubbers
Part 2: Casting the Unicorn
In part 1 of this how-to guide, we showed you how we created the silicone mold using
Hapsil™ 360. In part 2, we will go through the steps and materials needed to cast a water clear part out of Ultraclear™ 480N-40, using Hapco’s X-80 Molding Chamber and X-Vac™ Chamber.
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
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.
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.”
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.
