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Make a saving through a family bargain
Did you know that you could save yourself up to 40 percent of your costs if you identified parts that can be injection moulded as a family in the same mould?
Family parts, just like other multi cavity parts, share the same mould. Since a lot of the cost of injection moulding is in machining the mould, then putting two or more parts into one will be quoted as a single part instead of multiple parts. It won’t quite be two for the price of one, because there are other considerations, but you will still make a substantial saving.
But before you rush off to ask your supplier for a family mould, there are some restrictions due to the nature of the injection moulding process.
Part size matters
The injection moulding cycle time is usually dictated by the part size and thickness. If one part is bigger or thicker than the other, then their cooling times will be different, and this could affect part shrinkage. You may also have a filling unbalance between the two parts leading to potential defects.
It means that for a family mould to work, you need parts that are a similar size and have a similar geometry.
You also need to think about the overall size of the parts. If you want to save costs, then they all need to fit into a standard size mould. At Protolabs for example the family of two or more parts needs to fit into a standard mould size of either 12 x 4 inches or 8 x 6 inches.
Same volumes
All the parts also need to be produced in the same volume. So if, for example, you need the top and bottom of a part, but in practice the top needs replacing more frequently because it gets damaged, then producing them in a 1:1 ratio may not be your best answer.
There are ways around this. You may opt for two of the tops to be produced in a multi cavity mould for example. Or mould one bottom and two tops in the same mould – as long as the sizing and geometries allow this.
Complexity is not a great idea for multi cavity or family moulding, however, so we would always recommend that you work closely with us before assuming that you can do this.
Material considerations
The next point is more obvious; if you do opt for family moulding then both parts need to be manufactured from the same material.
Some materials also flow better than others and are therefore more suitable for family or multi cavity moulding. Liquid silicone rubber is often a good choice, because it flows freely and provides a flexible material with excellent strength and dimensional stability. It performs well in extreme temperatures, is resistant to chemicals and is biocompatible.
Gating
Regarding flow, it is also important to consider the gating.
The flow must be controlled and managed, and there is a difference between single cavity and multi or family cavity moulds.
For the former Pin style and hot tip gates are often used to help solve challenges with complex part geometries and reduce gate vestige – the small remnant of runner material that must be trimmed from the finished workpiece.
These are rarely if ever used on family moulds. Here edge gates are the better option, because you have far more flexibility in where to place it, which is vital when trying to squeeze multiple parts into a mould. It also has a larger vestige, which might mean that more trimming is needed, but crucially this helps absorb residual flow stress around that section of the mould.
Gate placement is also important when you are creating a multi cavity tool and something that you need to aware of if you are considering sharing parts in one mould. We would not recommend designing your own runner and gate system, but instead work closely with us to achieve what you want.
Part design
In addition to the specific factors that you must consider for family tooling, just like any other injection moulding you need to think about the general design principles for this process. Such as draft, wall thickness, material choice and surface finish. In fact, as the number of cavities in a mould increases, then these factors become even more important than before.
It is therefore worth revisiting our Design Tip on leveraging low-volume injection moulding. Protoquote, our design for manufacturability analysis (DFMA), will help you in this process, but we would also advise you to work closely with our application engineers.
If you are unable to put through the family parts within the same upload then be sure to speak to your account manager or application engineer to highlight that you would like your parts reviewed for family tooling opportunities.
Click here to read more design tips on creating plastic and metal parts for 3D printing, CNC machining, and injection moulding processes.