Breakage and rejection are two of the most common problems in ceramic manufacturing processes and It can come from a wide range of issues. Most ceramic manufacturers try to reduce their breakage and rejection as low as possible because small defect in a ceramic product can devalue the product by more than 50% or even mean a loss for a premium grade producer.
When we talk about breakage and rejection in ceramic manufacturing, , it can generally be down to body cracking. Most cracks are created during the forming and/or drying processes. When stresses are present within a drying ceramic item, cracks can appear to relieve them. These stresses appear when a ceramic product does not dry evenly. If one part of the product dries faster than another part it may also shrink faster than the other. Thus, when the latter part needs to shrink and dry, a crack can appear to relieve the stress. Small drying cracks will normally grow during firing, especially if significant firing shrinkage occurs.
To sum up, a cracking problem in the manufacturing process can occur in 2 stages:
1. Causes of cracking before firing:
- Raw material (Plasticity/ strength/shrinkage/purity)
- Forming process (pressing pressure/alignment of pressing mould/extruder)
- Drying process (drying time, temperature inside dryer/ air circulation)
- Conveying /handling process
2. Causes of cracking during and after firing
- Firing temperature
- Firing cycle
- Thermal expansion of body components
- Cooling rate
We can say that the cracking will happen when “ STRESS > STRENGTH”. Thus we need to understand more about stress and how to reduce it whilst also looking at how to improve green strength in our bricks and roof tiles.
Stress in ceramic manufacturing can be separated into 2 groups:
- External stress: There is stress from extruding /pressing /handling and conveying processes. We can call this “stress from production process”.
- Internal stress: There is stress from the raw materials:
- Thermal expansion during firing
- Moisture evaporation rate during drying
- Shrinkage and expansion when increasing or decreasing temperature
- Changing of internal structure of clay
- Chemical reaction during firing process
How to Reduce Breakage and Rejection
The simple way to reduce cracking is to improve strength (flexural and compressive MOR) and/or reduce stress. As mentioned, to reduce external stress we have to control and/or invest in improving our production process.
To reduce internal stress, we would need to change clay formulation or clay sources. This might be a difficult task, as finding a good quality clay can be time consuming and troublesome as the quality and properties can vary depending on the source. So, the most cost effective way to reduce breakage and rejection is to improve both green and dry strength by either of the following approaches-.
- Change the body formulation by adding higher percentage of high plasticity clay.
- Using a clay conditioner.
A high plastic clay will have an increase in fine particles which may lead to higher shrinkage than other clays with larger particle sizes. This in turn can make them more unstable during the firing process. It can sometimes be difficult to obtain a balance between strength and internal stress cracking during the drying process so the addition of a clay conditioner can be seen as quick and simple way to improve both plasticity and strength of the ceramic body.
The use of clay conditioners have shown a significant improvement in both green and dry mechanical strengths. This strength increase allows the clay products to move through all parts of the production process with reduced losses.
In many cases, binder addition to bodies is essential as without them some production processes would be impossible. For example , in the pressing process of powders, adding an organic binder makes possible a forming method that is independent of the plasticity.
A normal body binder must have several characteristics:
- It must leave a minimal amount of ash after firing
- It must easily burn out at low temperature
- It cannot be abrasive
- It must improve the mechanical strength of dry pieces
- It does not cause bodies to stick to moulds
- Its dispersion must be easy
- It cannot be toxic
- It does not affect the glazing stage of manufacturing
- It must be as inexpensive as possible.
There is not just one solution to reduce breakage and rejection during brick and roof tile production so it is important to test which one will work better for your type of clay and process.