Zirtec

A Fascinating Process

Steel Shots are one of the most important blasting materials available on the market. It was one of the first artificial abrasives produced and, after exhaustive operational research, recognized its quality of efficiency, uniformity and economy.
Initially they were made of cast iron. They were easily fragmented with high consumption, attacking the blasted surfaces and equipment. Even so, they have already begun to replace the traditional sand avoiding its operational deficiencies and the risks that cause the operators health.
Those produced in malleable iron or steel were only marketed in Brazil in the 1960s, dominating over 70% of the current market. The main advantages they feature are:

– Low operating cost: Support 300, 400 or more cycles as opposed to 2 or 3 sand max.
– Cleaning Speed: The higher density, the possibility of being driven by turbines reaching higher speeds and the consistency of their dimensions for a long time result in a durable and long lasting operating efficiency.
– Uniformity in finishing: For the same reasons, the surface finishes are of constant uniformity.

– Less wear: Abrasion on nozzles, turbine blades and equipment is less than when using sand.
– Lower dust generation: As steel or malleable grit does not break easily, the dust generated is restricted to removal from the surface of blasted parts, simplifying abrasive purification systems and reducing initial investment.

Shot Types

Cast Iron

They are still employed but their use has been progressively reduced. They are of lower cost but a careful comparative study considering, besides the direct consumption, the maintenance expenses, has proved to be unfavorable to those of cast iron.

Malleable

Generally produced cut wire (length equal to diameter) are quite durable and represent about 10% of total consumption.

Steel

They are offered with a wide range of composition and dimensions. As for the shape, they are spherical (“shot”) and angular (“grit”).

Virtually dominate the market for their high quality and stringent specifications. Depending on the purpose, they undergo quenching and tempering processes which give them characteristics of hardness and malleability.
The most commonly used dog shot for cleaning are low hardness (40 and 49 Rc) or cut wire (31 to 39 Rc). High hardness (up to 66 Rc) is mainly used for shot peening or grit sometimes required in certain anchor problems or in rollers. It is always preferable to choose smaller diameter grits that are more perfect and more economical.

A mixture of particle size or even spherical to angular is generally used. The following average mixing values’ (IX) can be considered as follows:

Cleaning of Castings

Recommended dimension ……………………… 60%
Particle size 1 pt smaller …………………… 20%
Size 2 pts smaller ………………….. 20%.

Stripping

Recommended dimension ……………………… 75%
Particle size 2 pt smaller …………………… 25%

Abrasive recycling and purification systems should be efficient in removing fines generated during operation as they reduce efficiency, especially in turbocharged equipment.

Cut Wire
16	17	18	19	20	21

Øx Length (mm)
1.6	1,5	1.7	1,0	0.9	0.8

Spherical Shots (Shot)

SAE	Ø mm	ASTM Screen	Ø Medium mm
S 660	2.38 to 1.68	8 to 12	2.0
S 550	2.00 to 1.41	10 to 14	1.7
S 460	168, 1.41	12 to 16	1,4
S 390	1.41 to 1.00	14 to 18	1,2
S 330	1.19 to 0.84	16 to 20	1,0
S 280	1.00 to 0.71	18 to 25	0.9
S 230	0.84 to 0.59	20 to 30	0,7
S 170	0.71 to 0.40	25 to 40	0,6
S 110	0.650 to 0.30	30 to 50	0.4

Grid Shots

SAE	Ø mm	ASTM Screen	Ø Medium mm
G 12	2.38 to 1.68	8 to 12	2.0
G 14	2.00 to 1.68	10 to 14	1.7
G 16	1.68 to 1.19	12 to 16	1,4
G 18	1.41 to 1.00	14 to 18	1,2
G 25	1.19 to 0.71	16 to 25	1,0
G 40	1.00 to 0.42	18 to 40	0,7
G 50	0.71 to 0.30	25 to 50	0,5
G 80	0.42 to 0.18	40 to 80	0,3
S 110	0.650 to 0.30	30 to 50	0.4

Recommendations for using steel grit for cleaning

It would be too exhausting to list and detail the applications of the types of grit and each of its commercial grit sizes. The tables below contain general applications which, as examples, provide guidance for generalization.

Granulom.	Ø medium mm	General applications of spherical shot
S 660	2.0	Removal of foundry sand on iron parts. Stripping of large parts. Engine blocks for tractors, housings
S 550	1.7	Castings in general with large dimensions. Truck Engine Blocks, Rail Material
S 460	1,4	Forgings and heavy and medium castings. Pieces with thick walls, over 20kg. Car engine blocks.
S 390	1,2	Sand removal and blasting of small and medium cast iron parts such as small engine blocks. Pickling of forged parts in general and thick plates. Application to thick-walled pipes and to columns and structural beams.
S330	1,0	Castings and forgings in general. Rarely used on sheet metal and drawn except for walls above 3/8 “.
S 280	0.9	Forged castings of small and medium sizes such as valve camshafts and tappets.
S 230	0,7	Same as the S 280 but smaller as valve and tappet rocker arms.
S 170	0,6	For small or thin parts such as drums and bicycle frames. Replaces S 110 in the indicated applications only causes greater roughness.
S 110	0.4	Sand removal and blasting of high precision castings and forgings. Can be applied in cast brass. Recommended for bike or motorcycle frames.

Granulom.	Ø medium mm	General applications of spherical shot
G 12	2.0	Has few applications.
G 14	1.7	Heavy foundry as turbine bases and structures joining rail axes
G 16	1,4	Stripping of plates over 1 “. Heavy casting.
G 18	1,2	Coarse plate stripping (over 1/4 “). Shipbuilding, oversized pipes, truck and train cold shoes.
G 25	1,0	Stripping sheets and pieces of iron with thicknesses over 1/4 “as used in metal structures. Railcar chassis. Brake pads.
G 40	0,7	It is the most widely applied grain size. Preparation for anchoring paint or rubber or plastic coatings. Preparation for galvanic coatings. Stripping and general cleaning on parts, sheets or sections with thicknesses greater than 1/8 “.
G 50	0,5	Same as the S 280 but smaller as valve and tappet rocker arms.
G 80	0,3	Sand removal of castings as non-ferrous materials. Preparation for galvanization. Stripping of 1 to 2 mm sheets and small parts of relative precision. Preparation for galvanic coatings or anchoring.

Comparative Economic Study – Sand X Steel Shot

The low cost of sand compared to steel shot leads to the erroneous conclusion that it would be more economically operational to use it.

The sand when employed under pressure blasting (shot blasting machines) at 7kg / cm2 (100 p.s.i.) breaks up rapidly.

After the first cycle, 70% of the total is pulverized with only one second cycle allowed before the material is completely replaced.

Steel Shots are much more durable. The spherical endures last from 400 to 450 cycles, with the loss volume about 160g / m2, and the angular from 300 to 3550, with wear being estimated at 180g / 2.

The price of sand only covers the Single Tax on Minerals and that of the sand levy IPI and ICM, both recoverable.

That said, the operating cost per square meter would be:

Approximate Value (ORTNs)	Spherical shot	Angle Shot	Sand
Cost per ton.	129,12	129,12	5,62
IPI Taxes%	4	4	-0-
ICMm Taxes%	17	17	-0-
Actual cost per ton.	102.00	102.00	5,62
Consumption per m2 (KG)	0.16	0,18	21
Cost per m2	0,016	0,018	0,118
Comparative Cost	1	1,13	7,37

In addition to direct cost, other aspects have to be considered.
– Cost of collecting and transporting certain volumes of dust.
– Productivity of angular shot is 15 to 200% higher than the right.
– Sand produces silicosis requiring strict precautions to protect operator health.
– The use of sand in cabinets or cabins requires greater investment in doubling at least the exhaust capacity and filtration area of dust collectors.
– Of course sand is indispensable for outdoor operations. However, operational efficiency, more than seven times lower cost, quality of service and other economic and partner aspects fully justify investments in booths or cabinets to confine the process, allowing for the recycling and purification of the shot.