What is PU sole footwear ?

+7 votes
How polyurethane sole shoes are made and how PU Shoes plants function
asked Aug 11, 2017 by 50 (190 points)

3 Answers

+5 votes

PU sole footwear


The PU Sole footwear manufacturing process can be briefly described as below. 

Manufacturing of Synthetic Leather uppers and straps.

Pouring of PU Liquid into moulds of shoes by pouring machine.

Finishing & packing.

Manufacture of Uppers :-

To manufacture uppers from synthetic leather sheet, the Upper sheet is pasted with PU liner cloth by the lamination machine.There are various types of synthetic leathers available in different colors,designs and thickness. uppers cut from the synthetic sheet materials by putting on platform of upper cutting hydraulic machine and then placing knife on the sheet. The cut components are then joined together and stitched by industrial sewing machines. After stitching, eyelets are inserted into the uppers by eyeleting machine and insoles (socks) are stitched by string lasting machine.

Pouring of PU liquid into moulds :-

In this process , first of all ready uppers are lasted On the last of Shoes mould manually and then moulds are closed. These closed moulds are then kept on the mould stations conveyer, which moves in anticlock wise direction. In this way the moulds keep moving along with conveyer. The conveyer stops for a few minutes under the arms of pouring machines where the PU Liquid (mixture of Poly isocyanate and polylol) is poured into hot mould by the nozzle fitted into the tubes carrying PU Liquid which comes through pump from the containers of Poly isocyanate and polylol.

The whole arrangement is fixed on the stand of the machine (arms). Before pouring the liquid, moulds are cleaned by MCL Liquid and then mould-realizing agent is applied inside the mould. The moulds are heated by passing though the hot chamber, which is fitted, on conveyers. The setting of PU liquid in hot moulds starts gradually and after setting of PU, moulds are opened manually. Shoes are then removed from the mould and 

another upper is lasted on the last of mould for PU casting. The computer of the machine controls the volume of PU liquid, pouring time, time period and mould temperature.

Finishing & packing

After casting PU sole, shoes are shifted for checking, & trimming and then printed sponge insole is inserted into the shoes. The finished shoes are packed into Corrugated Boxes and finally into carton.

The process flowchart of manufacturing PU Footwear.

chart3

chart3

answered Aug 11, 2017 by 46 (390 points)
+4 votes

Polyurethane soling process in shoe technology can be done in two ways

Direct Injection Process (DIP) PU

In this process first upper is positioned on to the holding last of mould and then  PU 

in liquid form is Injected into the closed mold (holding the upper) at high pressure. 

This process is known as “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 shoes may be fabricated by 

injecting the reacting monomers directly into a mould by High Mixing speed of over 

18000 RPM. This gives better grain structure and thus better physical properties.

PU Direct Injection Process Moulding Machine

PU Pouring Technology

This is comparatively simpler and cheaper process to manufacture shoes.The basic 

principle involved in this technology is the mixing of two liquid chemicals, Polyol 

and Isocyanate.In this process using a mixing head the liquid mixture is poured at low 

pressure into an Open Type Aluminum Mould. After that 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 with the support 

of Conveyor Type Fabricated Mould Holders or with the help of  Rotary Machine. These 

machines normally have 32, 40, 60, 90 or 100 stations as per production required. This 

is comparatively 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. This is a more economic option.

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.

PU Pouring Head

answered Aug 15, 2017 by 61 (440 points)
+3 votes

Polyurethane systems for footwear soling can be based on either polyester or polyether 

polyols. Both types of systems have individually unique performance characteristics 

and  and system selection is determined by end use of the shoe sole product. 

Polyesters tend to be cheaper, lower in viscosity and more reactive. Polyether polyols 

 give better abrasion and chemical resistance properties, are higher in viscosity and 

are higher priced. Polyester polyols are common in the Coatings, Adhesives, Sealants 

and Elastomers (CASE) polyurethane applications. The shoe sole industry is one area 

where some applications use polyester polyols to achieve the desired physical 

properties.Based on our need we can make adjustments to the mold release chemistries 

to provide better release properties and improved surface finish (uniformity, gloss, 

etc.) as required by the customer.

Polyester systems are generally fast to react due to the fact that they are solid at 

room temperature and must be heated prior to use. The need for elevated temperatures 

to maintain a liquid form can make handling of polyester polyols difficult. Since the 

foam system is heated, mold temperature and cure times are generally reduced. Another 

advantage is that polyester systems are less expensive than a polyether system.  

However, soles made with polyester polyols are less durable and can be susceptible to 

biological (bacteria/fungi) attack. These attributes are best suited to footwear 

markets with rapidly changing designs and where consumers frequently purchase new 

footwear styles to keep up with current fashion trends.

Polyester systems are generally fast to react due to the fact that they are solid at 

room temperature and must be heated prior to use. The need for elevated temperatures 

to maintain a liquid form can make handling of polyester polyols difficult. Since the 

foam system is heated, mold temperature and cure times are generally reduced. Another 

advantage is that polyester systems are less expensive than a polyether system.  

However, soles made with polyester polyols are less durable and can be susceptible to 

biological (bacteria/fungi) attack. These attributes are best suited to footwear 

markets with rapidly changing designs and where consumers frequently purchase new 

footwear styles to keep up with current fashion trends.

Polyether based polyols are more resistant to oil and chemical attack and are more 

temperature resistant. The foam system is liquid at room temperature and does not 

require any special handling techniques. Molding temperature and cure time is usually 

increased (compared to polyester) for polyether-based foam systems. Soles manufactured 

with these foam systems are well suited to markets where durability is important, such 

as safety shoes.


Polyurethane sole shoe-manufacturing techniques and can be devided into four major 

categories. These categories are single density unit soles, dual density unit soles, 

single density direct attach, and dual density direct attach. 

Single Density Unit Soles


For single density unit soles the polyurethane foam is poured directly into the mold 

cavity (open pour) and a lid is secured over the cavity. At the completion of the cure 

cycle, the lid is removed and the completed part is de-molded. Single density unit 

soles are then attached to the shoe upper to complete the shoe manufacturing 

process.Shoe uppers are lasted onto the shoe lasts or upper holders and sole is pasted 

with special adhesives. There are various machines and equipments required to complete 

this process.Soles is pressed with pressing machines.  In many cases, single density 

unit soles are subjected to some type of post-mold operation to obtain the desired 

finished appearance on the part. Basic mold release agent requirements for this type 

of process include release performance, low build-up in the mold cavity, and 

compatibility with the post-mold finish operation and final shoe building process.

Dual Density Unit Soles


The dual density process requires bonding a midsole to an outsole in the same molding 

process. The outsole polyurethane foam is poured into the mold cavity, and a dummy 

last is used to close the mold. The dummy last is a type of false lid designed to 

cover the mold cavity, creating the proper thickness for the outsole while leaving 

room in the mold cavity for the midsole portion of the part. At the end of the outsole 

cure cycle, the dummy last is removed, and the midsole foam is poured on the backside 

of the outsole. A new lid is used to secure the mold, and a second cure cycle is 

completed. The molded soles may be subjected to post-mold finish operations and, as 

described above, are attached to the upper to finish the shoe. Release agent 

requirements for dual density unit soles are similar to those described for single 

density unit soles with one important difference: the mold release agent must be 

applied to the dummy last in order to separate it from the outsole. As a result, the 

release agent can be transferred to the outsole, but it must not interfere with 

adhesion between the outsole and midsole foam layers.

Direct Attach Processes


As the name suggests, the direct attach process is used to mold the sole directly to 

the upper material, eliminating the need for gluing or sewing to produce a finished 

shoe. Direct attach requires that the upper is secured to the top of the mold, and the 

polyurethane system(s) is injected into the mold cavity. For dual density direct 

attach, a dummy last is used to form the outsole, and the upper is secured into the 

mold prior to midsole injection. Scrap produced using a direct attach process is very 

expensive due to the fact that an entire shoe is wasted. It is also very difficult to 

complete a post-mold finish process on direct attach unit soles without affecting the 

upper portion of the shoe. For these reasons, mold release agent selection is 

critical. Direct attach mold release agents must produce a defect free urethane part, 

as well as impart a uniform gloss appearance. Gloss requirements are defined by the 

manufacturer, but can cover a broad range from low (matte) to high (shiny). Other 

important performance attributes for direct attach release agents include release, low 

build-up in the mold cavity, and dual density adhesion.

answered Aug 15, 2017 by 66 (520 points)
very informative answer. thanks
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