We often receive common questions concerning our equipment and products. Peruse the list of typical questions below to see if perhaps your question has been answered before.
Q: What is Double Mixing?
A: Double mixing involves mixing the A&B components thoroughly in one container and then transferring the mixture into a new, clean container. This process leaves much of the unmixed material that hides in the corners and sides of the cup behind, ensuring a more thorough mixture. Unmixed material can show up in the part as soft spots or unsightly swirls/streaks. This process is extremely important when casting silicones and water clear materials.
Q: What is Cure Inhibition in Silicones?
A: Some materials can have chemical incompatibilities when poured into or up against other materials causing an uncured, sticky mess. For example, platinum silicones may show inhibition characteristics against natural rubbers, cyanoacrylates, tape residue, sulfur based clays, and some urethanes. Whenever in doubt, a small side test is always recommended.
Q: What is Cured Inhibition in Urethanes?
A: Certain urethane systems may be affected by the type of silicone mold you cast into. Tin based silicones (condensation cured) are known to cause issues with many Shore A Elastomer urethane systems and also Aliphatic urethanes (e.g. our Water Clear materials). Silicone mold material must be carefully selected depending on the urethane system you plan to cast. Casting a urethane into an incompatible silicone can be an expensive disaster; both money and time.
Q: Why do I need to Mix Individual Components/Containers?
A: With many urethane and epoxy systems, the individual components in a container may need to be mixed and/or stirred prior to weighing out on a scale. With storage time and ambient temperature changes, ingredients in the material may separate out and need to be remixed. It is of critical importance with fire retardant materials to pre-mix before every use or the heavier fillers may settle at the bottom. Filled or pigmented materials may also need stirring prior to weighing out on the scale for the same reasons. Water Clear materials are very sensitive to cold storage temperatures and may need to be both heated and stirred to ensure a clear end product.
Q: What happens if I want to Demold early?
A: Casting materials need time to chemically crosslink and to gain physical properties. Removing a part from a mold early or casting in cold temperatures increases the risk of breaking a part due to brittleness or low tear strength. With most materials, adding heat can speed up demold time but the mold material must be able to withstand elevated temperatures without deforming. Some of the Shore A Aliphatic systems (e.g. Shore A Water Clears) will have adverse affects if you heat them too much before they have time to gel. See How Do I Work with Water Clears.
Q: What is Work Time?
A: The time you have from the moment you begin mixing to the point you can still easily pour material into a mold. Once the material starts to gel (or get higher in viscosity) the material does not have good flow characteristics and may not produce a good part. Always be aware of the working time rated on the data sheet and listed mass it is calculated at. Adding more mass (e.g. 400g vs. 100g) will shorten the working time. Mixing an appropriate batch size can assure you have time to vacuum and pour your material. Always choose an appropriate working time based on how difficult your cast is and how much material will be mixed prior to casting.
Q: How does Temperature affect Casting Materials?
A: Ambient temperature and storage temperature can have a drastic affect on materials. Working time, demold time, viscosity, and physical properties of the material are all affected by temperature. In cold winter months working with thermal set systems (urethanes, epoxies, etc.) can present challenges not seen in more moderate seasons. Materials can be much higher in viscosity making mixing, degassing, and pouring more difficult. You can also experience brittleness on demold. This is not the fault of the material; it is the nature of chemistry for these materials. Extra care should be taken to pre-warm materials prior to mixing and the same goes for molds. Warm material poured into a cold mold can have undesired results, especially if the material has a long working time. Also, heat can work to your advantage if you need to process faster but it can also work against you. Pre-heating materials can lower viscosity making it easier to mix and pour but it will shorten your work time. Sometimes going with a longer work life and heating the material can give you the benefit of lower viscosity without sacrificing valuable work time. Elevated temperatures can also greatly increase strength of materials. See Post Curing for more details.
Q: What is Post Curing?
A: Post curing is a confusing subject for many people. The majority of BJB materials are designed to cure at room temperature and will continue to improve physical properties over several days. For some applications this may be sufficient but there are some instances where we need additional performance out of the same material
A post cure at elevated temperatures increases the level of cross-linking in two-component thermosetting systems. In flexible materials this typically increases tear strength and tensile strength. For rigid polyurethanes and epoxies the main improvement is an increase in the Heat Distortion Temperature (HDT) but you will also see a reduction in Cold Flow/Creep over time. Don't be fooled by claims that a rigid thermosetting material does not have to be post cured; it's a half truth to say that. The truth is you don't have to post cure, but don't expect the optimum performance properties stated on the Technical Data Sheet. Even running a "mild" elevated temperature post cure can have benefits on the material. We have many customers who do not want to subject their tools to extreme temperatures (above 150°F/65°C) so they run a mild post cure at 120°F/49°C to achieve a certain degree of improvement over a room temperature cure.
Note that even silicone tools will improve physical properties and mold life if you run a post cure prior to putting the tool into service. But be aware that high heat can swell silicones and may not be a suitable procedure for high tolerance tools. Post curing is best done in a properly controlled parts oven (never a household oven). Some materials can have undesired results if subjected to extreme elevated temperatures (swelling, bubbles, deformation) so always check the recommended procedure on the Data Sheets or consult BJB's Technical Department.
Q: What is the difference between Aromatic and Aliphatic urethanes?
A: Aromatics are one of the more common chemistries in the world of casting urethanes. They tend to be more economical to produce, have good physical properties, and range from low durometer elastomers to rigid plastics. Their downsides tend to be poor UV resistance and lower chemical resistance. The Aliphatic urethanes have outstanding UV stability, color stability, and chemical resistance. Their down side tends to be longer gel and demold times, low heat distortion, low tolerance to colder casting temperatures, and higher material costs. Blends of Aliphatic and Aromatic urethanes can exhibit excellent UV resistance, good heat distortion, and good physical properties.
Q: How Do I Work With Water Clears?
A: Aliphatic Water Clear (WC) materials are a wonderfully unique material. They have unrivaled clarity and when pigmented they are the most color stable material we manufacture. They offer outstanding UV resistance; some of our WC systems can withstand years of outdoor exposure. Note that Water Clear refers to their clarity (also called Water White) and does not mean they are water-based or water soluble. Working with them does require a few essential steps to ensure success. First, they are not tolerant of cold casting temperatures. Clarity and physical properties can be severely affected if cast below 70 degrees F. Molds must also be preconditioned to minimum temperature (75-85°F) or heat may be pulled from the curing material. They also require a good vacuum system to pull the trapped air out of mixed material prior to pouring into a mold. BJB Water Clears do not self release entrapped air. Without pulling a vacuum on the mixed material, a cast part may end up looking more like a glass of carbonated soft-drink than a glass of crystal clear water. Casting the part in a pressure tank/vessel can also aid in reducing bubbles in a part by compressing bubbles down to the point they are not visible anymore (see Vacuum and Pressure). Casting in thin sections may also require additional heat to make up for the lack of internal exotherm (less mass equals less self generated heat). Water Clears operate best if preconditioned to a minimum of 75-85°F range and a max of 100-110°F keeping in mind that higher temperatures will shorten work life. Refer to the technical Data Sheet or call BJB for more assistance regarding proper temperature settings.
Q: How do Vacuum and Pressure affect casting materials?
A: When you are hand mixing casting materials, air bubbles are inevitably going to be stirred in. Viscosity, temperature, and surface tension of the material will determine how well the air will or will not self release from the liquid. Other than a few of our low viscosity systems, the mixed material will need to be placed in a vacuum chamber to remove those air bubbles prior to pouring in the mold; also called de-airing or de-gassing. The vacuum expands the trapped air causing the bubbles to grow, rise to the surface of the material and release. After a period of time these bubbles decrease in quantity meaning that air is effectively being removed from the material. This air would otherwise cause voids and potential mechanical issues with your cast part. Cold materials will have higher viscosities and may be more difficult to degass. Raising temperatures lowers the viscosity which will aid in pulling air out of the material but may also reduce working time if taken to extremes. Pressure casting can be an alternative or secondary process to vacuum ‘degassing’ casting material. Pulling air out of the material ensures the material isn’t latent with bubbles as it goes into the mold, but air can be re-introduced into the material as it is poured in from splashing or mold features that trap air (e.g. sharp corners). In some applications, vacuum and pressure must be used in conjunction to produce air free parts. Typical pressure values, when material is degassed, are in the 40-50psi range. Higher pressures may be required if material is not degassed. Pressure does not remove air; it compresses air bubbles down to the point that they are not visible anymore. If pressure is released too soon (before material has had time to set) the bubble could potentially reappear.
Q: Where can I download handling guides for your products?
A: We have a short list of handling guides available for download in Adobe Acrobat PDF format. Please choose one of the following:
Shore A & D Polyurethane Handling Guide
Water Clear Handling Guide
Foam Handling Guide
Silicone Handling Guide
SkinFlex Handling Guide
Epoxies, Surface Coats, & Adhesives Handling Guide
Q: Why does my material look thick and grainy?
A:Some systems can have a condition called “Chemical Freezing” even at temperatures above water freezing (32°F or 0°C). On the “A” sides of some systems you may look into a fresh bucket of material and think that something is wrong because it looks “grainy” and thick (almost like apple-sauce). On some “B” sides of materials, you may see layered separations from top to bottom. With our Water Clear materials, you may notice that it looks cloudy or has marbled striations. If your material is shipped by air, the contents can experience cold temperatures in the belly of an airplane. In winter time, shipping warehouses and delivery trucks are rarely heated. So while your material is made in the moderate climate of Southern California, once it leaves our dock it may travel through some nasty weather to get to your dock. Another possibility is that you may have also stored the material in an uncontrolled climate until its final usage. Even storing a container of material directly on a concrete floor can have ill-effects. The heat transfer from material to the cold concrete can be quite severe. If these conditions occur, we recommend that you put the material in a warm area of the shop or, with the cap slightly cracked, into an oven at modest temperature: 100-120°F for 1-4 hours. You will then want to re-blend by shaking or if possible stirring with a metal or plastic spatula. Wood sticks can contain moisture and contaminate the material so avoid using those. Some shops will even build small warming boxes to place newly received or production material in them to prevent any cold weather issues. Do not make parts if your material is in this condition. Use the above techniques or call our Technical Department for more help.
Q: What is the purpose of a Dry Nitrogen Gas Blanket?
A: To help preserve polyurethane materials in storage, dry nitrogen gas is often dispensed into the container before resealing. Nitrogen is a dense, non-flammable gas that will settle on top of the material (displacing oxygen) and form a protective barrier/blanket preventing contamination from humid, ambient air. Dry nitrogen can be purchased from many welding supply sources. Any nitrogen gas used to purge a container must be considered “dry” by standards in the nitrogen gas manufacturing industry. Some nitrogen sources could potentially have higher levels of moisture in them so be sure to request dry-nitrogen. Many people also ask about using “dry air” products in aerosol cans (like computer cleaner) but we have found that they have a limited effect on extending shelf life of materials and it is best to go with dry nitrogen.