Accuracy and flexibility
Since its introduction in the late 1960s, polyurethane (PU) has established itself as an important soling material. It combines excellent physical properties with light weight to produce footwear that is extremely durable and comfortable to wear.
It is for this reason that it is popular for use in safety footwear by direct moulding on to the lasted upper in either a single or dual density format. Nevertheless, extreme accuracy in terms of lasts and moulds are required due to its very low viscosity, so tolerances of around 0.2mm are called for to ensure no material is exuded during the injection cycle.
The milling machines used to produce lasts for this process have special holding systems that allow the entire surface of the last to be milled including both toe and heel. This results in lasts that are so precise they can be used for direct injection soling using PU without the risk of flash. This is how German last manufacturer Fagus-GreCon produces its HPM lasts, designed for use in conjunction with rotary PU moulding machines and fitted with specialised top plates that are compatible with automated production systems and Industry 4.0.
The need for change
As we are all only too well aware, the covid-19 virus has hit the world very hard. It has also revealed the need for some changes in the footwear industry including the production of safety and work shoes. As many industries shut down or only operated part time, demand for this type of footwear fell dramatically and the resultant worldwide slump in sales has impacted heavily on company profitability and inevitably brought increased pressure on costs. Every measure to reduce the latter and increase efficiency therefore counts.
This applies especially to direct soling where there is still a great deal of untapped potential for improvements. At first glance, high investment costs prevent many companies from consistently increasing the degree of automation of their production facilities. For example, the use of a rotary table moulding machine is a given but, in many cases, automated material supply through robot support is omitted. The necessary investment sums often simply exceed the financial possibilities of the companies concerned. It may therefore well be worthwhile to divide the automation process into several phases. Enormous potential for increasing efficiency can be obtained simply by switching from conventional to HPM (high-precision milled) lasts.
Intelligent last selection
Very often, classic aluminium lasts are used for direct injection moulding. They are very accurate but heavy and relatively expensive compared to conventional plastic ones. Fagus says that, by changing to its HPM lasts, advantages of both types are combined and enormous improvements in efficiency and quality can be immediately achieved on a rotary table even without automated material supply:
• Higher productivity due to reduced machine stops for setting lasts/moulds
• Optimal fitting shoes as they stay on the last longer
• They can be adjusted in length giving flexibility in production requirements
• Prepared for a possible future increase in the level of automation
• Metal parts can be reused on future orders and savings on investment costs.
Advantages of HPM lasts
With HPM lasts, it is possible to adjust their position backwards and forwards by 2mm. Furthermore, there is also material in the hinge joint which can be removed or added to, therefore adjusting the last length by +/- 0.5mm to 2mm. This can be a great benefit in having the flexibility to cope with varying upper substances. As these lasts are fully milled, the possibilities are endless as to what can be achieved, such as anatomical last bottoms, logos, edge milling, toe cuts and last markings for upper location etc.
Fagus-GreCon has also developed an anti-sticking sealant called Fagus Antistick which can be added to the mould bite line area. This can be used for producing sandals or to give more compression allowance for upper variance. The benefit of Antistick is that PU does not stick to it as in other rubber bitelined lasts. Furthermore, HPM last back parts can be completely removed which makes the lasting and de-lasting of uppers, especially boots, so much easier.
However, the key point with HPM lasts according to Fagus-GreCon is that the last is positioned at a zero point at the very start of construction. All future lasts in new developments use the identical zero point so that no manual setting of lasts is required as all round table station last holders are set identically using the Zero setting mould. This is why the company claims that HPM lasts not only offer such a degree of accuracy but also excellent manufacturing flexibility.
Lasts for use in direct PU injection moulding have to be milled to the finest tolerances.
All Credits: Fagus-Grecon
