Island Scientific - Technical Note 1065

Technical Note No: 1065

Subject: General Introduction to Resin Casting and Encapsulation

This is no more than a brief guide. Always refer to the resin makers resin instructions for detailed procedures to be followed. There are a group of chemicals - known as 'resins' which by suitable treatment can be transformed from parable liquid to hard solids. By filling suitable moulds with treated resins we can produce Resin Castings.

Different types of resin have different properties. Addition of extra components give a variety of electrical, mechanical optical or decorative properties to the finished casting.

Such operations vary in scale from jewellery making on the kitchen table up to mass production of items such as 'Cold Cast Bronzes', valve bodies, electronic components 'potted' in a casting etc.

The term 'Vacuum Casting' is often referred to for such processes. This topic is fully covered in later paragraphs.

MOULDS AND MOULD MAKING MATERIALS

To commence producing castings we need a suitable mould or an original component from which to make a mould. Mould making is a complete subject on its own, dealt with elsewhere. We assume for now that moulds have been already made or obtained. Moulds can fall into one of several categories.

1) Box	This comprises a simple shape i.e., square. The sides need to be slightly tapered, i.e., wider at the open top. Into such a mould we could pour our material and when set solid pop out the casting. These can be purchased ready made for:- a) Jewellery and decorate objects. b) Encapsulating boxes for electronic components.
2) Latex Skin Mould	These moulds are made from liquid rubber latex which is cured to be a flexible skin up to 3mm thick over the original pattern, i.e., the item we wish to reproduce. The original needs to be fairly simple in shape so that the mould - which is reasonably flexible can be peeled back and off the original and off subsequent casts. Popular type of mould for making chess pieces.
3) Latex Skin Mould (Backed)	This is similar to 2) for larger casts. The thin skin of the mould is supported by a shaped fibre glass/resin case. Sometimes this outer case is in several parts held together by nuts and bolts or clamps. Such Moulds are often used for concrete or other cement mixtures. e.g., garden ornaments.
4) Vinamould	This material is re-useable which is its chief advantage. It needs to be carefully heated (well over 100^oC) when it melts and can be poured over the original to create the mould when it sets. Moulds are usually solid Vinamould self supporting or possibly with a simple outer case. Three grades of Vinamould are available, soft, medium and hard.
5) RTV Silicone Rubber	RTV stands for 'room temperature vulcanising'. When RTV Silicone rubber liquid is mixed with a suitable catalyst it slowly solidifies to a rubbery solid. Whilst liquid, it can be poured over the original to form either a solid mould, or a skin mould with backing to be added later. Some grades of RTV Silicone are thixotropic, i.e., gel like and can be buttered over the mould with a spatula. This technique is especially suited to large items, e.g., busts. Backing material is often casting plaster. Silicone moulds have a limited life, the material is expensive but detailed reproduction is very good, and the silicone withstands temperature better than other materials.
6) Polysulphide Rubber	This is a black material used the same way as RTV Silicone. It is cheaper to use but may not be suitable for use with some resins.
7) Other Materials	Other materials may be possible for special purposes, e.g., encapsulation or individual projects.
8) General Comments	Your moulds need to be designed for vacuum casting and degassing to be used. Rarely can one produce reliably good castings without vacuum. Two part liquid moulding materials, i.e., silicone and polysulphide will need to be degassed under vacuum to make bubble free moulds. Using a RTV Silicone mould which was not vacuum degassed can give rise to surface defects on the casting. Many mould materials require a coating of mould release, eg., wax polish. RTV Silicone is self releasing for most resins.

RESIN VARIETIES FOR CASTING

There are several manufacturers of resins suitable for casting. Each produces a range of resins with different characteristics. Yes , the choice is large.

Most Casting Resins fall into one of four groups, i.e., Polyester, Epoxy, Polyurethane and Acrylic.

1) Polyester	This is the cheaper material. It is used with accelerator to cause it to solidify. Popular for paperweights, Resin 'Bronze', (otherwise called cold cast bronze) imitation marble and general decorative items. Polyester resins have a strong smell and give off an inflammable heavy vapour in the uncured condition.
2) Epoxy	This is a more expensive resin. There are many varieties with different properties. Used for electronics encapsulation tooling, special purpose castings, valve parts etc. It is tougher and more hard wearing than Polyester. It is used in the same way, ie., two parts mixed together.
3) Polyurethane	These resins tend to be used for special applications. Thin, reasonably strong sections, small figurines, military modelling components. Certain grades have strength enough for small thin components to be handled without breaking. It is a useful alternative for short runs to plastic injection moulding. However, once mixed it has a short working time. Some grades need to be heated before casting, shrinkage is generally low. Polyurethane resin before curing is susceptible to moisture. It must be kept in sealed containers and resealed after opening, only after a dry nitrogen purge of the air space in the container. Never use a wood spatula with polyurethane as the moisture in the wood could affect the resin.
4) Acrylic	The best known acrylic resin is ICI grade called 'Perspex'. Special equipment and techniques are needed to cast in acrylic. It is not for the amateur or beginner. The material needs heat (100^oC) and pressure 10 Millibar for a period of hours to form a tough clear solid. Even so, shrinkage is high and must be allowed for. It is used mainly for hard wearing encapsulations eg., bar fittings in pubs and restaurants. Special purpose enclosures which are clear can be made from it. It is reasonably straightforward to machine with care.

FILLERS AND ADDITIVES FOR RESIN

We can mix any one of a variety of materials with our chosen resin to give special properties to the cast pieces. The more usual additives are as follows:-

1) Colouring paste

A wide variety of colours are available usually as a thick paste. make sure that the colourant selected is suitable for the type of resin to be used.

2) Metal Powder

Sintered metal powder can be mixed with the resin to create a metallic appearance. As the metal powder is heavier than the resin, it will tend to fall to the bottom if using low viscosity resin. However, the metal powder usually remains suspended in the resin long enough to enable application. To keep costs to a minimum this mixture is used only as a surface layer. The body of the cast is made up of cheaper material (see later notes). Typical metal powders are >sintered= (NOT flake). They include:

Bronze or Copper to give Bronze finish

Lead to give Weight

Nickel Brass to give Old Silver finish

Iron to give Magnetic qualities and cast iron like

appearance

Brass to give Brass/Gold finish

Aluminium to give Aluminium finis

3) Other Powders

Other non-metallic powders can be mixed with the resin. As before mix with resin for surface appearance. Mix with bulk of resin for making up the bulk of the casts. Among the powders used are:-

Marble Dust - Marble coloured appearance

Slate Powder - Slate colour which is also a cheap filler

Calcium Carborate - White, low cost

Aluminium Trihydrate - Special purpose to obtain onyx like effects

Silver Powder - To give electrical conductivity

Abrasive Powder - Certain type of industrial abrasive block

Mica Powder - Better electrical resistance

Asbestos - No longer used hazardous when inhaled

Glass Beads - Special Effects

The use of fillers in the proportions about 3 parts to 1 part of resin reduces the shrinkage in the final cast. It is important that any filler used does not adversely affect the properties of the resin, its curing time or subsequent strength. The mix with filler results in a much thicker i.e., more viscous mixture which will need vacuum degassing to remove the trapped air.

EXOTHERMIC CHEMICAL REACTION OF RESIN MIXES

As resin mixed with accelerator and hardener react chemically, a solid is formed. At the same time the mixture becomes hot due to the chemical reaction. Massive castings tend to crack or distort unless special care is taken. The heat must be allowed to escape during reaction such that the temperature of the mass does not go too high. This can be controlled by using less accelerator but there is a lower limit for the amount. Filled resins tend to be cooler as the filler absorbs some of the heat generated. Also the chemical reaction speeds up as the temperature increases. Thus a runaway reaction can occur. Experience will guide you as to size of casting and amounts of accelerator to use.

Resin manufacturers quote a range of percentages for accelerator addition. Whereas small thin sections can be left to cure in a warm place, heavy solid casts may need to be cooled.

ENCAPSULATION

Solid objects are often embedded in resin for one of several reasons e.g.,

1) Electronic Components	Can be encapsulated to protect them from the atmosphere e.g., damp, dust, air. This is used often in military or tropical applications. Encapsulation in an opaque, i.e., non-see through resin gives security. It is not then possible to identify the components inside by simply looking. Coil or transformer windings subject to AC frequency can be fixed in place and not vibrate. Delicate components can be protected from hazards due to handling, i.e., broken connections greatly reduced. However, if an electronic component fails then it is difficult and often near impossible to repair or replace it.
2) Biological Specimens	Fragile bio specimens can be encapsulated in a clear resin to make up teaching aids of museum pieces.
3) Paper Weights, Trophies and Awards	A variety of objects are encapsulated in clear resin to make paper weights and ornaments, e.g., coins, shells, dried flowers, mineral specimens.
4) Jewellery	Small embedded items can be used to make pendants, earrings, key fobs. In these examples a wire loop is cast with the loop external, for fixing purposes. Any object being encapsulated must be able to withstand the external heat and pressure of the process. Items must be warm, dry and solid, i.e., not hollow.
5) Engineering Components	Metal strengthening parts can be embedded in resin to give the finished cast extra strength and rigidity.

WALL PLAQUES

Flat cast items are easily made in one sided moulds for the production of wall plaques. This can be as a cast or mounted in suitable frames.

EQUIPMENT REQUIRED

To commence casting, you will need the following:-

1) Moulds - plus:	Mould release agent, stand to support the moulds.
2) Resins - plus:	Catalyst, Colouring, filler, metal powder (if used)
3) Measuring equipment:	Liquid measure, Hypodermis (for very small amount of catalyst), weighing scales to say 5 Kgs.
4) Mixing Equipment:	Bowl, bucket (for larger amounts), paper cups (for smaller amounts), paddle or spatula, electric mixer, clock or timer.
5) Island Scientific Vacuum Degassing Equipment:	Island Scientific Vacuum Degassing Outfit. Comprising motorised high vacuum pump, connecting hose, vacuum tank, lid, seal, gauge and controls. Lift out tray. (See Island Scientific Technical Note Numbers 1006, 1007, 1009, 1010 and 1011.
6) Working Area:	Warm pre-heated area ideally central heated, bench or table, cleaning tissue or paper towel, waste bin (metal with metal lid), cleaner (acetone), power sockets, ventilation or extraction, secure storage cool and dry for resins and chemicals, secure storage for acetone and inflammable cleaners.
7) Finishing Equipment:	Cleaner (acetone), Linisher (or wet and dry paper, to smooth bases), steel wool (to clean resin bronzes), Chemicals for Patination of Bronze, Bucket and lid for acetone, Scissors, polishes and colourants e.g., boot polish, Wax polish and cloths, Packing tissue, Boxes, labels, self adhesive felt or baize.

HEALTH & SAFETY

Work Area - NO FLAMES.
Beware of gas water heaters, fires, fan heaters.
NO SMOKING notices to be observed.
Barrier cream for hands and wrists. Possibly gloves, disposable.
Safety Glasses
Apron or overalls
Hand cleansing cream
Gloves (strong for using wire wool on resin bronze)
Fire extinguishers
Absorbent granules (in case of spillage)
Possibly - hard hat and face visor
Dust extraction system for mechanical linisher

NB: Never clean your vacuum tank lid (acrylic) with acetone.

GENERAL OBSERVATIONS ABOUT WORK AREA

1) Do not allow food on the benches or tables.

2) Be careful where exhausted air discharges. It will smell of resins, acetone, etc., and may cause a nuisance if large amounts are used.

3) Ensure that the vacuum pump exhaust gases are ducted outside by a suitable means.

4) Always maintain good housekeeping. In particular, clean benches, tables and floor.

5) Ensure use of the resins and materials in strict date order so that they are used before shelf life expires.

6) In the cold weather the vacuum pump will be difficult to start, as the oil in it will be cold and thick. Try to use thinner oil in the winter if you experience problems, i.e., Island Scientific Grade 15 or even Grade 8.

7) Take care with disposal of waste material.

TECHNIQUES AND METHODS

We deal with the seven main methods.

A) Simple cast with filler

B) Embedded object - cast in clear resin

C) Resin bronze (cold cast Bronze)

D) Wall plaques

E) Vacuum Encapsulation of Electronic Assemblies

F) Vacuum Casting

G) Autoclaving

A) Simple Cast With Filler

1) Support your mould open end upwards on the bench or in a carrier frame as appropriate.

2) Layout the tools and material required.

3) Switch on the Island Scientific Vacuum Pump, close off the vacuum valve, open the vacuum release valve. Remove the Island Scientific Vacuum Chamber lid (put to one side).

4) Weigh or measure out the resin and filler powder and accelerator.

5) Mix together by hand or electric mixer the resin and filler powder in a large mixing bowl (as the mix will swell). Always follow the resin makers information about quantities, temperatures etc. Measure accurately the accelerator. Use scales, or a Hypodermic or small measuring cylinder. This must be accurate. Too much and the resin will set too quickly and heat too rapidly. Too little and it will take much too long to harden.

6) Place the bowl of mixture in the Vacuum Chamber. Replace the lid, close the Vacuum Release Valve. Open the Vacuum Valve. As the level of vacuum increases the mix will swell up. Note: The mix can swell up to 10 times its original volume). At the full vacuum the surface of the mix will appear to erupt as the now enlarged air bubbles break free and are pumped away. After 30 seconds more, degassing should be complete, close the vacuum valve. Open the vacuum release valve and allow the air back into the Vacuum Chamber. Remove the lid and the bowl of degassed resin mix.

7) Carefully fill the mould the resin mix, tilt as needed to minimise entrapped air. Leave about 10% of the mould unfilled.

8) Put the partly filled mould into the Island Scientific Vacuum Chamber and degas as before. This time only a few bubbles will rise up. The mix should not swell up as before. After about 15 seconds at full vacuum, degassing is completed.

9) Remove the partly filled mould and top up with more resin.

10) Put aside to harden for time needed.

11) When hardened (do not leave much longer than needed), carefully remove the mould. Latex moulds can be peeled off by wetting the outside with soapy water and folding back. Two part moulds are carefully removed; take care not to damage the mould. Put the mould back together. Put the casting aside to fully harden.

12) Finish the casting as required.

NB; See Island Scientific Ltd Technical Note Number 1011 for full details of Vacuum Degassing and filling procedures.

B) Embedded Objects in Clear Resin

Prepare your mould and resin as before, with colourant if required. Part fill a mould with vacuum degassed resin, degas again needed and allow to gel, i.e., start to harden. When gelled enough, place on the surface, the object to be embedded. Top up the mould with vacuum degassed resin, degas the whole mould. Put aside to set. When hard enough remove from the mould. Finish by sanding/polishing the top surface as needed.

C) Resin Bronze

Mix a quantity of resin plus hardener and then mix in bronze metal powder. Up to four parts metal powder to resin can be used. This mixture is vacuum degased and then carefully spread all around the interior of the mould where it will become the outer visible layer of the finished item. For latex one piece mould, the mix can be rolled around by hand. Vacuum degas before it starts to gel. Avoid allowing liquid filled resin to sink down to bottom of the mould. With two part moulds, the resin mix can be brushed on before the two parts are closed up. Again degas before the resin gels. Once the resin/metal powder mix has set, the mould can be filled with more resin and cheap filler simply to add bulk.

It is advisable to add some colourant to approximate to the colour of the finished item. This will help to disguise thin spots. When the cast has set, remove from the mould and set aside to cure for at leave 3 days. The appearance will be dull chocolate. Remove the outer surface film of resin using steel wool and water by hand. Gently abrade the resin to revel the bronze finish. Linish the base and cover with green felt. The >bronze= can be polished with metal polish and lacquered or chemically patinated to give an aged effect. Chemicals used include Potassium Sulphide and Ammonia Sulphide (take great care with these). Finish with wax polish and buff with a soft cloth.

D) Wall Plaques

A Bronze Plaque or shield can be produced using a similar procedure. The mould must be strong and backed with fibreglass, wood or such like. The filled degassed resin can be brushed all over the mould and vacuum degassed again in the mould. When the surface layer has set then the back up resin can be applied. To give the finished item strength you can add fibreglass cloth as used for car body repairs. Embed this in the resin backing. If required, cast in a wire hook to hang the plaque. If it is not to be framed, then possibly a plywood back cut to size would be suitable to embed in the resin backing. To avoid excessive finishing, always prepare the mould edge so as to give the desired appearance of the plaque when viewed edge on.

A few moulds of small plaques, ready to be filled can be made available when casting other items. Should you mix too much resin, then the surplus can be poured into these moulds. Finishing is as before.

E) Electronics Encapsulation

As some electronic circuits are susceptible to moisture, vibration and changing atmospheric conditions, we often need to encase all the components in a suitable resin, e.g., Epoxy or silicone Rubber. If the components are able to withstand full vacuum, the vacuum encapsulation carried out as for Island Scientific Vacuum casting is advisable. The resin selected must have suitable properties of electrical resistance, clarity, impact resistance etc., as required. In particular, take care that the exothermic, i.e., heat generated by the curing resin will not upset the components. The wires or cables coming from the circuitry should be suitably positioned and such that moisture cannot creep back along the wire or sleeving.

F) Vacuum Casting

The majority styles of detailed complex moulds of a decorative item or industrial casting cannot be filled satisfactorily pouring resin in open air. The air entrapped in the mould may not be able to escape. The method to use is Vacuum Casting. The mould is under vacuum, i.e., no air is present. The resin can be also inside the vacuum chamber ready mixed ready to pour by operating pouring mechanism from outside. This is suitable from small quantities. If several small moulds are to be filled then these moulds can be placed on a turntable in the vacuum chamber operated from outside. The resin is poured into each mould in turn all under vacuum.

For large amounts of resin you can admit resin from outside the vacuum chamber through suitable pipework. The actual techniques depends upon the ease of cleaning out the resin afterwards.

In all the above cases, the filling and release back to atmosphere must be completed before the resin starts to gel or thicken. Some contraction may appear to happen with complex moulds and cavities as the atmospheric pressure pushes the liquid resin into every corner.

G) Autoclaving

To reduce cavities caused by air bubbles with difficult resins and/or moulds, a second process after vacuum encapsulation can be used. This is especially important for example with electrical coils of very fine wire. This process is to subject the encapsulated article in the mould to external pressure, whilst the resin is still liquid. This is done in a pressure vessel or autoclave with say, nitrogen gas at 7-10 bar. This pressure also reduces any bubbles formed by the exothermic heating. Autoclaves are however more expensive to buy and operate than vacuum systems.

FURTHER READING

For the beginner in resin casting decorative objects the Company Trylon sell a good range of moulds and materials with comprehensive technical data.

Railway Modellers could also read about making model buildings The Model Railway Journal No. 54 Pages 105 to 107.

The manufactures of most RTV Silicone Rubbers produce good leaflets on mould making.

Island Scientific Ltd have a wide range of Technial Notes covering every aspect of vacuum, i.e., degassing and encapsulation, maintenance, Health and Safety etc.

NB: The information in this pamphlet is offered in good faith and is based on our general experience. The method of use of Island Scientific Ltd equipment and materials are outside our control, the responsibility to ensure that the equipment is properly used and suitable for intended application rests with the user. No responsibility will be accepted by this company nor any person or other company associated with this leaflet for loss or consequential losses as a result of them using this information.

For advice, information, general literature, prices or a discussion of your needs, telephone 01983 855822, fax 01983 852146 or
E-Mail: sales@island-scientific.co.uk.

A full list of Technical Notes is available free on request.