Why we should consider Direct Injection Process as better option in comparison to Pouring Process for manufacturing high performance shoes with PU Sole for Police, Military and Security Forces?
Some of the important factors that should be considered before taking a final decision to choose a technology for PU Soling
PU is made by mixing together two liquid components (Isocyanate and Polyol) in predetermined proportions, which react together to form the solid cross-linked PU material.
Uniquely, PUs Utilise simultaneous polymerisation and shaping of the part.
Production of consistent end products depends on mixing, in precise ratio, the ingredient chemicals and maintenance of the processing temperatures. As the liquid Isocyanate and Polyol react to form the PU, the liquid mix becomes increasingly viscous eventually forming a solid mass.
Success is due to good formulation 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.
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 choose the best technology at this stage.
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 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.
The mixed liquids (ISO & Polyol to form PU) hardens and permanently fuses into one piece with the upper
This is possible because of the dramatic speed at which monomers polymerise to form network polyurethanes, a process that is so rapid, that articles may be fabricated by injecting the reacting monomers directly into a mould.
In case of pouring method, upper is clamped down on mould after mould is filled with already processing liquids. 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.
I would also like to mention few points in respect of PU processing and advantages of DIP over Pouring process.
Polyurethane can be processed by
Slow process: Casting (Pouring) into an open mould
Fast process: Direct Injection Moulding (DIP) in a closed mould
Due to the need for a controlled and rapid polymerization process, DIP (RIM) is best suited for condensation type polymers with favorable polymerization kinetics. Examples are polyamides, epoxides and especially polyurethane (PU), which represent more than 95% of total RIM production.
Processing of components has three stages
In case of Direct Injection Process these three stages are complete in one integrated and highly controlled machines.
Quality of end product will depend on the above three stages of formation of PU
Advantage of DIP:
Volumetric Control: Each size of shoe will need different volume of mixed chemicals. In case of DIP, volume injected for each size fitted mould is precisely controlled independently. Since the material is injected in closed mould there is no possibility of material splashing out of mould.
Practically, in case of pouring process, material is poured by bringing head over the mould manually and at times some volume loss is there due to splash etc. Short volume will cause low density product. Therefore different shoes can have different density.
In case of uncontrolled Pouring process (as normally used in India) Poor Metering will result into off ratio mixing of components. This would result into lower tear strength & Tensile Strength.
b. Mixing: Good mixing quality will decide quality of end product.
“Reynolds Number (Re)” is the measure to judge the mixing quality. More the “Reynold Number” better is the mixing quality
Re = 4Qρ/dμ > 150
Q is the volumetric flow rate
d is the Nozzle diameter
ρ is the density
μ is the viscosity of the fluid
Advantage of DIP: Material is mixed at high-speed and injected with high pressure as compared to pouring process, therefore volumetric flow rate is higher. Resulting in higher “Re” means better mixing quality
In case of DIP Nozzle Diameter is less as compared to the diameter in case of pouring head. Therefore “Re” will be higher, again better mixing quality.
c. Moulding: Moulding will depend upon balanced ratio of the ingredients and their mixing quality.
DIP Advantage: due to better metering of components and better mixing quality. Further to this; since material is injected into a closed mould there is no atmospheric effect on formation of PU after mixing.
It is important to understand that formation of PU (moulding) start immediately (almost instantly) once mixing is started.
At this stage, we must also understand the advantage or need of high speed mixing. In case of DIP; chemicals are mixed at 18,000 RPM whereas in case of Pouring mixing is done @ 5-6000 RPM.
Slow flow of prepolymer (mixed components just before moulding) into the moulds results into voids / air pockets.
In case of pouring machines, mixed chemical is poured at slow speed and by the time the mould is filled and mould lid is closed with upper, formation of PU has already started and fluid viscosity has increased. Therefore material will flow slowly into the mould during moulding. This would lead to poor bond strength.
Further, since the flow is also dependent on mould temperature and there is no temperature control in the pouring conveyors that are normally being used by Indian shoe manufacturers.
d. In case of pouring process, two much of release agent is used in moulds, which result in air pockets in the PU sole.
e. Air Pockets / Voids, would result in the following problems
i. Non uniform Density
ii. Lower Compression Set
iii. Lower Hydrolysis resistance means lower shelf life.
iv. Uncomfortable in long wear. Rough surface and stones on the walking are can be a problem.
f. High mixing speed in case of DIP gives better grain structure resulting into better physical properties.
DIP machines have the mould carrier with a mechanism that after injection of PU into the mould, the bottom stamper is lifted with 50kN clamping force for creating “bubble free” tread sole.
This mechanism is not there in case of fabricated mould carriers used for pouring machines by most of the Indian manufacturers.
Bubble Free (Air Pockets free) has its own advantages as explained above.
Clamping force is limited in case of pouring machines. There are always possibilities of more air pockets / bubbles in PU Sole made by this process.
Shell mould with flap frame for immersed last and moulds with undercut are required to make bucket type of sole as required in specifications of shoes. These types of moulds are not used in these crude fabricated mould carrier conveyors.
Desma is one of the largest machine manufacturers for PU Soling. On the basis of many advantages of DIP machines over the Pouring Machines, DESMA Does Not Qualify Pouring Process technology for Safety and military footwear.
In case of Direct Injection Process, Sole becomes integral part of the upper as the material is injected directly onto the upper in closed mould, resulting in better bond strength as compared to other soling methods like Pouring, Stuck On etc
In case of pouring method mould temperature can not be controlled due to its open condition and results on final product 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, moulds are closed with upper already clamped on mould and liquid is injected directly onto the upper, the curing is 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.
High speed blending result into strong molecular structure, whereas in case of pouring loose molecular structure. Loose structure will result into low resistance to hydrolysis. (Low shelf life)
Since the molecular structure is very close in case of DIP, the infiltration is good; the combination with other components is good resulting into better bond strength
Dr.B.N.Dass, of CLRI made a presentation on DIP and Pouring on 05/07/2013 at the seminar on PU held by FDDI at Pragati Maidan. We would like to highlight the relevant part as under:
It must be understood that Mr.Dass made a comparison on the basis of PU UNIT SOLE (to be pasted on upper by stuck-on process) made on highly controlled Pouring Machines vis-à-vis Direct Injection Process for PU Soling Directly onto the upper.
It is very important to note that Indian manufacturers of shoes by PU pouring process are not manufacturing shoes on these types of pouring machines. They are simply using pouring heads to pour PU into open moulds fixed on simple fabricated conveyors. Therefore the comparative analysis made by Dr. Dass is not relevant in comparing the two processes of manufacturing.
I would therefore like to highlight only the following relevant part of the presentation in respect of DIP for manufacturing shoes.
I. Advantages of PU Sole formed by RIM (Direct Injection Process)