Are hidden production problems killing your budget and timeline? High rejection and rework rates silently eat away at profits and lead to frustrating delivery delays that can shut down your assembly line.
Rejection rate is (Scrapped Parts / Total Parts Produced) × 100. Rework rate is (Reworked Parts / Total Parts Produced) × 100. These two key metrics are the most direct measure of a die casting supplier's process stability and true cost-effectiveness.
In my 20 years on the shop floor and as an engineering consultant, I've learned that these two numbers tell you almost everything you need to know about a supplier's operation. A Purchasing Director and a Supplier Quality Engineer should view these rates as the true health report of a production process. Understanding how they are calculated is the first step, but understanding what they reveal is how you choose a reliable long-term partner.
How Do You Measure the True Cost of the Rejection Rate?
Your supplier quotes a low piece price, but are you seeing unexpected costs and delays? An uncontrolled rejection rate can mean that a large portion of your material and machine time is being thrown directly into the scrap bin.
The rejection rate, also called the scrap rate, measures the percentage of parts that are so defective they cannot be fixed. This number represents a total loss of material, energy, labor, and machine time for that part.
The calculation itself is simple: Rejection Rate % = (Total Parts Scrapped / Total Parts Produced) × 100. The real work is in the details. "Total Parts Produced" usually means all parts that come off the die casting machine and have their runners and gates removed. "Total Parts Scrapped" are the units that fail a critical inspection and are sent to be remelted. This could be due to internal porosity found on an X-ray, a failed pressure leak test, or a dimensional error that is outside the tolerance for rework. For a Purchasing Director like Simon, every percentage point in the rejection rate is a direct hit to profitability. For an SQE like Jure, a high or unpredictable rejection rate is a huge red flag indicating that the process is not stable or under control. I remember a new structural part for a Canadian client where the initial rejection rate was over 10% due to porosity. By adjusting the injection speed and adding vents to the mold based on our simulation data, we brought that rate down to under 1.5%. That is the kind of process control you should expect.
Is a High Rework Rate a Hidden Factory Costing You Money?
Your supplier promises on-time delivery, but shipments are consistently late. A high rework rate creates a "hidden factory" inside the main plant, absorbing labor and capacity and making delivery dates completely unpredictable.
The rework rate measures the percentage of parts that have defects but can be repaired to meet the specified standards. While better than scrap, a high rework rate indicates an unstable process and adds hidden labor costs.
The formula for rework rate is: Rework Rate % = (Total Parts Reworked / Total Parts Produced) × 100. A "reworked part" is one that initially fails an inspection but can be fixed. In die casting, this often means manually deburring excessive flash, straightening a slightly warped section, or machining a feature that was cast slightly out of spec. While it’s good to save a part from the scrap bin, rework is a dangerous metric to ignore. It is a source of inefficiency and inconsistency. Each reworked part is handled manually, which introduces variability and adds significant labor cost that isn't part of the automated casting cycle. I once visited a potential new supplier for a project and saw nearly 30% of their workforce standing at benches, grinding and filing parts. Their piece price was low, but their rework rate was clearly killing their efficiency. My team focuses on designing molds and processes that prevent defects in the first place. A good parting line seal in the mold, for example, prevents flash and eliminates the need for most manual deburring. A low rework rate is a sign of a truly capable engineering partner.
Conclusion
Controlling rejection and rework rates is fundamental for any aluminum die casting company. It is the key to ensuring profitability, delivery stability, and the customer satisfaction we strive for with every part.
