The investment casting process differs greatly from the billet forging method of iron production. Equipment is very different as is the metal fabrication. The finishing operations between casting and forgings are very similar however most investment cast irons are produced from stainless steel. No protective plating is required for stainless steel. The head gets polished and is good to go. It is possible to cast carbon steel. The process shown below is the same for both stainless and carbon steel castings with the exception that most carbon steel irons will also be chrome plated.
Investment cast iron manufacturing begins by shooting wax into a mold. Above is a typical wax injection station where the temperature and pressure the wax shooting into the mold are controlled.
Foundries require minimum order quantities for production efficiency. Hundreds (or thousands) of waxes are produced at a time. There are cost inefficiencies to produce lower quantities.
Waxes are inspected to meet weight criteria in addition to cosmetic perfection. The can be no voids in the wax part and all graphics engravings must have excellent clarity.
The waxes are then assembled onto a "tree". The heads are joined to the tree via the gates which on the above model are located on the sole heel and toe. Gate positions can vary depending on clubhead design.
The trees are then transported to the slurry room.
There are various stages of slurry. Above is the first dip slurry. It is a very fine texture that can easily get down into the details of the grooves and engraving graphics.
The tree is dipped into the slurry to provide an even coating around the structure. The slurry needs to dry between subsequent dips. Each successive slurry coating is coarser than the first dips as it builds a thick shell around the wax tree.
The slurry has to dry between dippings. Air needs to be circulating around the slurried trees in order for them to dry properly. Temperature and humidity are carefully controlled.
Following the series of slurry dips and drying (approximately one week) the trees are placed into a furnace turning the slurry into a very hard ceramic shell.
Once the ceramic shell is cured the tree and club heads that were originally wax are now liquified from the heat. Above the molten wax is being siphoned into tanks.
We now have a hollow ceramic shell that has all of the original geometry and surface details from the original wax parts. Molten steel will be poured into these shells to create golf club heads.
Above is a pallet of virgin stainless steel.
This is how recycled stainless steel is received at the foundry.
The ceramic molds must be heated up before molten steel can be poured into them, otherwise the molds would shatter. Here the ceramic molds are placed in a furnace to heat them to a temperature that can handle the molten steel.
The steel is melted in a crucifix so that it can be poured. There needs to be efficient movements between the person handling the steel and the person handling the heated molds.
Steel is being poured from the crucible into the iron head mold. This is not a pleasant job on a hot summer day.
With the mold filled with liquid steel it must be set aside to cool down.
Freshly poured club heads generate a lot of ambient heat.
Club heads continue to cool down. Those on the exterior cool down the quickest while the interior club heads will cool at a slower rate. This is one reason there is variance in the metallurgy of golf club heads. Some hosels can be adjusted more easily than others. This is why additional heat treatment is necessary.
Once the steel has cooled down to room temperature the ceramic shell is removed. A motorized chisel is used to break the ceramic shell away from the steel club heads and tree. One of the least appealing jobs at a foundry.
Here the metal tree is sand blasted to remove any remaining ceramic shell.
The next step is to removed the club heads from the tree. The heads are cut at the gates which leaves excess material on the gate areas of the club head. It will need to be polished away.
The club heads are heat treated to normalize the metal. Different steels require different heat treatment processes. Heat treatment ovens can vary in size. This is a smaller version above.
After heat treating the hosels get bored to accept the appropriate shaft dimension.
In many cases score lines are milled into the face. Many investment cast irons already have the score lines in the wax parts. Naturally the cost to mill in the score lines is more expensive and time consuming.
The heads are now ready for polishing. The surface of the cast irons has a texture to it than needs to be smoothed down. This process is called 'removing the orange peel.' Heads are ground to weight, dimension specs and cosmetic target.
The appropriate surface finish is polished onto the part. Mirror and satin finishes are the most common these days.
Here the heads are cleaned for final cosmetic preparations.
There is a good amount of hand labor involved in the taping and masking of the club heads. The score line area will be glass bead blasted and the rest of the head must be protected with tape. A similar process may also require that the cavity be blasted or painted. Sometimes a club must undergo multiple taping operations.
Here a worker glass bead blasts the scoreline areas of the irons. Different blast media will result in different surface textures.
Factory workers handling final head inspections, shrink wrap protection on the heads and package the product for shipment to the customer.
The final production steps involve adding cosmetic paint fill to the engravings and applying cavity medallions if necessary.
The final result of a quality product will be free of flaws, visually appealing and a highly playable golf club for many years to come.