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Direct Injection Process (DIP) and Pouring Method for PU Soling onto Footwear

 

 “let there be no bare feet…”

Selection of Polyurethane

Consider the requirements which the application will demand of the PU with respect to chemical and physical properties.

Based upon an understanding of what controls these properties select a few PU systems.

The properties of a PU are largely controlled by the chemical nature of the system and how it is processed so it is prudent to consult specialist suppliers and processors at this stage.

Direct Injection Process (DIP) PU

The PU, in liquid form is Injected into the closed mold (holding the upper) at high pressure. Thus we get the term “Direct Injection Moulding

The PU hardens and permanently fuses into one piece with the upper. There is absolutely no chance of separation.

Direct injection moulding or Direct Moulding points to the fact, that the soles are moulded directly onto the upper without use of adhesives or stitching.

This is possible because of the dramatic speed at which monomers polymerise to form network polyurethane, a process that is so rapid, that articles may be fabricated by injecting the reacting monomers directly into a mould.

High Mixing speed of over 18000 RPM resulting in better grain structure and thus better physical properties.

PU Direct Injection Process Moulding Machine

“PU Direct Injection Process Moulding (DIP-PU) Machine”

PU Pouring Technology

As the name suggest this technology uses the method of pouring the liquid mixture into an open mould.

PU Pouring Head

“PU Pouring Head”

The basic principle involved in this technology is the mixing of two liquid chemicals, Polyol and Isocyanate, using a mixing head and pouring the liquid mixture at low pressure into an Open Type Aluminum Mould, thereby PU remains in open condition for sometime.

With P.U. pouring technology, there are two options:

One, where the mixing head is stationary, and the mould-holders move (Conveyor Type Fabricated Mould Holders or Rotary Machine); these machines normally have 32, 40, 60, 90 or 100 stations, as per production required, and are costly options.

In the other type, called BANANA (thanks to the shape of mould-holders), the mixer is moved manually, while the mould-holders/ moulds remain stationary; in such a case you could have 6, 12, 18, 24 and up to 32 stations / moulds and is a more economic option.

“Type of PU Pouring Machine”

PU Pouring Machine

DIP Vs Pouring

Pouring Method

Possibility of blending Waste to lower cost but also result in lower quality
Possibility of blending solid additives (such as PU waste, cork, barium sulfate and many others) with Polyol and Isocyanate within the mixing chamber

PU-Pouring-Head-Crossection

Liquid Mix remains in open condition resulting in lower quality
The basic principle involved in this technology is the mixing of two liquid chemicals, Polyol and Isocyanate, using a mixing head and pouring the liquid mixture at low pressure into an Open Type Mould, thereby PU remains in open condition for sometime.

Possibility of blending Waste to lower cost but also result in lower quality
Possibility of blending solid additives (such as PU waste, cork, barium sulfate and many others) with Polyol and Isocyanate within the mixing chamber

Low Pressure Mixing
Mixing and Pouring is done at low pressure that result in poor results as compared to Direct Injection Process, in particular bond strength is badly effected.

PU remains in open condition for sometime.

PU-Pouring

Use of Pouring Method in making parts
Pouring technology is suitable for making small PU Parts and is not recommended as a soling method for footwear. This technology is generally used to manufacture soles to be used in footwear with stuck on method.

Direct Injection Process (DIP-PU)

Sole Become integral Part of Upper
In case of Direct Injection Process, Sole becomes integral part of the upper, resulting in better bond strength as compared to other soling methods like Pouring, Stuck On etc.

Better Grain Structure of PU
Due to higher mixing speed of over 18000 rpm as compared to pouring method, better grain structure is obtained resulting into better physical and chemical properties.

Consistent Quality
Injection at high pressure into the close mould results in better flow of liquid into the mould that gives better results due to least atmospheric contact and self curing of mixed liquid.

Injection moulding machines used into the production of shoes are highly controlled machines in respect of temperature control, mixing pressure, mixing ratios etc. therefore better and consistent results are obtained.

Others

  • In case of pouring method mould temperature can not be controlled due to its open condition and result may vary from time to time.
  • In case of pouring method, upper is clamped down on mould after mould is filled with liquid is already processing. Clamping down the upper on open mould containing liquid for soling is not automatic and delay of few seconds would lead to variation in results.
  • In case of DIP, mould is closed with upper already clamped on mould and liquid is injected directly onto the upper, the curing is its formation of PU is done in closed mould with upper. This gives better consistency in results.
  • In case of pouring method, since moulds are in open condition, there are possibilities of foreign particles like dust, etc. entering from the atmosphere into the sole liquid resulting in poor quality sole.

In view of the merits of DIP, FDDI has recommended this technology as

  1. A proven and reliable technology for making shoes with PU sole.
  2. Better technology for use in manufacturing shoes with PU Sole to be used by Police Personnel.
  3. Based on merits of DIP, BPR&D has approved this technology and have recommended Various Departments to purchase shoes made out of DIP-PU Sole

Comparison between Direct Injection PU and PU Pouring

Specification PU direct soling
Direct Injection Process
PU pouring
Ejective power The material pump and the centrifugal force while the SCREW is operating in high-speed. Weight (Gravity)
SCREW R.P.M. More than 18000r.p.m. /minutes 5000~6000r.p.m.minutes normally
Material Pressure 4-5kg /c㎡ 0 (Gravity)
Mold internal With hermetical space, so the material has pressure itself.(The sole plate has pressure) With opening space (because the mould is open), so the material only has expansion pressure.
Position of the injection nozzle In the side of the mold On the upper opening of the mold
Molecular structure Blending at high speed, so the structure is denser. Blending badly, so the structure is
loose.
Combination The structure of the molecular is very close, so the infiltration is good; the combination with other components (upper material) is good. The structure of the molecular is loose, so the infiltration is bad; the combination with other components is not good.
Suitable products Shoe types, such as Industrial Shoes (Those have to be bended often and to be used in rugged conditions) Chair (for clean room), chair arm, and the wheel (Something don’t have to be bended often)
Recognisation International recognisation from agencies like SATRA, EN, BIS etc. Part of International standards for footwear and also BIS No recognisation by any organization, so no literature or journals available for this process of making shoes.


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