In designing a die for moulding plastic products, a great proportion of the work is very tedious because it does not require any creative effort. The very first step in building up a general die assembly involves many creative ideas and skills but once this is done, the next step, the production of engineering drawings for individual die components, is a routine matter of data extraction because all components can be derived from the assembly. This latter step is repetitive and time consuming. These factors make it very prone to human errors. The purpose of this project has been to analyse the design process with a view to giving computer assistance wherever possible. Based on this analysis an interactive computer-based design process is proposed. The approach has been pragmatic and the process designed so that it can be followed by a designer skilled in the conventional design process. The programs, written in Fortran, were designed for use with the GT-44 graphics unit on-line with the 24K PDP-11 computer. The major difficulty has been the three dimensional description of the die cavity and die components. Various description methods were studied and in general, the method chosen for a particular application depends very much on the geometry of the objects under consideration. They generally involve piece-wise representation of surfaces e.g. Coon's patches, Bezier and Ferguson patches. These were deemed to be unnecessarily general for this application and prohibitively extravagant in their use of computer time and storage. Consideration of typical dies for plastics showed that they were usually constructed from simple geometrical shapes with flat, cylindrical and conical surfaces. A method, called the block-building method, was developed. In this method, a library of primitive objects having simple shapes was defined. Primitive objects consisted of: rectangular blocks, cylinders, truncated cones, truncated pyramids, wedges and fillets. These primitive objects can be used to build the required object in the same way as houses are built from simple, rectangular bricks. While I was developing the method independently at this university, I.C. Braid published his first paper, "The Synthesis of Solids Bounded by Many Faces", describing the same principle. His work has been referred to extensively in this project although it was limited to a smaller class of object shapes and differed in other details. Objects were built by suitable additions and subtractions between primitive objects or other objects. To do this, there were two algorithms called merging and intersection. Merging merges two objects together provided that they have a pair of flat, coplanar faces. When two positive objects are merged the action is similar to cementing them at their coplanar faces. However, when a negative object which is completely inside a positive object is merged into it the result to remove the part of material having the shape of the negative object from the positive object. Intersection finds the object resulting from the intersection of a negative object and a positive object. The two original objects do not necessarily have to have any pair of coplanar faces. The resulting object has all the material inside the positive object but outside the negative object. Intersection can also be used to find the union of two positive, intersecting objects. In this case, the two objects are negated first, then intersected and finally negated again. To conclude the project, a very simple example for a compression die was given to demonstrate the feasibility of the computer-aided design process.