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 (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”

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”

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

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.

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.

 

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

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

 

Footwear tips for Monsoon

Monsoon, the rainy season, in India is like a festival. After severe heat people are waiting for the rain and when it comes, people can be seen dancing in rain and rejoicing in many ways.

A pleasant weather is not the only thing that monsoon brings for us. It is well equipped with not only the higher chances of fungal and bacterial infections, but also the chances of slipping on wet surfaces.

Last week after a day’s rain in Mumbai my daughter slipped on water covered stairs causing bruises and grounded her for a day. She missed her Mumbai Darshan organised by NMIMS, Mumbai for freshers of the college.

Following on, I thought I’d write about footwear you can use for the coming rain season.

Mentioned below are few tips that will help you in buying right footwear for monsoon and keeping your feet happy and infection free this season.

Flip-Flops made of rubber or materials from PVC are ideal for the rains, because they are washable, dry easily and easy-to-maintain footwear. They are available in a range of designs and colours and are suitable for both girls and guys.

Full Plastic Footwear are low-cost options for rainy season and are widely used in heavy rains. Nowadays they are available in varied style generally looking like stylish leather shoes.

Crocs are a better option for the rains. They are essentially rubber, and their grip is also better, so they won’t slip from your foot when both are wet with rain water. Colourful options with holes on top make them a style statement for monsoon.

With a formal outfit, you can wear sandals with straps as they would be firm on your feet. Go for slip-resistant sandals and check that your wet feet don’t slip even inside the footwear.

Open toes or closed shoes: Open toes or peep toes are available in waterproof material from PVC. They come in colours like red, pink, blue, silver and gold. Flat Ballerinas or closed shoes made from Lycra look great and survive monsoon.

Gum Boots: There is nothing better than rain boots and Gum Boots to protect your feet in heavy rains. This waterproof footwear is usually knee-high, water-proof and are mostly made from rubber or PVC. These boots protect the feet from mud, dirt and water during monsoon.

Avoid

• Avoid wearing heels in rainy season. But, if you just cannot resist wearing heels then go for slip-resistant sandals of heels not over 2 inches.

• Avoid wearing leather footwear as much as possible since the material gets easily infected with fungus and bacteria. They have a tendency to support bacteria because of the moisture.Still have passion for leather footwear, look out for Waterproof shoes. The waterproof range of well-known brands is quite stylish and perfect for a rainy day. Formal shoes made of waterproof leather and shoes with foam base that are odor-resistant are a huge hit with office-goers.

Here are some tips while purchasing your footwear for monsoon:

• While purchasing plastic footwear, make sure that the pair doesn’t have any sharp edges as those could give you shoe bites.

• For this season particularly, whether you are buying flat sandals or heels, look for footwear with good grip that hold feet in place and do not slip when the feet and shoes are wet.

Don’t

• During monsoon do not wear sports or canvas shoes as they may attract the growth of bacteria.

• Don’t keep your shoes and sandals under the bed or inside the shoe rack when it is wet. This gives birth to bacteria’s. Let them dry under the sun to keep away the bacteria’s from breeding in your shoes.

Mentioned below are few tips that will help you keep your feet infection free in Monsoon.

• Camphor powder is an effective antiseptic. Mix it with talcum powder and apply before wearing shoes and after opening them.

• Rubbing onion juice between infected toes helps to cure infection faster.

• If you develop itch due to mud water, wash feet in a mild solution of lime juice and vinegar to reduce the itch.

• If your wet feet have tendency to cause odour, mix 5 ml of peppermint oil in a tub of lukewarm water with handful of neem leaves and soak your feet for 10 minutes. Dry and spray talcum powder on it. This will help in reducing the perspiration plus relieve you from odour-causing bacteria.

• Sometimes in this season our nails become soft, brittle and start to clip. You can prevent this by adding a few drops of olive oil and lemon juice to a tablespoon of honey and apply this mixture to the nails before sleeping to treat brittle nails.

• At the end of a humid rainy day, soak your feet in cold water and salt solution. Scrub, clean and wipe dry.

 

Azo Dyes in Leather

‘Hide’ (or Animal Skin) needs to be tanned and treated with certain chemicals before it achieves the characteristic ‘leather look’. Chemicals are also needed to preserve the hide before tanning, or while the hide is in transportation.

Azo compounds are compounds containing the nitrogen or azo group (a bivalent group –N=N- united by two aromatic groups). Due to the brilliant colours of aryl azo compounds, they often find use in dying and pigmentation, and are thus commonly known as Azo Dyes. Around 70% of dyes used for colouring leather are azo dyes. You can read more about the chemistry of azo dyes here.

While most azo dyes are suitable for use, there are some that contain aromatic amines which release carcinogens (cancer causing substances). Exposure to them is harmful to human health. In many developed countries of the world, particularly European countries, use of certain azo dyes has thus been restricted. This has been done to protect consumers from substances hazardous to human health.

If you are an exporter of textile or leather products to EU countries, you have to ensure that your product does not carry any of the banned azo dyes. Also, every individual part of your product must comply with the limits. For example, shoe laces in footwear need to comply with the limit of 30ppm, as well as leather parts of the footwear.

Responsible manufacturers across the world can protect their workers and consumers by avoiding use of the 22 harmful aromatic amines as recognised by the EU. Azo dyes which do not release these aromatic amines should be used.

Click here to read the EU legislation and learn which azo dyes are harmful.

 

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.