Quite often, when first approaching the world of injection moulding, people struggle to understand why the costs involved in making a mould are so high.
When our potential customers ask this question, it’s useful to encourage them to reflect by asking them another question:
“If you were organizing a solo road trip into the dunes of the Sahara desert or into the cold wilderness of Alaska, would you put your faith in a Smart (with all due respect to this car) or would you go for a 4X4 Jeep, a sturdy vehicle fitted with high-performance technology and special tyres?”
This is the maybe the first step into understanding why an injection mould is more expensive than other technologies.
When you think about it, these moulds are tools that have to withstand being injected with molten plastic under extremely high pressure (from 600 to over 1,800 bar) a countless number of times.
This pressure, multiplied by the area of the plastic part being moulded, creates enormous forces that try to open the mould. If this were to happen, the injection moulding process would fail because the molten plastic would not be able to fill the mould cavities, cool and solidify inside. This is why injection presses exert a clamping pressure on the moulds (between 2,000 to 3000 bar) to hold the mould closed as the part cools down.
An injection mould must be constructed to withstand both of the forces to which it is repeatedly subjected: the injection pressure and the clamping pressure.
Now it becomes clear that moulds are incredibly complex mechanical systems that require:
- • specific construction materials
- • special technology and machinery
- • dedicated engineering
- • highly qualified personnel for their construction, assembly and operational testing
Now let’s take a look at the various aspects we have just mentioned.
Injection mould materials
The materials used to make injection moulds range from aluminium to hardened steel.
Aluminium tends to be used for making simple prototypes or for low production volumes. This is because aluminium’s relatively low strength makes it quicker to process for making moulds but it also limits service life expectancy. Aluminium moulds are generally used to produce a few thousand to a few hundred thousand parts with relatively simple features.
Pre-hardened steel is suitable for moderate production volumes and for more complex moulds. Pre-hardened steel moulds are much stronger and durable yet still soft enough to be machined using conventional processes such as milling and turning. Pre-hardened steel moulds are generally used to produce between 100,000 to 500,000 parts. They can have a wide range of features (sliding cores, complex geometry…), which are not achievable in aluminium because these complexities would eventually break the mould.
Hardened steel moulds Hardened steel moulds are used for high production volumes and are long-lasting, consequently they are also more expensive because heat treatments to make the material harder are involved in the component manufacturing process.. Tempered steel moulds are designed to produce a million or more parts. The hardness of these moulds makes them resistant to wear,which is part of the injection moulding process, and abrasion cause by the plastic material (especially when using reinforced thermoplastics).
If you would like further information on this topic, you can download our “Mould Classification” pdf from our Download page.
Injection mould technology
Single-cavity injection moulds offer very low tooling costs and maximum precision, with, however, very high unit costs.
Multi-cavity moulds are used to increase production volumes and reduce the unit cost.
And finally, family moulds offer a lower cost for producing the mould and moulding the parts. However, these also come with other issues linked to the need to optimize the moulding cycle, correctly combining times, cycle and injection pressure in order to correctly mould all the different elements in the mould.
Injection mould design
Designing an injection mould starts with a checklist of part specifications, including:
- • Aesthetics: colour, transparency, shine, opacity, etc..
- • Material: strength, tenacity, hardness, chemical and environmental resistance.
- • Interaction between parts that need to interlock: coupling and tolerances.
- • Demand and unit cost objectives.
This process leads to precise decision-making concerning the characteristics that the mould must have and will influence its design:
- • Material: aluminium, pre-tempered or tempered steel.
- • Number of cavities.
- • Considerations on melt flow.
- • Mould parting lines and any sliding cores.
- • Finish: polished, textured, incorporated text and graphics, etc..
- • Dimensional precision and tolerances.
- • Cooling stages.
- • Ejection system.
- • Designing of guides or systems without guides.
The next step involves actually designing the mould. The next step involves actually designing the mould. In this stage, highly qualified designers use complex and expensive software programmes to:
- • Make 3D models of the products and mould components.
- • Analyse the flow of melt through the mould, thus determining the position of the sprues, part ejectors and cooling lines.
- • Plan the CNC toolpaths.
- • Draw up an accurate and precise bill of materials for all the components and materials needed to make the mould.
Injection mould construction
Once the design has been completed, it is time to start making the mould where highly qualified mould makers are involved in various challenging steps. Any error would result in substantial expense to repair part of a mould that maybe has already undergone machining operations over a number of weeks and would otherwise be nearly ready.
The main processes involved in making the mould include:
- • Milling and turning
- • Heat treatments
- • Grinding and sanding
- EDM
- • Polishing and texturizing
When all the parts that make up the mould have been completed, the next step involves assembling, installing and testing the mould. All the parts need to line up with the utmost precision to achieve a satisfactory aesthetic result during moulding and to ensure that the mould itself does not wear down quickly or break.
The mould must be able to handle the high-pressure injection of molten plastic while at the same time it needs vents to let air out and avoid part burns.
Furthermore, although predictions on the behaviour of the plastic once moulded are made during the design stage, there may still be variations in the actual end result Consequently the mould has to be tested to make sure it works properly and that the moulded products meet the project specifications initially agreed upon.
Summary
So, as you have seen, injection mould design and construction is a process that requires time and the use of highly technological machinery and software. What’s more, highly specialized personnel are needed to design and make moulds. It is a very demanding operation in terms of skills, ability and maximum attention to details.
This will always appear to be extremely costly, unless you decide to take a broader view and consider this expense in relation to its result: unparalleled sophistication of design and aesthetics in the injection moulded product (and total repeatability in mass production) accompanied by low production costs.
For any further information or queries, don’t hesitate to contact us.