When casting epoxy or polyurethane resin materials there are many different factors that will determine how successful the result will be. The Learning Zone aims to guide you and provide you with some useful tips when casting the products available on Resins-Online.
Mixing and Dispensing
Twinpacks are pre-weighed resin and hardener contained in a tough flexible film, separated by a removable clip and rail.
Once the clip and rail is removed the resin and hardener can be thoroughly mixed within the bag and is then ready for use.
Mixing will normally take ~ 3 minutes depending on the operator and viscosity of the material. Twinpacks are ideal for small to medium production runs, prototyping and on-site or field use.
For best results the resin and hardener must be mixed to an accuracy of +/- 2%. Use the mix ratio stated on the technical data.
Do not deviate from the stated ratio as this may have an adverse effect on the properties and cure time.
Mixing Bulk or Kits
Using suitable protective clothing, pour the resin into a mixing vessel and add the correct quantity of hardener. Small quantities may be mixed by hand, i.e. a stick or spatula, and larger quantities should be mixed with a mechanical device e.g., a paint stirrer.
Some systems contain inert filler that may settle during storage. This is called sedimentation. If sediment is found it is
essential the sediment is re-dispersed in the original container before being used. Failure to do so may result in defective
product. Long-term sedimentation will be aggravated by storage above 25°C and should be avoided. Light sediment may be
re-dispersed by gently mixing with a paddle or spatula. In bulk or kit form evacuation may be necessary after mixing for best
Mixing of Three Part systems
Pour the pre-weighed hardener component into the resin vessel and drain thoroughly. Mix the resin and hardener components thoroughly by hand using a broad bladed implement or preferably mechanically with a paint stirrer on slow speed. (Approximately 1-2 minutes). Add approximately half the contents the extender / filler and stir. When the mix becomes even, add the remaining filler / extender and continue mixing for a further 1 – 2 minutes.
Stir well until the material becomes homogenous, paying particular attention to the sides and bottom of the container, as this is particularly important. (Low viscosity mixes will take less time and more viscous products may require more time. If in doubt continue mixing).
Mixing time will also depend upon the temperature of the system and size of the kit. 3-5 minutes would be an average;
however operator experience can reduce the time. The rule, however, "if in doubt - mix again" as insufficient mixing will result
in soft spots of uncured material that may lead to product failure.
For best results, mix and pour the material before the usable life of the system.
Dispose of the empty kit containers in accordance with local authority requirements. Any excess material that has been
properly mixed and cured can be classed as inert and safe.
It is essential for best results that the cartridge is ‘balanced’ before use to ensure correct mixing. This is achieved by loading
the cartridge into the gun before addition of the mixer element and pumping the gun to push a small amount of the contents
forward. Wipe the excess from the cartridge tip and add the mixer. The cartridge is now ready for use.
Spillage & Cleaning
All equipment contaminated with mixed material should be cleaned before the material has hardened. Robnor Resins TS130 is suitable non-flammable cleaning agent, although other solvents may be found suitable. TS130 will also remove cured material provided it is allowed to soak for a number of hours
Processing and Curing
This is the time normally in minutes that the mixed resin is ‘usable’.
This is the time normally in minutes that the mixed resin is ‘workable’.
This is the time to double the initial mixed system viscosity.
Viscosity is the resistance to flow. A high viscosity resin has high resistance to flow e.g., treacle. A low viscosity resin has low
resistance to flow e.g. water.
Curing is the process by which resins react to produce a solid infusible mass. The reaction of resin and hardener usually
involves the liberation of heat called exotherm. This is generally controlled by careful formulation and the addition of fillers and
additives that suppress excessive heat build up. It is however dependant on the ambient temperature and the mass of material used. Smaller masses and lower ambient temperature will extend cure time and larger masses and warmer ambient temperatures will reduce cure time. Hotter temperatures may be used for faster cure but will result in higher post cure shrinkage and higher cure exotherm. Experimentation and testing is suggested to avoid side effects. For maximum properties a post cure may be required - call Robnor Technical Service Department for advice.
Please consult the technical data sheet before using Resin-Online products.
In many resins the performance of the system can be enhanced by a process called post curing. This involves a secondary
cure process over and above the normal conditions to provide enhanced performance. In general post curing will improve such properties as; chemical resistance, temperature stability, dimensional stability, voltage breakdown resistance, water resistance and increase glass transition temperature. Post curing is mainly used on epoxy systems but can also be used on some high performance polyurethanes to enhance them still further. In general post curing occurs at 10 – 20.C above the maximum expected service temperature.
Typical post curing (depending on system), may involve a normal cure plus:
16 hrs at 100.C or
8 hrs at 120.C or
4 hrs at 140.C or
2 hrs at 150.C
Most epoxies and polyurethanes generally ignite above 415°C.
Decomposition products will consist of carbon, carbon dioxide, carbon monoxide, hydrogen cyanide nitriles and water, and are best avoided.
Mixing under vacuum is the most effective way to prevent air entrapment. Alternatively the mixture may be deaerated under
vacuum – allowing at least 200% ullage for the foam to expand. Some systems may benefit from warming but this should be
done before adding the hardener. Brief degassing under 5 – 10 mbar improves the mixture homogeneity and enhance the
dielectric and aesthetic properties of the resin.
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Moulding and Casting
The use of a vacuum minimises the risk of air entrapment during mould filling and the resin mixture should enter the mould in a steady steam. Fore best results the mould should be vented to avoid air entrapment and voids. Warm moulds generally give better castings. In general when using epoxy and polyurethane materials the mould will need to be treated with a mould
release agent. The quality of the moulding is largely determined by the choice of casting material and the quality of the mould
Allow material to fully cure before machining. Machine using these guidelines:
Lathe speed: 150ft/min
Tools: Carbide Top Rake 6°(+/- 2°) Side/Front 8°(+/-2°)
Feed Rate (rough): Travel speed .020. Rough cut .020-.060
Feed Rate (Finishing): Travel speed .010. Finish Cut .010
Polishing: Use 400-650 grit emery paper wet.
Use suitable dust extraction as necessary.
Varnishes and Coatings
Many objects may be coated using Resin-Online varnishes and coatings either by dipping, brushing or spraying. Pneumatic
guns should be used at room temperatures. Drainage retention on steel sheet is in the region of 3% with a non-diluted varnish.
When coating a diluted varnish (15% dry extract), the thickness is about 15 to 20 microns.
Filler or Extender
In some instances unfilled materials may require modification with filler or extender to give certain benefits.
Adding filler or extender will:
Increase viscosity, i.e. more difficult to fill moulds and evacuate.
Increase thermal conductivity (except using glass bubbles)
Increase general mechanical properties but lower impact strength
Increase dimensional stability
Increase thermal conductivity
Can make flame retardant (at certain levels)
But will reduce:
The cost per kilo
Stress on curing
Properties that remain relatively unaffected include:
Filler should be added to the resin by the use of a mechanical stirrer prior to adding the relevant hardener. For best results
evacuate the mixed product.
Pigments are added to a resin to give a desired colour. In general the pigment is simply stirred into the resin until uniform at a rate of between 2 to 10% by weight. Filled systems generally require more pigment to mask any effect the filler or extender
may have. For best results evacuate the mixed product.