A Liquid Silicone Rubber material is a two-part platinum-cured elastomer that can be injected into a mold cavity to manufacture a part. LSR is very versatile in the elastomer industry and is seen across a whole spectrum of parts from consumer products to medical devices and everything in between.
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The operating temperature range for LSR can be between -65°C to150°C, which short peaks to 260°C. With post cure, it offers very low compression set and can meet low outgassing down to TML < 1 percent and CVCM < 0.1percent. These polymers are translucent, which means they can be pigmented to match any color requirement. LSR also has quick cycle times and less raw material handling &#; leading to quicker delivery times and greater throughput.
To help manufacturers unleash the full potential of this material, we compiled everything you need to know about liquid silicone rubber.
There are a couple of different ways to make the base polymer for an LSR. Like all elastomeric materials, there needs to a polymerization reaction to create molecular chains. Vinyl and methyl siloxane groups are polymerized to form polydimethylsiloxane chains. These chains provide strength and flexibility to the material. This process will change it from the liquid, almost watery starting components, to the more viscous basic form of the LSR. It&#;s in here that a lot of other properties can also be designed into the material, as well.
Once that main polymer gets created, the material gets reinforced normally with fumed silica. Unlike the organic materials where the filler of choice is carbon black or clays (pigmented materials), LSR materials use other silicates to reinforce the polymer chains and add tensile strength to the material.
From here, the material is split into two halves and the catalyst (platinum) and crosslinker are added to separate halves. This greatly increases the shelf life of these materials. When blended together, there&#;s a short working life of LSRs, averaging around eight hours. By splitting these halves, the material can potentially be stored for years with little worry of it curing during that time.
There are many types of LSR materials. Some include:
: In the later stages, an LSR will get
stripped of extractable
for medical applications, which have very sensitive requirements to avoid interacting with tissues. Removing the extractables also increases the strength of the material. These grades are typically broken down to restrictive (implant less than 30 days) and unrestricted (implant greater than 30 days). The composition of these grades are normally the same, but it is the level of biocompatibility testing that was performed on the material. Many vendors establish FDA master files to help customers get approval in their medical device.
: Incompatible fluids can be mixed into the components. Once the silicone cures, this material will bloom to the surface to create a slippery surface. Then, 1.5 to 3 percent of a phenyl fluid is added to the silicone depending on the rate of fluid the customer wants to bloom.
Conductive-grade carbon black is added to the LSR components. These grades have very good mechanical properties compared to metal particle filled silicones and can get down to 9 ohm-cm volume resistivity.
Adhesion promoters are added. This offers primerless adhesion to various substrates. The material works well on glass filled nylons, stainless steel, aluminum and polysulfones. It offers a perfect solution for composite seals to reduce assembly components or provide even better sealing between two mating components.
: Barium sulfate is added to allow the silicone part to be detected under X-Ray. This works well for medical applications such as wound drains and catheters.
: Fluorine is reacted to the polymer chain and can be blended typically at 60/40 ratio or a 100 percent
fluorosilicone LSR (FLSR
) polymer. The benefit is a highly resistant silicone for fuels and oils. Typically, these FLSR won&#;t meet current MIL standards for fluorosilicone. At the present time there is no FDA food grade or medical polymers.
: Most LSR rubber will meet a UL 94 HB rating without any additives. Higher ratings, like V1 or VO, require additives that release water when decomposed.
Just like with an ethylene propylene where a terpolymer with double bonds is added, a manufacturer of LSRs can determine the level of crosslinking of the elastomer by controlling the amount of functional groups that are added as cure sites in the material. This results in higher reactivity to produce very quick cycle times for mass production of silicone parts.
When a high-consistency silicone is brought into our facility, it almost always needs some sort of labor to transform it into a moldable size and shape. In most cases, all that&#;s required is the material be sized and shaped for the mold cavity &#; which is referred to as performing the material.
Some materials have an extra step of adding cures to the material (which were left out to increase the shelf stability of the material). An LSR removes all of those steps. Here is a look at the LSR manufacturing process:
1. LSR A + B components typically come in a 5 gallon pail or 55 gallon drum. A process controlled pumping kit presses down on the buckets to force the material into static mixer and feed into the injection unit.
2. Then, streams of pigment can be added and controlled. In the injection, press material will be mixed together additionally with a screw and pushed forward to create a &#;shot&#; of material, or the amount needed to fill the mold.
3. After the material is injected into the mold, it will then stay clamped together to cure the LSR. Then, either an operator or a robot will remove the cured product from the mold and the cycle repeats. It is a very quick efficient process, that typically averages 30 seconds.
4. Conversely, with an organic material or high consistency silicone, the average cycle time to get a part made is around six or more minutes. Peroxide cured materials take even longer, commonly around nine minutes. LSR can lead to a significant cost savings in labor over a high consistency silicone material.
5. Secondary steps like post bake can help reduce volatilities and improve compression set. And the deflashing process removes unwanted flashing. It&#;s then cleaned to remove foreign material and packaged for shipment.
For more information on liquid silicone rubber, connect with us on Twitter or LinkedIn or contact our engineering team for assistance.
Everything You Need to Know About Liquid Silicone Rubber
was last modified: byFor working with our cast products, we describe the most commonly used casting materials and pour methods for making molds.
To help you with the first use of new materials, the following instructions and recommendations provide the best chance of success:
Schouten SynTec® have all kinds of flexible mold materials &#; for both industrial, medical, private and commercial use, such as polyurethane, silicone, resins, gypsum, concrete and alginate products. SynTec products usually consist of two components. The components vulcanize, cures, at room temperature (RTV: Room Temperature Vulcanization).
Polyurethane rubber or resin is a relatively inexpensive material. Silicones (on an additive basis) are slightly more expensive, often twice the price of most cheap silicone in the market (Condensation silicone, our SG C-sil) or polyurethane rubber. Gypsum / concrete is the cheapest and is therefore often used as a material. Gypsum and concrete (concrete, gypsum mortar and gypsum powder) are available in many formulas for various purposes. See our extensive plaster information. Polyurethane resins are also pourable, but the use of this material is limited by the stiffness and shrinkage upon curing of this material.
Metal alloys with a low melting point can be cast in a heat-resistant silicone rubber. Epoxy resins and polyurethanes, the two most used two-component synthetic resin systems, offer great applicability.
Polyester synthetic resin is a two components molding resin and relatively inexpensive. Especially if sand, limestone or wood pulp are added as filler. Big disadvantage is that polyester, styrene contains, this is a carcinogen. There are already styrene-poor polyesters on the market.
Synthetic resins have the disadvantage of rapid flammability, a strong odor (in liquid form) and a shrinkage (especially with large objects) during the curing time. After reading some literature and performing some experiments, you learn to recognize the differences and the advantages and disadvantages of the various materials.
Make sure that each mold and casting compound is unique and that this requires a tailored treatment of the materials, which may differ from other materials that you have previously worked with. Pay attention and time to a good preparation. First make a small test of the materials you will be working with before the material is used on a large scale. With this method you avoid unnecessary time and loss of materials.
Molds are generally made of liquid casting rubber. The casting rubber is poured over the model that is surrounded by a box or support mold. Another method is to brush with a brush or spatula of thick rubber on the model. The mold shell ensures that the entire mold remains in shape until the rubber has cured.
The most flexible times are casting molds. The easiest way to produce a mold is to build a box around a model so that the casting material can be poured into it and then harden into a hard and solid rubber mold.
Complex molds can be made in parts. Each part is then poured separately. When all parts have been poured and hardened separately, the model is taken out and the mold parts reassembled.
Most rubber types can be thickened so that a brush mold can be made. The thickened rubber can be applied to the model with a brush or spatula. A liquid rubber is not suitable for this because the rubber flows away from the vertical surface. This mold is then built up in layers. By adding the Eurosil thickener almost any silicone can be used to make a brush mold. The silicone gets a higher viscosity due to this additive.
Some products, such as latex, are only suitable for brush molds. brus molds are generally faster to produce and provide the user with a better visual inspection of the rubber being applied. Another advantage is that when making a brus mold, less material is needed than when casting the mold. With a brus mold, a support mold / mold shell must be made always (from, for example, plaster, polyester or resin).
You must be careful that the original model does not get stuck to the undercuts of the support template. All undercuts must be removed unless the mold shell consists of several parts. Flexible support jigs often offer a good solution with regard to the undercuts mentioned. Filler pieces are used to create seams and dividing lines if required. Usually it takes more time to make a syrup mold because of the layer of build-up, drying time and the amount of work.
When the mold is ready and you are going to pour a model, it is important that the mold and the mold shell are positioned level. This prevents mold from flowing out of the mold and being lost. The mold is filled to the top with cast material. If there are too many air bubbles in the casting material, you can vibrate the mold slightly, it is of course better to ensure that you mix the casting well without air bubbles. Then just let it cured.
Air bubbles in the casting material can be prevented by mixing the molding material under vacuum or by mixing and possibly pouring under vacuum. We do recommend this.
It is important that the castings are removed from the mold in time, always follow the instructions in the technical data sheet (TDS). Never demold it too quickly, but certainly not leave the castings in the mold for several days. In this way, detailed castings of high quality can be manufactured.
Use release agent or a sealer if necessary. Consult our sealer and release menucard for the resources about the agents.
What can go wrong?
Below we have described a number of problems that can arise when creating a mold and model.
Material that does not harden at all, remains soft or sticky, is often the result of a wrong mixing ratio. We always recommend carefully weighing the materials before use. Carry out correct calculations and build a double check with regard to the mixing ratio as described on the labels and the data sheets.
Before you start, consult whether you should use release agent or which release agent you must use. Do not use a release agent if this is not necessary. Pay particular attention to the mutual material combinations. For example: PU (polyurethane) adheres to polyurethane. Never use too much release agent, this can lead to a porous and greasy surface of your casting.
Large temperature differences and the rapid deformation can be the cause of deformations of the mold or cast model. At too low a temperature (below 10°C) the processing time and the curing time are drastically delayed. Because of this, it takes much longer before you can demold the mold and model. The viscosity can also increase with this lower temperature.
If the temperature is too high (above 30°C), the processing time and the curing time are accelerated. This can be an advantage if you want to work quickly. General advice: maintains an operating temperature of 20° C to 25° C! Temperature fluctuations can lead to air bubbles and dimensional changes. The viscosity can also decrease.
Molds can be lost through a leaky mold, time is wasted and the mold usually fails. Always check in advance if a leak can occur. Close corners and seams well with, for example, a dry clay or sealant. PAY ATTENTION!
Never use a &#;wet&#; clay (river clay) in combination with Polyurethane or Silicone on an Additive basis. The moisture in the clay disturbs the hardening of the rubber, the rubber remains soft and sticky and hard not completely. Use Plasteline synthetic clay for this! Only with Silicones on a condensation basis you can use a &#;wet&#; clay. Clamps and straps can also prevent leaks.
Avoid delays because there is no good preparation after two components are mixed together. It is advisable to keep the clock..
Strips of uncured material or air bubbles, which can be seen on the surface after hardening of the rubber, may be the result of insufficient mixing of components A and B.
Unmixed rubber is often on the side and on the bottom of the mixing cup during mixing. Mix the components carefully and carefully. Slightly scrape along the edges and over the bottom of the mixing bowl with a spatula. Do not pour everything into the mold or mold formwork. The edges and bottom can best be considered as &#;waste&#;.
Some components of polyurethane or silicone must be shaken or well stirred before use. Sludge can sit in the packaging, and the oils often float so always check and shake or stir! This is always indicated on the packaging.
Unmixed material can result in insufficient or total no cure. Air bubbles and weak spots can also arise in the mold.
&#; very strong
&#; many applications
&#; shrinkage nil
&#; release agent needed&#; Moisture sensitive
&#; Two Comp.
&#; plaster&#; concrete
&#; Cement
&#; Wax
&#; Casting&#; brush on
C-Silicone
Condensation / tin cured
&#; No release agent needed&#; affordable
&#; Very strong
&#; many applications
&#; More expensive than PU (polyurethane)&#; 1% shrinkage max. during curing.
&#; Two Comp.
&#; Gypsum&#; PU resin
&#; Polyester
&#; Wax
&#; Expoxy
If you want to learn more, please visit our website Liquid Casting Rubber.
&#; alloys with low melting point
&#; casting&#; Brush on
Silicone Addition / Platinum cured &#; No release agent needed
&#; Shrinkage nil
&#; Very Strong
&#; durable
&#; More expensive than silicone condensation&#; sensitive to dirt and moisture
&#; Plaster&#; concrete /Cement
&#; PU resin
&#; Polyester
&#; Wax/Epoxy
&#; acryl resins
&#; alloys with low melting point
&#; casting&#; Brush on
The most used casting materials from Syntecshop
Benefits Disadvantages Applications Plaster &#; Goedkoop
&#; Éénvoudige verwerking
&#; Niet giftig
&#; Veelzijdig inzetbaar
&#; Snelle uitharding
&#; Overschilderbaar en in te kleuren
&#; Breekbaar&#; Alleen voor binnen
&#; Beelden&#; Ornamenten
&#; Reliëfs
&#; Kunst objecten
&#; Holle beelden
Concrete
&#; Sterk&#; Goedkoop
&#; Geschikt voor buiten
&#; Éénvoudig verwerking
&#; Zwaar&#; Ruw oppervlak
&#; Alleen om te gieten
&#; Buitenbeelden&#; Balustrades
&#; Beton industrie
&#; Architectuur
Polyurethane resins &#; Sterk&#; Éénvoudige verwerking
&#; Snelle uitharding
&#; Veelzijdig inzetbaar
&#; Mengbaar met metaal- poeders
&#; In te kleuren
&#; Niet goedkoop&#; Brandbaar
&#; Geur
&#; Zonder voorbehande- ling niet overschilder- baar
&#; Gevoelig voor vocht
&#; Kunst objecten&#; Ornamenten
&#; Reliëfs
&#; Holle modellen
&#; Metal look mogelijk
Epoxy resin &#; Zéér sterk&#; Lange levensduur
&#; Hittebestendig
&#; Duur&#; Moeilijk te mengen
&#; Gevoelig voor vocht
&#; Modellen voor vacuüm- vormen&#; Diverse toepassingen
Polyester resin &#; Goedkoop&#; Snelle uitharding
&#; Sterk
&#; Te gebruiken in combina- tie met glasmat
&#; Bevat styreen&#; Sterke geur
&#; Niet UV bestendig
&#; Hoge krimp
&#; Veel toepassingen o.a. kunst objecten, repara- tie voor b.v. boten etc.Most used methods for making rubber molds
Technic Benefit Disadvantages Applications Casting ÉénvoudigSnel
VolumeGewicht
Mallen Modellen Beelden etc&#;. Brush on ÉénvoudigMateriaal besparend Geschikt voor moeilijke objecten
Kost meer tijd dan gietenSteunkap nodig
Laag op laag vraagt meer aandacht
Stroopmallen Dipping (Latex) Goedkoop Simpel Sterk Kost veel tijd door laag oplaag
Krimp 10%
Stroopmallen Condooms, Ballonnen, schoenindustrie etc&#; Hollow castingPlaster and resins
Snel & LichtGemakkelijk
Geschikt voor moeilijke objecten
Kan opgeschuimd worden
Machine nodigBreekbaar
Holle modellen Kunstobjecten Gipszuilen Etc&#; Injectionresins
Hoge kwaliteitSterk
Maatvast
Kost veel tijdMachine nodig
Kennis en ervaring gewenst
Technische modellen en onderdelen Vacuum en pressure casting Hoge kwaliteit Géén luchtinsluiting Maatvast Kost veel tijdMachine nodig
Kennis en ervaring gewenst
Prototypes Figuren/beelden Technische modellen en onderdelen
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