For large mold tools, plate steel from tolled or forged stock is often not available in the sizes that may be required. Even when plate steel is available in these sizes it is seldom economic to machine cavity forms directly into such large pieces of metal.
Deep drawn forms would also be extremely difficult to machine on components like refrigerators, wheelie bins and TV surround cases. Additionally, to machine such large forms would result in up to half of the original steel billet being machined away5 making it an uneconomic process. To overcome this problem, large mold tools would be cast from suitable steel, which is usually very similar to the plate steels previously discussed for smaller tools.
The cavity form is cast in a similar manner to other metal casting techniques using a pattern in a sand mold. It is common practice to incorporate during the casting process complete water temperature control channels and cored ejector pin policy and similar features that would be difficult or expensive to machine in the finished casting. A specialist foundry would carry out such casting work.
Although the steel used for casting are similar to those used for plat, there are slight differences because the structure of cast steel makes it not quite as strung as that of tolled or forged steels. Casting-grade steels are therefore selected to give as fine a crystal ^traitor as possible for maximum quality and strength.
An important factor is that the cast steel should be easily weldable, as pits, blow holes and shrinkage holes frequently occur in the surface of the casting* Long-term damage from thermal shock is common in cast steels and results in surface cracks appearing. Welding also normally repairs these.
The sand casting process is used for producing large mold tools of up to about 4 tones per casting (per mold half) resulting in finished molds of up to 10 tones in weight.
A pattern (usually wood) is placed on a bed of sand and further sand is packed around it until the sand reaches the parting line. Special parting sand is dusteel over the surface of the sand at this point to enable the two halves of the mold to be separated more easily. The top box is then placed on top of the lower box and also filled with sand.
Riser to provide a head of molten material and allow escape of gases.The runner and riser are inserted into the top box and the molten metal is poured into the gate. After the steel has cooled, the box is split at the parting line and the casting is removed. It is from this casting technique that the terms used in injection molding.