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2013 in review

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Posted by on January 6, 2014 in Uncategorized

 

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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Polyurethanes – What Goes Into PUs?

  • —A PU is made by mixing together the ingredient chemicals (Isocyanate and Polyol) in predetermined proportions, which then react to form the polymer PU.
  • —Uniquely, PUs utilise simultaneous polymerisation and shaping of the part.
  • —The production of consistent end products depends on mixing, in precise ratio, the ingredient chemicals and maintenance of the appropriate processing temperatures. As the liquid Isocyanate and Polyol react to form the PU, the liquid mix becomes increasingly viscous eventually forming a solid mass. The reaction is exothermic and therefore heat is involved.
  • —Other ingredients will be included in the polyol blend, for example the catalyst which controls the rate at which the liquid mixture reacts to become solid.

Success is due to well chosen technology

There are no hard and fast rules for obtaining the optimum PU end product, success is due to good formulation selection with well chosen and appropriate processing parameters, technology and mould geometry. The process by which liquid polymers are converted to elastomeric or glassy solids is fundamental to the manufacture of PU products.

Selection of a Polyurethane

  1. —Consider the requirements which the application will demand of the PU with respect to chemical and physical properties.
  2. —Based upon an understanding of what controls these properties select a few candidate PU systems.
  3. —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.

Basic Polyurethane Chemistry

  • —The simplest PU is linear in which the hydroxyl compound and the nitrogen compound each have a functionality of two. This can be represented by the following:

Isocyanate + Polyol = Polyurethane

  • —The isocyanate can react with different chemical groups, so the final properties of the polymer will vary according to the reaction route taken.
  • —Therefore the formulation of a PU must take into account every possible reactive constituent. PUs may have a very widely varying structure depending on the type of isocyanate and the type of reactive hydrogen components present in the formulation.
  • The presence or otherwise of the various groups along the urethane linkage will control the end properties of the polymer.
  • The curing of a PU can be regarded as the formation of a network, also called cross-linking, the extent or degree of cure is often expressed as the crosslink density.
  • The extent of cross linking may vary and will be reflected in the final properties of the PU, ranging from longer, linear chains of flexible elastomers and foams to the rigid, heavily cross linked polymers.

Chemical Structures

isocyanates

polyols

Posted by:
Prem Mehani 
for Pinza Footwear
Email. pmehani@yahoo.com

 

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Tips on buying footwear – 6 things to keep in mind while shoe shopping

Comfortable and well fitted shoes help in keeping a lot of foot problems away. But how to know if the shoe fits? Read on to find out.

Buying Time

Ever felt the need to take of your shoes during certain times of the day, when even the otherwise perfect pair seems tight? You are not alone. Our blood circulation increases as we walk around during the day, causing feet to swell-up.  It is thus advised that you purchase your shoes during work hours (noon) or in the evening (around 7-8pm), when blood circulation is at its peak. You will ensure you buy a well fitted shoe that is comfortable all day and not just early in the morning.

Right Size

It is important to wear the right size to prevent foot problems such as painful corns, calluses etc. Always have your feet measured by the salesperson, and correctly determine the size of both your feet. Do this every time you’re out shopping for footwear, as shoe size is never a constant across brands or even shoe type.

If your feet are not of the same size, buy the shoe that fits the larger foot. You can purchase an extra insocks or insole to fill space in the other foot. If you do not wish to buy an insole, simply peel one off from an old pair and paste it using adhesive in the new shoe.

Also, people with wider feet should look for shoes with broader toe area to prevent deformation of feet. Some international brands have sizes varying by width of foot too.

Toe and Heel

Your toe should not be squished against the shoe, there must be a little gap between the shoe and the thumb. Also, the toe should lie flat and not crunched by the sides of the shoe. Unless you are woman buying pointy toes, go for the broader toe ones that don’t cram your fingers.

The ankle should not feel friction from behind, and the shoe should fit snugly around the heel. When you walk, your heel should not slip or come out of the shoe, but neither be so tight so as to cause blisters.

Take a Walk

Always wear BOTH the shoes and take a walk around the store. If the shoe bites or pinches in any way while walking, it will continue to do so even when you leave the store. And no, contrary to what the shopkeeper might say, the shoe will NOT “open-up” or “break-in” with regular use. A good, high quality shoe should never de-shape. Id it is not comfortable in the store, it will not be after a week of wear either.

Check the Material

If you wish to buy a comfortable, long lasting shoe, check the quality of materials used. Leather is a very comfortable material that allows your feet to breathe, but if you are against buying leather products, you can go for PU coated synthetic shoes. Ensure you purchase synthetic shoes from reputed companies, as only few have the technology to manufacture high quality synthetic material. Also, if you are not buying genuine leather, don’t opt for a leather “look” (or raxine shoe). It is much lower on quality, and thus comfort.

Buying standardised and comfortable footwear prevents a lot of injuries and foot problems.

Tie the Laces

A common problem is breaking of laces after a few days of wear. While buying the shoe, pull and tie the laces like you normally would, and watch them for any sign of withering. Often, the laces break simply because the eyelets they go through have pointy edges that eat away the thread.  You may check the eyelets for smoothness, or purchase extra laces for emergencies.

 
 

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