Why classify defects
A scrapped casting has already consumed metal, cores, energy and labour. But scrap only becomes useful if you know why it happened. The first step is a shared classification of defects: if moulding calls “blowhole” what inspection calls “porosity”, the statistics will never show where to act.
The main casting defects
Gas defects
- Blowholes: smooth-walled cavities, isolated or in clusters, often near the top surface or around cores. Typical causes: mould moisture, core binders that generate gas, insufficient venting, low sand permeability.
- Gas porosity (pinholes): small, widespread cavities, often just below the surface. Typical causes: gas dissolved in the metal (for example hydrogen in aluminium), metal not degassed, metal–mould reactions.
Shrinkage defects
- Shrinkage cavities: irregular voids in heavy sections or hot spots, caused by contraction that is not fed during solidification. Typical causes: undersized or badly placed risers, inadequate gating system, incorrect pouring temperature.
- Shrinkage porosity: fine, widespread cavities, often only visible after machining or with non-destructive testing.
Inclusions
- Sand inclusions: mould or core sand carried by the metal. Typical causes: erosion, poorly compacted moulds, fragile cores, insufficient mould cleaning.
- Slag or oxide inclusions: non-metallic material carried from the melt. Typical causes: insufficient deslagging, turbulence during pouring, no filters.
Filling defects
- Misrun: the metal does not fill the whole mould. Typical causes: low temperature, slow or interrupted pouring, thin sections, insufficient metal.
- Cold shut: two metal streams meet without fusing. Typical causes: low temperature, long flow paths, discontinuous pouring.
Cracks
- Hot tears: cracks that form during solidification, often at changes in section. Typical causes: contraction hindered by moulds or cores that are too rigid.
- Cold cracks: cracks caused by residual stresses, often after rapid knockout or impacts.
Mould-related surface defects
- Metal penetration into the sand, scabs and buckles, veining (finning): linked to sand quality, compaction, temperature and refractory coatings.
Dimensional defects and breakage
- Mould mismatch or core shift, distortion, breakage at shakeout or cutting: often originating not in pouring but in moulding, core setting or later operations.
From defect to cause
Three pieces of information are needed to trace the cause:
- What defect it is (shared classification).
- Where it recurs: on which item, pattern, heat, alloy, period, operator.
- What it costs: number of pieces and the operation at which they are scrapped.
A Pareto analysis (a few causes explain most of the scrap) shows where to act first.
How it is usually handled
Scrap is often counted at the end of the shift or week, without a cause or with causes written freely. Statistics are produced in spreadsheets, rarely, and are not linked to heats and lots.
The limits of spreadsheets and generic ERP systems
- Non-standard causes: each department uses different names.
- Scrap not linked to heat and lot: impossible to tell a metal problem from a process problem.
- Late statistics: the problem is seen when it has already cost weeks of scrap.
- No comparison over time: you cannot see whether a corrective action has worked.
How FOND/WISE handles it
FOND/WISE lets you record scrap with its cause directly on the shop floor and analyse it over time.
- Scrap declaration in the MES: pieces scrapped by operation and order, and scrapped materials (cores, filters…).
- Scrap statistics by item: breakdown by defect cause, with year-on-year comparison.
- Non-conformities linked to the item, with an alert in the technical data sheet when one is opened.
- Full traceability: from a defective casting back to heat, analysis, lot and operations, and from a heat forward to all castings affected.
- Notes and images per operation: inspection instructions (for example “check for sand inclusions and shrinkage porosity”) are visible to the operator.
- OLAP and BI tools to cross scrap with alloys, patterns, periods and customers.
Key features
Scrap with cause
Declared on the shop floor by operation, order and item.
Statistics by item
Scrap causes as percentages, year on year.
Non-conformities
Opened on the item and flagged in the technical data sheet.
Back to the heat
From the defect to the heat, the analysis and the lot.
Inspection instructions
Notes and images for the operator at each operation.
OLAP analysis
Scrap crossed with alloys, patterns and periods.
Worked example
The scrap statistics for an aluminium item show this breakdown of causes over two consecutive years (share of total scrap, illustrative values):
| Cause | Year 1 | Year 2 |
|---|---|---|
| Breakage at shakeout | 5% | 40% |
| Porosity | 7% | 5% |
| Misrun | 4% | 13% |
| Distortion | 3% | 7% |
| Breakage at cutting / fettling | 22% | 35% |
| Other causes | 59% | – |
The most striking change is breakage at shakeout, up from 5% to 40% of scrap: a defect that does not originate in pouring but at knockout and shakeout, for example after a change of equipment, parameters or core. Without classification by cause the data would just have said “more scrap”, and the search for the problem would probably have started in the melt shop, the wrong department.
Frequently asked questions
What are the most common casting defects?
The most frequent are gas defects (blowholes and porosity), shrinkage defects (shrinkage cavities and shrinkage porosity), sand or slag inclusions, cracks, filling defects (misruns and cold shuts), mould-related surface defects and dimensional defects such as mismatch and distortion.
What is the difference between a blowhole and shrinkage?
A blowhole is a cavity caused by gas (from air, mould moisture or core binders) and usually has smooth, rounded walls. A shrinkage cavity results from metal contraction during solidification that is not fed by liquid metal; it has irregular, dendritic walls, often in the heaviest sections.
How do you find the cause of a defect?
By examining the defect (shape, location, surface), checking where it recurs and crossing it with process data: heat, analysis, temperature, cores, pattern, operator, period. That is why scrap should be recorded with its cause and linked to traceability.
Is there a standard classification of casting defects?
There are international classifications and atlases of casting defects published by industry associations and used as a common reference. What matters in the foundry is to adopt a list of causes shared by all departments.