Should I Use Polyurethane Mold Rubber or Silicone Mold Rubber?
You’re in the planning stages of your mold making project and you need to decide which rubber is more suitable for your process: polyurethane mold rubber or silicone mold rubber?
Each choice has benefits and downsides and there ar a good style of choices in every of the merchandise lines themselves (e.g., there ar over thirty individual choices inside the ployurethan rubber line — they vary by hardness, pour time, cure time, body and color, among alternative factors).
For the needs of this text, we’ll specialize in 3 general mildew rubber categories: ployurethan mildew rubber, tin-cured siloxane mildew rubber and platinum-cured siloxane mildew rubber.
General Advantages & Disadvantages of Each Rubber
Mold Rubbers: Advantages & Disadvantages
Type
Advantages
Disadvantages
Mold Making Methods
Higher strength
Low to moderate cost
Wide range of Hardnesses
Requires careful application of release agent
Moisture sensitive (in liquid form)
Pour
Brush
Spray
No release agent needed
Excellent chemical resistance
Higher cost
Shrinks on cure (~1%)
Cured rubber has limited storage life
Pour
Brush
Spray
Platinum-Cured Silicone Rubber
No release agent needed
No shrink on cure
Cured rubber has long storage life
Excellent chemical resistance
Higher cost
Cure inhibited by some surfaces
Pour
Brush
Spray
There are three major questions that need to be asked when deciding between polyurethane mold rubber and silicone mold rubber.
- What casting material am I going to use?
- How many castings do I need to make?
- What material is my model (a.k.a master) made of?
1. What Casting Material am I Going to Use?
Concrete — polyurethan mildew rubbers work terribly effectively and are the “go-to” choice for casting concrete. In some cases, a unharness agent isn’t needed once casting concrete in these molds, however, it depends greatly on the elements of the concrete combined. polyurethane rubber choices vary from a Shore Hardness of A20 to D45, which permits for a good style of concrete applications, as well as concrete statuary, type liners, countertops, stamping mats, and far a lot of.
Wax or Plaster — polyurethane mildew rubbers are a preferred choice for casting wax or plaster. Typically, softer choices within the line-up are used: 74-Series & Polygel® Series.
Polyurethane and Polyester Resins — siloxane rubbers are typically accustomed to forged these resins. In most cases, a unharness agent isn’t necessary and an outsized amount of castings are often made of one mildew once designed and used properly.
An EasyFlo 60 (polyurethane plastic) casting removed from a PlatSil® 73–40 silicone rubber mold [click here for a tutorial on making this mold]
Epoxy Resins — Silicone rubbers are sometimes preferred for casting epoxy resins because a release agent is not needed; however, the corrosive curing agent in epoxies quickly wears down silicone rubber and results in limited castings. Therefore, polyurethane rubber with a suitable release agent (e.g., Pol-Ease® 2300 Release Agent) is often used instead.
Polyurethane Foam– Silicone rubbers are usually recommended for use when casting polyurethane foam, as they last longer than polyurethane rubber molds and do not require application of a release agent (e.g., paste wax, PVA) prior to casting.
Most of these materials can be cast in either polyurethane or silicone molds; the options mentioned above are general recommendations based on popular preference. Based on the section below, you may decide on a different option.
If your casting material is not mentioned above, please give us a call to discuss your project.
2. How Many Castings Do I Need to Make?
Now that you have an idea of what products are most popular for casting certain materials, the next consideration is quantity:
Concrete, Plaster & Wax– In well-designed polyurethane rubber molds, hundreds of these castings can be made. In this case, it may not be worth it to spend more money on silicone rubber if you’re able to successfully cast a large quantity with polyurethane rubber molds (NOTE: Silicone rubbers may cause efflorescence in concrete castings).
Polyurethane and Polyester Resins– As mentioned within the previous section, siloxane rubbers are usually the most effective choice for casting these materials. Upwards of 1 hundred components are often forged in these molds. If you merely need 10–20 forged components, then you would possibly like better to use less-expensive polyurethane mildew rubber. you may get a restricted quantity of castings and can get to use a unharness agent. If the forged components got to be painted, the discharge agent would wish to be washed off, which may be tough.
Epoxy Resins — If a little number of epoxy castings ar required, synthetic rubber is also an appropriate choice and doesn’t need a unharness agent (which is useful if the forged components got to be painted). If a bigger amount of epoxy castings ar needed, polyurethane rubber with an appropriate unharness agent is usually the most effective choice.
Polyurethane Foam — In several circumstances, upwards of 1 hundred foam castings are often created in synthetic rubber molds. just like the ployurethan and polyester resins listed on top of, a a lot of economical approach (if you need fewer castings) would be to form a ployurethan mildew instead. A unharness agent, like PolyCoat (a silicone-based, semi-permanent sealer and unharness agent), ought to be applied to the mildew during this case.
A TinSil® 80–30 silicone rubber mold [left] is used to cast R-8 PolyFoam [right]
3. What Material is My Model Made of?
First, if the model is a human being or animal, careful consideration and investigation into appropriate skin-safe mold materials needs to be completed. Mold materials such as Skin-Wax, Hydrogel®, or PlatSil® Gels (a platinum-silicone rubber) may be viable options.
Some Other Considerations:
Certain mildew rubber materials can suffer from cure inhibition (i.e., failure of a compound to cure against a surface inside the suggested cure time) once applied to explicit model materials. as an example, platinum-cured silicones won’t cure properly once poured over sulfur-containing clay. they will conjointly suffer from cure inhibition once applied to bound plastics (e.g., some plastics utilized in three-D printing), polyester resins, some epoxy resins and any model that has had tin-cured synthetic rubber or latex rubber applied thereto within the past.
Inhibition problems will typically be overcome by terribly careful application of a waterproofing agent to the model (e.g., PVA, shellac, paint, Vaseline, PolyCoat).
Generally, polyurethane rubbers and tin-cured siloxane rubbers aren’t sensitive to cure inhibition.
Damage to Models:
No matter what the mildew material is, there’s invariably an opportunity that a model might get broken in how throughout the mildew-creating method. victimization siloxane rubbers might eliminate the requirement for unharness agents (which have the potential to have an effect on the model); but, the oils from siloxane rubbers even have the potential to have an effect on the model in how. it’s suggested to invariably perform a little check cure on the model before creating the complete mildew.
Every mildew-creating project is unique; please contact North American country with any queries you’ve got as you opt what rubber mildew material is best for your project.