1) PU Injection (Pouring) Machine
The heart of the line! A good PU injection machine will control:
- Mixing ratio
- Shut (amount injected)
- Material temperature
If you manufacture safety shoes (specially two densities PU soles), quoting a two componentes machine is a must, you’ll be let down by low-end machines that never inject correctly (mould with bubbles, density inconsistent, etc). That’s where, from my POV, 99% of “cheap line” problems arise.
2) Rotary Moulding Machine
Another way of saying “carousel system” which means that a table rotates and joins all the stations to a singled-out common workflow:
- Injection from previous station
- Curing (heating)
- Demould
Important factors:
- Of stations (12 / 18 / 24 being common)
- Cycle time (length) for each station
- Stability of the heating system
If someone tells output/h, never quotes the station number and cycle time in h/mold, it’s guessing selling.
3) Alu Shoe Moulds
Direct relation with quality of end product:
- How accurate are mould cavities machined
- Venting (air, binges) = how many loss defects due to no air venting.
- Mould surface treatment (affects both finishing and mould release).
Bad mould, defects galore, no matter how good the machine.
4) Preheating Oven for the Moulds
What it is, and why?
It’s where you preheat the mould itself so the PU will flow nicely into it (keeps finish lay all over the sole). Preheating of the mould before injection is vital; again, it’s a matter usually snubbed in order to save costs at installation.
5) Cooling System
After curing, careful cooling of the mould/sole is vital so the sole will “freeze” correctly.
This generally consist of:
- Cooling conveyor or cooling station.
- Air or water assisted cooling
Without proper cooling, your soles may:
- Deform
- Not be dimensionally accurate
6. Air Compressor System
We are relying a lot on pneumatic control in PU machines.
Whether for:
- Valve operation
- Mixing head or
- Automation of the entire system
Instability in air supply leads to inconsistent injection.
7. Raw Material Storage & Feeding System
It’s no use having a high speed PU shoemaking machine if you don’t control how you load pieces of material into it.
- Polyol & isocyanate storage tanks
- Temperature control units
- Metering and feeding system
For the more sophisticated plants, this may even be integrated with the automatic feeding of material into the machine as in BASF Haptex systems
8. Safety Shoe Upper Preparation Area
While this is no part of the PU machinery as outlined above, it is an adjunct necessary for total production.
It includes:
- Heating the upper
- Applying primer (if needed)
- Placing in position
Most bonding failure between upper and sole is NOT in the PU injection machine, it occurs at this point.
9. Post-Processing Equipment
Once you demould your shoes, they will need:
- Trimming
- Other finishing
- Inspection
That will determine whether your product is superior or substandard.
10. Optional Automation & Upgrades
If you have the budget and the desired volume you wish to achieve.
- Robot arm for easy demoulding
- Automatic mould release spraying
- MES to keep track of production
- Modules for remote trouble shooting
These are not a necessity but relieve the need for labour input and reduce variances in the product.
What Most Suppliers Won’t Tell You
A PU safety shoe “production line” is not simply about the things on a checklist.
Neither is it a matter if you already have equipment.
You should be focusing on:
- Ensuring that your machine produces at a rate that isn’t too fast or slow for the mould cycle time
- Balancing heating, injection and cooling to be just right and not one predominant feature, at the expense of others
- Making sure that YOUR material system is compatible
And that’s why simply having two factories with “similar equipment lists” does NOT “guarantee” similar results!
Final Thought
If you intend to start a PU safety shoes factory, understand this:
You shouldn’t be asking
“What machines do I need?”
You should instead be asking:
“How do I build a stable, matched production system?”.
Because you can always buy equipment if you want.
Designing a working production line is the challenge!
