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

06 Aug
 

 “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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14 responses to “Direct Injection Process (DIP) and Pouring Method for PU Soling onto Footwear

  1. Mamun ur Rahman

    August 6, 2012 at 3:28 pm

    ALL SAID AND DONE I AGREE THAT DIP PU PROCESS IS A BETTER CONSTRUCTION METHOD THAN PU POURING. BUT THE PRICE OF THE DIP PU MACHINE AND MOLDS SHOULD ALSO BE
    A MAJOR CONSIDERATION. I HAVE SEEN CHINESE SAFETY FOOTWEAR FACTORIES USING PU POURING MACHINE TO EXPORT SHOES TO FAMOUS US BRANDS. SO I AM PRETTY CONVINCED THAT IT IS THE PU TECHNOLOGY AND RIGHT HANDLING OF THE MACHINE IS WHAT MATTERS.

     
  2. Shoes and More

    August 6, 2012 at 5:35 pm

    I agree that cost of DIP Machine and Moulds is higher than PU Pouring machine. It is for this reason that Chinese manufacturers are not using DIP machines for PU Soling. But you should also understand due to advanced technology and better controlled processing of PU in DIP process, rejection rate is very less as compared to Pouring Process.That’s why there is higher rejection rate in Chinese Safety Shoes and the quality in general is not very high.

     
  3. lista de email

    September 4, 2012 at 9:01 am

    very nice information. keep sharing.

     
  4. Prasanna Rao

    October 31, 2012 at 6:47 am

    Thank you Prem bhai for such wonderful information & steps taken to provide quality shoes to the Police Dept. Wishing you all the best.

     
  5. Ashish

    October 31, 2012 at 11:57 am

    We are the leading manufacturer of safety and uniform shoes. Due to merit of DIP process we are manufacturing all our safety shoe with PU /PU-TPU sole and shoes for Police department with Direct Injection Process to maintain IS standard.

     
    • AAMIL FARIDI

      July 17, 2013 at 1:18 am

      hi here is AAMIL FARIDI from delhi where r u from??please contact me 9953225003

       
  6. S. S. Lahiri

    October 31, 2012 at 5:16 pm

    Though Shoe can be made by both DIP & Pouring but better quality of shoes can only be made by PU DIP process. The quality of mixing under high pressure is always better than had mixing or by PU pouring machine. Because of high pressure injection in PU DIP, the chemical immediately reaches the far end of the mould cavity before the start of cream time.

     
    • David Tuer

      July 23, 2013 at 8:04 pm

      These remarks are only valid if one is directly injecting solid urethane soles, as is the case in safety shoe manufacture. The injection of liquid urethane does not produce the massive pressures that injection moulding does and I think that there is a great deal of misunderstanding in this area. Polyurethanes are largely uneconomic unless expanded and working at a density of 600-650gs/L the urethane has to fill the mould by it’s own expansion. This gives a minimal pressure generation.The material is ot evenly distributed throughout the mould and as a consequence flow patterns can be formed and density variations can occur at various points in the sole. Direct soling by the pouring method ensures even distribution and the urethane expands vertically giving a more even textured foam and eliminating small bubbles of entrapped air at corners and edges of e.g. cleats and other air trap points.

       
      • sslahiri@libertyshoes.com

        November 14, 2013 at 12:10 pm

        Sir,

        I do not agree with the observation.

        Liquid PU when Directly Injected by a higher pressure machine we get far
        more better quality of shoes than Direct Pouring /casting process.

        With Best Regards ,

        Lahiri.S.S

        From:
        Shoes and More
        To:
        sslahiri@libertyshoes.com
        Date:
        11/14/2013 10:02 AM
        Subject:
        [New comment] Direct Injection Process (DIP) and Pouring Method for PU
        Soling onto Footwear

        David Tuer commented: “These remarks are only valid if one is directly
        injecting solid urethane soles, as is the case in safety shoe manufacture.
        The injection of liquid urethane does not produce the massive pressures
        that injection moulding does and I think that there is a gre”

         
  7. fazalenterpriseseo

    February 27, 2013 at 12:14 pm

    i gone through your website about Safety Shoes Manufacturers & Distributors
    its really amazing. if you want additional information about Safety Shoes Manufacturers & Distributors contact Us.

    Safety Shoes Manufacturers & Distributors.

     
  8. Fahad

    September 15, 2013 at 12:57 am

    i want to know something about pu machine
    like
    price, total value,c
    onsumption,material etc.

     
  9. Sandeep Shukla

    November 28, 2013 at 9:59 am

    The cosistency of DIP is very high than Pouring. PU is very senstive material.
    For Footwear DIP is the best way.

     
  10. pretechplast

    September 11, 2014 at 4:05 pm

    Your post is really good. Useful content. Very helpful to me. i have blog for Thermoset injection moulders in India
    see my products also

    Plastic parts manufacturer in India

     
  11. Jawad Ahmad

    January 8, 2015 at 2:50 pm

    Thank you for this post, it is surely very informative. Will somebody tell me what pairage we should make on molds on PU DIP method before replacing it with new ones

     

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