Shell Moulding Technical Memorandum - UNV-205
The shell moulding process provides a new technique for making foundry moulds and cores.
A mixture of dry sand and powdered SUPRA shell moulding resin provides a starting point for the process and from it moulds are built up in the form of shells of suitable thickness by dropping the mixture on to the neat metal patterns. These shells can be hardened in a matter of minutes.
Complicated shell moulds can be made as easily as simple ones and in addition, it is possible to work to closer tolerances and to save on finishing costs.
SUPRA Resin UNV-205 is a finely ground phenolic resin for use by the foundry industry in the shell moulding process.
The sand should be dry silica sand substantially free from clay (not greater than 0.75% clay) and other contaminants. For most purposes a fairly fine sand, of which most passes 60 meshes but is retained by 200 meshes, is suitable.
The resin is incorporated in a sand mix of which the following is typical:
Regarding the small quantity of resin which has to be mixed into a relatively large quantity of sand, very thorough mechanical mixing is necessary. It has been found that a blade or paddle type miser is satisfactory for this purpose. The distribution of the resin through the sand is materially assisted by mixing 0.05% of the wetting agent into the sand before adding the resin (5%). The ratio of resin to sand can be varied according to the specific properties required in the (5%). The ratio of resin to sand can be varied according to the specific properties required in the shell mould. In general 5% of resin gives satisfactory results but in some cases a small increase in the amount of resin used may be found advantageous. For instance, where fine sand is used it is usually found desirable to use a somewhat larger proportion of resin than with coarser sand.
The resin/sand mixture is loaded into a bucked which is mounted on trunnions so that it may be inverted quickly. A metal pattern, previously heated to 1850C - 2600C (3650F – 5000F) and lubricated with HYLAK parting agent S-1011, is fixed securely over the open top of the bucked and the whole apparatus is inverted. By this means the resin/sand mixture is thrown or dumped on to the hot pattern and allowed to remain there until a shell of sufficient, thickness is formed 10 – 40 seconds is usually sufficient for this part of the operation though, obviously, this time, which is known as the “dwell time”, must depend on the thickness of the mould required for any particular casting. The bucket is then returned to its original position and the remaining resin/sand mixture is ready for the operation to be repeated.
The pattern is now removed from the bucked and placed in an oven for the final hardening of the shell mould. This is usually carried out at a temperature of 2880C - 5400F – 10000F) and will take 1-5 minutes according to the shell thickness. After this treatment the shell mould is in the form of a hard infusible solid, which is removed from the pattern while it is still hot.
To complete the mould, two matching shells are fastened together by any suitable means such as clips, bolts or bent nails. The hot shells may be stuck together by using Supra Cement, UNV-205 in the following way:
A narrow band of the powdered resin is applied to the joint surface of one of the hot shells and the matching hot shell is placed on it quickly, sufficient pressure being applied, for atleast 30 seconds, to keep the surfaces together.
Casting the Mould
To set up the mould for pouring, it is generally adequate to embed it in sand to a sufficient depth to maintain it firmly in position during this operation. The metal is poured in the normal way. The mould being porous, allows free escape of gases. Back pressure is reduced to a minimum, resulting in sound casting and the elimination of surface blemishes.
SUPRA Resin UNV-205 and cement should be stored in a cool, dry place with containers securely closed.
Note: Whilst the above information is based on the result of experimental work in our laboratories and is correct to the best of our knowledge, users are recommended to establish for themselves that the process and materials are satisfactory for their requirements. Freedom from patent rights must not be assumed. No liability is accepted by us for any damage, injury or loss resulting from the use of this information.