The design of a tool or mould starts during the design phase of the part. The designer will need information about the part, such as, how will it be made, using which production process, which fibres and resin will be used, does the part have undercuts, how many parts need to be made and in what time period, will it be a prototype—small or large series, what are the part tolerances and what surface finish is required.
Dependent on the answers to these questions, the result will be a single or a double tool, a tool with inserts, the materials used for the tools, the number of tools, specifications for a heating system, the number of in- and outlets and their position, specifications for a clamping system, specifications for a demoulding system and specifications for an alignment system.
Types of tooling methods
Tools can be divided into two categories—direct and indirect tooling. They each have advantages and disadvantages. The preferred type will depend on quality, quantity, cost, lead time, expertise, etc.
Direct tooling method: The direct tooling method deals with moulds that are directly machined from a block of solid material. The materials used for this method range from blocks of polymer to tooling foams, metals and machinable ceramics. Moulds made by Additive Manufacturing would also be classed under the direct tooling method.
Indirect tooling method: The indirect tooling method starts with a master model. This master model can be machined or it can be an existing product. Once the master is prepared, the mould is cast or laminated onto it. These moulds are mostly made from GFRP (glass fibre-reinforced polymer) and CFRP (carbon fibre-reinforced polymer), but castable polymers, ceramics, concrete or nickel electroforms are also possible.
Materials for master models and moulds
Selecting materials is really a matter of selecting properties. The list of relevant properties can be grouped into five categories.
Coefficient of thermal expansion
Specific heat capacity
Maximum use temperature
Compatibility with resins and their curing agents
Compatibility with the solvents and release agents to be used
Tendency to corrode in the production environment
Ease of machining
Ease of integration with presses, ejectors and injections
Cure or thermal shrinkages or other dimension changes
Minimum and maximum gauge limitations
Availability of suitable suppliers
Their workload and competitive positions
Ease of modification
It is important to realise that the materials chosen for making the master model and mould will determine how the mould will behave during production, the number of parts that can be made from it, how much the final product will cost, etc. The selection affects the whole production chain. As such, a thoughtful choice made with the advice of experts will save time and money in the end. The most commonly used materials for master models and moulds are:
Unfilled and filled polymers: Precast blocks or other shapes of polymers such as PTFE, nylons - or mass cast systems such as filled epoxies, polyesters or syntactic foams - fall into this group. Filled and unfilled polymers can be used for moulds for relatively small parts of low to moderate complexity, where closure and injection pressures are limited and temperatures are low to moderate.
Tooling blocks: Light and stable material are intended for manufacturing master models or, occasionally, for use in direct tooling. Tooling foam is likely to be used for prototyping activities and for very short runs – for example, for master models.
Fibreboards: Fibreboards such as MDF (medium density fibreboard) and HDF (high density fibreboard) can be used for making master models and moulds. Fibreboards are similar in weight to tooling foams and both filled and unfilled polymers, but are sensitive to moisture. Fibreboards have often been used in the past. Nowadays, tooling foams and (un)filled polymers are more often preferred because they are more homogeneous and less moisture-sensitive. The main disadvantage of fibreboards is the limited layer thickness and their soft cores or hard skins.
Glass fibre reinforced polymer (GRP): Commonly used for manufacturing tools, often in combination with core materials. Electric heater mats or heater pipes can be integrated for controlling the tool temperature. GRP tools are widely used for low-to-moderate production volumes (up to 2,000 pieces).
The great advantage of GRP tools is that all the knowledge for making and repairing the tool is normally present at the composite processing company. However, tight radii are very difficult to obtain and tolerance build-up can be problematic.
Carbon fibre-reinforced polymer (CFRP): Tools made with CFRP are generally manufactured with prepregs via autoclave rather than by hand-lamination. CFRP tools are commonly used. However, as with GRP tools, tight radii are very difficult to achieve and tolerance build-up can be problematic.
Nickel electroforms: As with GRP and CFRP, nickel electroform tools are made with a master model. Nickel is electrodeposited on the master model, forming a thin metallic surface without the need for machining. The metallic skin is reinforced by a steel or composite structure. Nickel electroform tools give a relatively lightweight tool face that gives good long-term performance. However, other materials should be used for large or complex moulds.
Ceramics: Ceramics are a large group of materials, ranging from plaster and concrete to special blends of different types of materials. As with the previous mould technologies, a master model is also needed for ceramic materials. Ceramic moulds can be cost effective for small quantities or for prototypes. They however, are susceptible to wear and are difficult to repair.
Aluminium: Moulds in aluminium can be manufactured by milling, starting from block material or near-net shapes. The material properties can vary significantly. Aluminium can be used for moulds but is better suited for moderate part quantities (up to about 5,000 parts). Special care is needed during demoulding as aluminium is scratch-sensitive.
Steel: As with aluminium, steel moulds can be milled from block material or near-net shapes. Steel moulds are the most robust and high gloss surfaces can be achieved. They have the longest service life, but require a high investment cost and special care is needed to avoid corrosion.
Making the final decision...
It should be clear that the final decision always will be a compromise between different candidate materials, and that the importance of each criterion is case dependent. Decision-making techniques such as multi-criteria decision making (MCDA), Consensus and Promethee can be useful.
The article is reproduced with courtesy to Sirris
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