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Footwear and Leather Goods

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Have you ever wondered what it takes to process the leather your beautiful shoes or bags are made of? You are in luck! This article explains the step-by-step process of leather processing. We hope, once you finish reading this article you will have a better understanding of leather processing. I bet you will look at your leather shoe in new lights once you know all the intricate details tanners follow to produce a beautiful piece of leather.

Preparatory Stage:

First step is Hide collection. Hides are mainly collected from slaughter houses and transport to tanneries for further treatment. The precursor to a high quality hide is proper Flaying. Flaying is an art which is better left to the experts. A beautifully flayed hide will have minimum flesh left behind and be devoid of any puncture to the hide.

Raw hides are then collected by merchants and quickly salted to prevent vain marks from becoming permanent on the hide. It then can be preserved in a warehouse. If bactericide can be added the preservation is better.
Tanners collect the raw materials for their factory for processing.

The hides are then sorted manually by size.
They are then trimmed. It helps to get rid of unnecessary left overs which can consume chemicals in later stages. This helps to minimize waste of chemicals. 

Soaking, fleshing, liming:

The hides are then soaked in drums or in the pits. They soak the hides for 8-12 hrs. The drum speed used is 1-1.15. Occasionally the drum is reversed for better soaking. In soaking process water, washing soap (nisafol), sodium sulfide (for unharing), enzyme are used. Enzyme helps to get rid of/ dissolve connecting tissues. It reduces the overall soaking time. Enzymes need to be used in correct proportion and for correct amount of time. Sodium sulfate can be used for hair saving or hair devolving method. If hair saving method is used filter needs to be used to separate the hair form the draining liquid. 

The hide is then unloaded from the drum for fleshing. It can be done manually or by machine. Soaking helps this secondary fleshing process.then hides are loaded into liming drum.

Once the soaking is finished the hide is ready for liming. Avg hr of liming is 17-24 hr. Liming helps to inflate the hide which in essence helps the hide to absorb chemical used in later steps. Water, ca carbonet, liming auxiliary is added in the liming drum. Normally drum rpm is set to 2 -2.5 rpm. Occasionally the drum is reversed for better mixture. After liming process the weight and volume is increased by 40-50%. 

Tanning:

The hides are unloaded from the drum. It is now ready for tanning. The tanning drum size needs to bigger than liming drums as the weight and volume increase after the liming process. The rpm used in tanning is 6-8 rpm. Total duration is about 8 hrs. The processes involved are prickling, bating, and tanning. Each process require different chemicals. In tanning, chrome is used. Goal is to feed the chrome to the hide efficiently. Chrome amount is tested in water and in hide. The lower the chrome amount in water is better. Lower chrome in water means more chrome has been absorbed by the hide. After the 8 hr process the hide has become WET BLUE.

Above tanning process can be full vegetable, full chrome, or semi-chrome. Semi-chrome can be archived from full chrome. 

1% chrome is used for all types of tanning. 

In full vegetable tanning, 1% chrome is used for pre-tanning in this. two /three types vegetable tanning agents are used, mimosa, quebracho and chase nut oil. Mimosa increases fullness of the lather ( fills up voids). Quebracho increase firmness of the leather. In full vegetable chrome, 20 % mimosa and 10% quebracho is used. In mimosa there are two types. Mimosa GS and Mimosa SD. GS is used to retain the natural color. It is used in natural crust leather. If leather is is exported in died crust or finished form SD is used. SD has higher tanning power and produces little more darker color. Full vegetable tanning is used for bags and leather goods. It is more environmentally friendly.

In full chrome, 6-8 % chrome is used. Full chrome used to be used for rigid classic shoes. 

Semi-chrome can be archived from full chrome. It is used for furniture lather and shoes as well. You have to feed the hide 30% vegetable tanning agent.

The hide is then taken out of the tanning drum and put up on the horse saddle for water drainage and chrome fixation. It is left there for about a day.

Samming, splitting, shaving:

Next step is samming (throughfeed) where the water is squeezed out. Then hide is split into multiple layers (two for bd hides). One layer is called grain leather which is the outer side. The second layer is called split hide which is the inner side of the original hide. Split hide will not be shaved and are exported as it with little trimming. Grain hide will be then shaved for uniformity. (Next shaving is done where the hides is shaved to get an uniform thickness. )

Retanning and dying:

The next step is re-tanning and dying. The rpm of the drum is set to 12 or 14. (Note, there is another type of drum called super load drums which we are suggesting customers not to use in retanning phase. They are used to be single gear drums. However, those drums were not suitable for Bangladesh tanneries. )
There are three steps in this, retanning, fat/oil liquoiring/feeding, and dying. It takes about 6 hrs. During retanning process different chemicals are used. This process might be different from previous tanning process.

The hide is then taken out of the tanning drum and put up on the horse saddle for water drainage and chrome fixation. It is left there for about a day.

Samming setting and drying:

Next step is samming and setting. Samming is done to get more water out. Setting is done to remove rinkle and open up/ spread out the hide uniformly.

Hides are then put into dryer. There are multiple. types of dryers, vacuum, hang or tunnel, 
First vacuum drying is done. Then pole drying is done for faster production when natural humidity is high. Pole drying is more automatic process done in a machine. Pole drying is going out of fashion now a days. Alternatively, hides are hung from a chain conveyor for natural drying (factory roof) which moves through a tunnel. The tunnel has drying mechanism. 
Toggle drying is the next and final drying process. It is used for drying and expanding the leather. Expanding process increases hide real estate..

Once the drying is done all the moisture is out of the hide. The hide now goes for vibration staking. In this process the hide is pricked with staking pins. This process softens the hide. There might be two or more bridges in a staking machine. Each bridge consists of set of pins. Pin poking frequency as a whole or for certain area of the hide can be controlled. Once the hide has been staked and passed through all the bridges it finally goes through an ironing module which removes pin mark and gives an ironing effect. Vibration staking machine with four bridge is also used during leather finishing process. 

Once the vibration staking is done the hide is called CRUST leather. If crust leather is to be exported few more things need to be done. Crust leather is kiss split ironed to flatten and give it a better look. Measuring machine is used to measure the leather. 

Finishing:

First step of this step is buffing and dedusting. In buffing, the flesh side of the leather is scraped with buffing paper. This makes the leather smooth and uniform. The working width of buffing and de-dusting machines is 1800-2200 mm (for bd leather). Finishing can be done on full piece or side leather. When a hide is cut in half each piece is called side leather. 

For high grade leather, leather is sprayed with chemical (wax, lacker, ....) for ..... purpose. If you want to do aniline finish you need high quality (a, b, c grade)leather. For lower grade leather a base coating is needed. In base coating (smearing of chemical) hide coated with chemical. After base coating hide is sprayed with chemical. You can do corrective grain finish from low grade leather. You can do semi-aniline from medium grade leather. For low and medium grade, polishing and snuffing need to be done. 
 

 

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