Die casting and injection moulding sound similar and follow a related principle: a molten material is forced into a mould under pressure and solidifies there into the part contour. The decisive difference lies in the material – injection moulding processes plastics, die casting processes metals such as aluminium, zinc or magnesium.
For designers and buyers this raises a question that is settled early in development: which process suits the part? This article compares the two and shows when switching from metal to plastic pays off – and when it does not.
The key points at a glance
- Same principle, different material. Both are primary forming processes using pressure. Every further difference in process, tooling and part properties follows from the material.
- Temperature separates the processes. Metal melts are processed far above the temperatures used for plastics, which shapes tool steel, tool life and equipment.
- Plastic can integrate functions. Seals, grip zones and colours can be formed directly into the part in multi-component moulding, without a separate assembly step.
- Metal stays ahead on heat and stiffness. Where high continuous temperatures, heat dissipation or large forces occur, die casting is usually the right choice.
- Both need a production mould. The investment only pays off across the quantity – the business case decides, not the process itself.
Shared principle, different materials
Both processes belong to the primary forming methods: a liquid material is forced into a cavity under pressure and solidifies there. In injection moulding this material is a molten plastic, in die casting a metal melt. From this single difference follow all the others – in mould design, in the process window and in the properties of the finished part.
The key differences in detail
- Temperature: Metal melts are processed at considerably higher temperatures than plastic melts. This places greater demands on tool steel, temperature control and equipment.
- Stiffness and heat conduction: Die cast parts are stiffer and conduct heat far better – an advantage wherever components need to dissipate heat.
- Weight: Plastic parts are considerably lighter. In applications where mass matters, this is often the decisive argument.
- Corrosion and insulation: Plastic does not corrode and insulates electrically – two properties that would require additional coatings or components in metal.
- Design freedom: Colours, transparent areas and hard-soft combinations in two-component moulding cannot be reproduced in metal.
- Post-processing: Die cast parts more often require mechanical finishing, such as deburring or machining functional surfaces. Injection moulded parts frequently come out of the mould ready to install.
When die casting is the better choice
An honest comparison also names the limits of plastic. Die casting has the advantage when:
- the part is permanently exposed to high temperatures at which engineering plastics soften or age
- heat has to be actively dissipated, for example in housings for power electronics
- very high stiffness is required at small wall thicknesses
- the part carries structural loads with high point forces
- electromagnetic shielding is needed without additional measures
When switching to injection moulding pays off
Conversely, plastic injection moulding plays to its strengths where weight, insulation, corrosion resistance or functional integration matter. Typical examples are housings for electrical engineering, operating elements, connecting parts and components with integrated seals or grip zones.
The switch becomes particularly economical when several metal parts can be replaced by a single plastic component. What requires assembly in metal – a bolted-on seal, a separate grip element – is created in one operation in injection moulding. The saving then lies not in the part price but in the assembly step that no longer exists.
Thermal and mechanical loads in service always need checking, though. A substitution that works on paper can fail on continuous temperature or creep behaviour – and that belongs before the tooling investment, not after it.
The switch needs to be calculated
Changing the process is not purely a material decision. At minimum: what continuous temperature occurs in service? Are there sustained loads under which a plastic might creep? What tolerances are required, and can they be held once shrinkage is taken into account? And what annual quantity has to amortise the new mould?
In practice the switch rarely pays off on part price alone. It pays off when weight drops, corrosion protection becomes unnecessary and assembly steps disappear at the same time. Those three effects together shift the calculation significantly – individually they often do not.
What both processes have in common
Both require a precise production mould whose cost is only amortised across the quantity. And in both cases the mould design determines dimensional accuracy, surface quality and process reliability. Anyone who knows the cost drivers in mould making – cavity count, undercuts, tolerance requirements – can weigh both processes against each other reliably. Existing moulds can also be taken over from another manufacturer; the procedure is described under tool transfer.
Frequently asked questions
What is the main difference between die casting and injection moulding?
The material. Injection moulding processes plastics, die casting processes metals such as aluminium, zinc or magnesium. The basic principle – forcing a melt into a mould under pressure – is the same for both; every further difference in process, tooling and part properties follows from the material.
Can a die cast part be replaced by an injection moulded one?
Often yes, if the thermal and mechanical loads allow it. The switch becomes particularly economical when several metal parts can be combined into a single plastic component, eliminating assembly steps. Continuous temperature, creep behaviour and stiffness should be verified beforehand.
Which process is cheaper?
That cannot be answered in general terms. Both require a production mould whose cost is spread across the quantity. Plastic parts are often cheaper per piece and need finishing less frequently; what matters, however, is the overall calculation of tooling, part price, post-processing and assembly.
Are injection moulded parts less durable than die cast parts?
Not generally – it depends on the type of load. Metal is stiffer and conducts heat better. Engineering plastics, particularly glass-fibre reinforced grades, nevertheless achieve high strength at considerably lower weight and are corrosion resistant.
Does Schröder + Heidler also produce die castings?
No. We specialise in plastic injection moulding and the associated mould making. We are happy to assess whether your part is suitable for a plastic version based on your drawing.
Conclusion: the requirement decides, not the process
Whether die casting or injection moulding is the better choice depends on load, weight, quantity and functional requirements. Where heat dissipation and stiffness matter, metal stays ahead. Where weight, insulation and integrated functions are called for, plastic injection moulding is usually the more economical solution.
Would you like to know whether your part is suitable for injection moulding? Send us your drawing.