2 shots
Challenge
You’ve been given a 150x100x50mm aluminium block which is actually 2 blocks of 150x100x25mm.
You have to design a polymer widget for a brand intended to be a promotional item.
Then figure out how the mould [the 2 x 150x100x25mm] needs to be CNC machined to create the shape of the widget ready to be injection moulded.
So what do you do?
Design a book page holder for Penguin Books using a 2 shot injection mould with 2 slab inserts + 7 interchanging inserts.
How?
With 7 people milling and lathing the mould, inserts and pins in 5 weeks to injection mould at least 3 polymer widgets.
[NB. We individually designed a polymer widget and figured out how the mould tool was to be machined, we then came together as a group and chose 1 person’s design, a team member.]
Design Process
What mattered was the mould and if it could be machined, not the design of the widget itself. Essentially, using the widget, you ‘cut away’ its shape from the mould so when injected with polymer, it creates the 3D widget.
So in part, yes the design of the widget was important.
SolidWorks is a powerful thing, with the ability to simulate the milling process which was translated into programming for the 3-axis milling machines with the help of HSM Works.
Polymer flow was also checked to ensure the cavity filled fully, fill time and identify weak spots in the final widget, these are all dependant on the placement of the sprue and runner so was adjusted accordingly.
Engineering Drawings were produced so mould halves, inserts and pins could be machined.
Manufacturing Process
Using my good friend bluefoam [styrofoam] prototypes was made to ensure and test the programming and desired shape was created before moving to steel and aluminium for the functional mould tool.
Both mould halves and inserts, were fabricated using the CNC Milling Machines.
The pins were manufactured from silver steel and the excess material removed using the manual lathes and milling machines.
With 9 inserts, tolerances were small to ensure a seamless fit and keep flashing to a minimum.
Some figures
to prove, in an ideal world, the mould could mass produce 3M units:
3,025,452 widgets
manufactured within a
13 week
Just-in Time work period
4 cavities
set in a vertical rotary mould
9.9 s
fill time.
Reflection 💭
Even though there were 7 of us, we still struggled to machine all the pieces in time due to the quantity and availability of machinery.
By the time we got to injection moulding, we could not inject using 2 different colours as it had taken us too long, hence only being able to produce black widgets.
One of the slab inserts bowed as a result of the amount of material removed and perhaps even the material itself, mild steel. This meant other inserts did not sit flat and flush so further alterations were made to accommodate this.
We were already working within very small tolerances so our margin for error was even smaller. If I were to do this again, with the knowledge I have, I would first try to alter the current mould or widget to reduce the chance of the slab insert bowing or reinforce it. Second I would consider injecting the whole widget using polymer instead of using 2 metal prongs [as seen above] meaning less inserts so less chance of error. I think my last resort would be to choose a different design, but what we also failed to do was discuss the potential risks that might arise, and it definitely came to bite us in the bum down the road.
We were certainly ambitious and even though the mould and inserts were no where near perfect, I loved being about to witness how it was made: the whole manufacturing process from start to finish was incredible. Figuring out the mould was just like a backward puzzle. Seeing the simulations on screen to watching the milling machine carve out the shapes to the actual injection moulding machine was one of the best experiences and one of my favourite projects I did during my time there.
