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Defects in casting

Foundry industry in developing countries suffer from many casting defects . Even in complete controlled process , defects in casting are observed and hence casting process is also know as process of uncertainty .

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There is a various reason or sources which is responsible for the defects in the cast metal. Here we will learn more about different major types of casting defects.

Types:-

Casting defects can be classified as follows

Filling related defects
Shape related defects
Thermal defects
Defects by appearance

Filling Related defects:-

Blow holes:-

It is a kind of cavities defects, which is divided into pinhole and subsurface blowhole.
Pinhole is very tiny or small hole and subsurface blowhole only can be seen after machining.
When gases entrapped by solidifying metal on the surface of the casting , which result in a rounded or oval blowhole as a cavity.
This defects nearly always located in the cope part of the mold in poorly vented pockets and undercuts.

Causes:-

Resin-bonded sand

High release of excessive release of gas from core
Excessive moisture absorption by the cores
Low gas permeability of the core sand

Clay-bonded sand

Moisture content of the sand high
Low gas permeability of the sand
High sand temperature
High bentonite content

Remedies:-

Resin-bonded sand

Improve core venting and provide venting channels ,ensure core prints are free of dressing.
Reduce amount of gas or use slow reacting binders.
Apply dressing to cores,the slowing down the rate of heating and reducing gas pressure.
Dry out cores and store dry , thus reducing absorption of water and reducing gas pressure.

Clay bonded sand

Try to reduce moisture content of sand , improve conditioning of the sand
Improve gas permeability .
Reduce temperature of sand use sand cooler if necessary .
Reduce bentonite content , use bentonite with a high montmorillonite content , high specific binding capacity and good thermal stability.
Use low reacting lustrous carbon producer

Sand Burning:-

Burning on defects also called as sand burning, which include chemical burn –on , and metal penetration. These defects occur to a great extent in the case of thick- walled casting and at high temperature.

Causes:-

Clay- Bonded sand

Low lustrous carbon content
Proportion of low melting point substance too high

Molding plant

Uneven mold compaction

Gating and pouring practice

High temperature of liquid metal
Uneven distribution of in flowing metal with resultant over heating

Remedies:-

Clay-bonded sand

Increases proportion of lustrous carbon producer, which result in positive separation between mold and metal.
Use pure silica sands

Molding plant

Ensure uniform heat compaction and try to increase heat removal from the molds.

Gating and pouring practice

Even out incoming metal flow
Reduce pouring rate
Reduce liquid metal temperature

Sand inclusion

Sand inclusion are also called as scab or blacking scab and it look like small or middle holes with sand grains in the internal or on the surface of casting. This casting defect formed during abrasion of the mold surface by the metal flowing past and the associated thermo-mechanical stresses.

Causes

Molding plant

Break up of mold section during stripping of patterns
Uneven compaction of molds

Clay-bonded sand

Low compatibility
Very low bentonite content
High inert material content
High lump content
High content of lustrous carbon producer

Resin- bonded sand

Low core strength
Excessive core mismatching

Gating and pouring practice

High pouring rate
Ladle too high above pouring basin
High pouring time

Remedies

Molding plant

Check mold for pressure mark if require add pressure pads
Carefully blow out mold cavities
Increase pattern tapers and radii, improve pattern plates
Ensure uniform mold compaction , avoid over compacted sections.

Clay bonded sand

Raise compact ability and plasticity of the sand
Increase bentonite content.
Reduce inert dust content

Resin bonded sand

Increases the strength of the core, use greater proportion of binder.
Avoid core mismatching

Gating and pouring practice

Avoid high pouring rates and impact of metal stream against mold walls.
Improve distribution of gates.

Cold lap or cold shut

Cold shut also called as cold shut .It is crack with round edges.Cold lap is because of low melting temperature or poor gating system.

When the metal is unable to fill the mold cavity completely and thus leaving unfilled portion called misrun.

Causes

Lack of fluidity in molten metal
Faulty design
Faulty gating

Remedies

Adjust proper pouring temperature
Modify design
Modify gating system

Misrun

When the molten metal is unable to fill the mold cavity completely or molten metal solidify before filling the mold cavity completely and leave a space in mold called misrun.

Causes:-

Lack of fluidity in molten metal
Wrong design and wrong gating

Remedies

Maintain proper pouring temperature
Modify design and gating system

Gas porosity

Gas porosity is nothing but the formation of air bubbles within the casting or pin hole on the surface of the casting.

When the molten metal start filling the cavity , before that air is present in the cavity .When we poured molten metal air start compressing as more and more metal streams in to the cavity and the pressure rises.

When cavity full it becomes dispersed as small sphere of high pressure air. The swirling flow can cause them to become elongated.

Causes:-

Low metal pouring temperature
Low metal fluidity
Slow pouring
Slag on metal surface
Interruption to pouring during filling of the mold
Thin metal section
High gas pressure in the mold arising from molding material having high moisture

Remedies

Increase metal pouring temperature
Modify metal composition to improve fluidity
Remove slag from the metal surface
Reduce gas pressure in the mold by appropriate adjustment to molding material properties and ensuring.
Modify casting design and avoid thin section
Use insulating coating and ensure that molds are adequately pre-heated.

Shape defects

Mismatch defects

Mismatch in casting is common defect, when casting does not match at the parting line or improper alignment of upper and lower portion of the casting.

Causes:-

When cope and drag parts of the mold not remaining in their proper position
Misalignment of flask

Remedies

Use proper closing pins and molding boxes
Check pattern mounting on match plate and rectify , correct dowels

Distortion or wrap

Distortion due to wrap age is known as warp defect.

Causes:-

It occur time in a casting that partially or completely liberates residual stress.

Remedies

Common practice in iron casting is normalizing heat treatment to remove residual stress.

Flash defect:-

Its also known as flashing, fins or burns ,it also defined as when unwanted or excess material is attached to a molded or cast product .

Causes

Improper clamping of the flask can also creak flash defects
When part of the die have insufficient strength
Damage die component and die faces
Cavities offset from center of platen
Wear or damage in toggle mechanism
Hydraulic valve or seal leak

Remedies

Reassembling the molds and core and there should be a enough weight on the top part of the mold so that the two parts fit together tightly.

Cracks and tears

Cracks can be seen with the naked eyes and it appears in casting from a number of reason.

Causes

When molten metal does not have sufficient strength to resist tensile force during solidification.
It mostly caused by poor mold design.
because of thermal imbalance in the die
undercut or damage in the die cavities

Remedies

try to reduce pouring temperature
provide feeders
give sufficient cooling time
reduce sharp corners

Shrinkage

This defects occur when filling material is not available for shrinkage as the metal start solidify.

This defects again split into two parts :-

1)open shrinkage defects

2) closed shrinkage defects

Open shrinkage defects:-

This defect is open to atmosphere and easier to notice ,they can be caved surface or pipes that enter into the surface of the casting.

Closed shrinkage defects:-

This type of defects are difficult to spot, and they are hidden within the casting and causing structural impurities. Under this defects isolated pool of liquid metal is form inside solidified metal which are called hot spots and shrinkage defects usually forms at the top of the hot spots.

Causes:-

When density of die casting alloy in liquid state is less than its density in solid state. There for when alloy change its phase from liquid states to solid state, it always shrinks in size.
When pouring temperature is too high

Remedies

To avoid shrinkage porosity ensures that liquid metal under pressure is continue to flow into the voids as they form.
Apply principle of directional solidification in mold design.

Sink Mark

Sink mark are unintended depression on the surface of molded part ,this marks are very small and often quite visible because they reflect light in different directions to the part.

Sink mark occur when there is a thicker section of plastic compared to the general wall thickness. The Greater the difference in thickness, the more shrinkage is concern.

Causes:-

This marks formed mostly because of thermal contraction or shrinkage during cooling.
After the material on the outside has cooled and solidified, the core material starts to cool . its shrinkage pulls the surface of the main wall inward, causing a sink mark.
If the skin is rigid enough, deformation of the skin may be replaced by formation of a void in the core.
High volumetric shrinkage
Insufficient material compensation
Short cooling time

Remedies

Optimize the runner system design
Try to use different material
Try to reduce volumetric shrinkage
Change the part geometry
Optimize packing profile and control the packing pressure

Defect by appearance

Metallic projection

Metallic projection also called as joint flash or fins and it occur at the parting of the mold or whenever two elements of the mold intersect.

Causes

Because of incorrect assembling of mold and cores
Insufficient weight of the mold clamping of the flask may produce the fins.
Blowholes and pinholes are produced because of gas entrapped in the metal during the course of solidification.

Remedies

Correct the assembling of mold and cores
Provide sufficient weight on the top part of the mold so that the two parts fit together tightly.
Make adequate provision for evacuation of air and gas from the mold cavity.

Discontinuities

Discontinuities are also called as hot crack and hot tears and it is the formation of shrinkage crack during the solidification of weld metal.

When metal in liquid condition it is weak and the residual stress or tensile stress in the material causes the casting fails as the liquid metal cools down. The failure of casting in this case is looks like cracks and its called as hot tear or hot cracking.

Causes:-

Damage to the casting while hot due to rough handling
Improper mold design

Remedies

Care in handling mold while it is still hot
Proper mold design can eliminate this defects
Elimination of the tensile stress from the material of the casting

Incomplete casting

Incomplete casting is common casting defect it occur when molten metal is unable to fill the extremist of the mold and its also called partially complete casting.

Causes :-

Insufficient alloy is used
High viscosity of fused metal or metal is not so hot and so metal is not able to enter thin parts of mold.
Casting pressure is low
Insufficient venting of the mold or improper burnout

Remedies

Have sufficient metal in the ladle to fill the mold,check the gating system.
Increase temperature of the alloy and rise liquids temperature
Decrease the viscosity and surface temperature of the alloy

Incorrect dimension or shape

When geometry of metal casting is incorrect this could be due to unpredicted contractions in the part during solidification or this occur when the metal does not shrink properly .When the cast sets, it is the wrong size or it is warped or bent.

Causes

When rigidity of the pattern is insufficient to withstand the ramming pressure applied to the sand, this result in an elastic deformation of the pattern and a corresponding , permanent deformation of the mold cavity .

Remedies

Assure adequate rigidity of patterns and patterns plates, especially when squeeze pressure are being increased .

Defective surface

When the liquid metal is leak out of the mold and the defect appears as lines which trace the flow of the streams of liquid metal. This defects are also called as flow marks and they are easy to remove and always reveal a buckle underneath.

Causes

Oxide film which make cottage at the surface , partially making the path of molten metal flow from the mold.

Remedies

Increase the liquid metal temperature
Modify the gate size
Reduce the pouring temperature

Rat tail and buckles

When the temperature of molten metal is too high it caused the expansion of a thin outer layer of molding sand on the surface of the mold cavity.

Causes

Caused by compression failure of the skin of the mold cavity because of excessive heat in molten metal.
A buckle is a more severe failure of the sand surface under compression.

Remedies

Provide proper expansion to mold to avoid compression layer.

Author:-

Gopinath Murthi

The Pantanal, the world's biggest tropical wetlands, is burning at record-shattering pace this year as drought-fueled fires devastate its vegetation and celebrated wildlife in an environmental catastrophe.

The region, which sits at the southern edge of the Amazon rainforest, is known for its immense biodiversity, drawing wildlife lovers from around the world with its jaguars, jabiru storks, giant otters, caimans, toucans, macaws and monkeys.

But in recent months, the images emerging from the region have been of charred animals' corpses and flames stretching clear across the horizon.

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'I've been here 20 years, and this is the worst situation I've ever seen,' Felipe Dias, head of the environmental group SOS Pantanal, told AFP.

© Ibere PERISSE Fire raging in the Pantanal wetlands, Mato Grosso State, Brazil, on August 28, 2020, in a handout image picture released by Projeto Solos Fires raging in the Pantanal, the biggest tropical wetlands on Earth, are threatening a nature reserve known as the home to the world's largest jaguar population, Brazilian authorities said on September 8, 2020.

Stretching from Brazil into Paraguay and Bolivia, the Pantanal is criss-crossed by rivers, swamps and marshes.

More than 2.3 million hectares (5.7 million acres) -- an area 388 times the size of Manhattan -- have gone up in flames in the region so far this year, according to the Federal University of Rio de Janeiro.

There have been 12,567 fires in the Brazilian Pantanal in 2020, setting a new annual record for the number of fires less than nine months into the year, according to satellite data collected by Brazil's national space agency, INPE.

The damage is 'irreparable,' and especially devastating for animals, said Juliana Camargo, head of wildlife conservation group AMPARA Animal.

'Very few animals survive. The ones that do often suffer very severe effects. They're burned to the bone, they often have to be euthanized, or die of hunger and thirst,' she said.

'The worst part is when the people on the ground fighting the fires tell us, 'There's nothing we can do, everything is going to burn.' The only hope is for it to rain, but that's not expected until November.'

- Wetlands running dry -

Local volunteers have rushed to help the teams of soldiers and firefighters deployed to battle the flames.

Many of them depend on the region's ecotourism industry, which has been battered by the twin crises of the coronavirus pandemic and the fires.

This week the flames reached a nature reserve known as the home to the world's biggest jaguar population, Encontro das Aguas State Park.

The disaster is being driven by extreme drought.

As it happens, the world's biggest tropical wetlands are not that wet these days.

Rainfall in the Pantanal plunged by half for the period from January to May this year, usually the height of rainy season.

Many areas that typically flood with the rains were left dry.

High temperatures and strong winds have fueled the fires.

But there are other factors in play, too.

Farmers and ranchers are increasingly introducing non-native crops to the region, which burn more easily than native vegetation, said forestry engineer Vinicius Silgueiro of the Life Center Institute (ICV).

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Some clear their land using the slash-and-burn method, lighting fires that can then grow out of control.

Far-right President Jair Bolsonaro's government has meanwhile failed to crack down on the problem, environmentalists say.

'There's a widespread sense of impunity, a weakening of environmental protection agencies and a reduction in funding' for environmental programs, said Silgueiro.

- 'New normal'? -

Bolsonaro, who took office in January 2019, faces criticism for presiding over a surge in fires, not only in the Pantanal but in the Amazon.

Studies show deforestation in the Amazon is having an impact on rainfall in other regions of Brazil by shrinking the rainforest's so-called 'flying rivers': vast clouds of mist that are carried by the wind and dump water across a large swathe of South America.

'It's too early to know if the droughts in the Pantanal in recent years are directly linked to that,' said Silgueiro.

'But there's no denying things are different from before. I'm from this region. I remember when it used to rain in August and September. This year, it hasn't rained since June.'

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Droughts like this year's risk becoming the 'new normal,' said Tasso Azevedo, the coordinator of Mapbiomas, a collaborative research group that tracks environmental data.

'That would be really tragic,' he said.

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'Because in the Pantanal, if you have fire after fire in the same place, the vegetation can't grow back.'