Case Study: Cast Lead Strip

Production of lead strip by battery manufacturers is the first step in the plate making process. Accurate material measurement is especially crucial at this stage as this is when the highest volume (and dollar value) of lead is being processed. The table below inc ludes estimates of the amount of lead that’s consumed during the lead strip production process, using examples of a typical high-volume and low-volume manufacturing facility.

Line Data

Lead Strip Parameters
Strip Quantity	4 mm
Strip Width	97 mm
Strip Thickness	0.820 mm
Strip Casting Operation
Line speed	18 m / min
Daily Runtime	8 hrs / day
Operational Days per Year	300 days / year

Annual Lead Usage

Cost of Lead	$1.13 / kg
Density of Lead	11.34 g / cm3
Lead Production	31450.8 kg / day
	$35,539 / day
	$10,661,824 / year

When you’re processing such a large volume of material, even a small percentage of scrap is amplified and can have a significant impact on your OEE.

Because lead strip production is done early in the manufacturing process, defects that occur at this stage can cause further, compounded issues downstream. For example, strip that’s too thin can lead to broken wires in the punching and expanding process. Strip that’s too thick can lead to line jams and excessive tooling wear that could otherwise be avoided.

Inconsistent and inaccurate strip thickness causes individual grid weights to vary, which in turn affects the weight of the pasted plate. When the pasted plate is weighed, you can’t tell if the weight change is due to an error in the grid weight or an error in the amount of paste applied. Lead alloy is X% more dense than lead oxide so a minor change in grid thickness can cause a major error in paste plate weight. Inaccurate lead strip thickness translates into inaccurate lead oxide application which ultimately results in battery performance issues.

The Challenges of Continuous Casting

There are two primary methods used in lead strip production: continuous strip casting (also known as chill casting) and lead rolling lines. This case study focuses on the chill casting method. Note, however, that both methods are subject to process variations that are difficult to observe and equally benefit from improved thickness control.

 

The drum speed factor

Let’s begin with a brief description of the continuous chill casting process. Molten lead is brought into contact with a chilled rotating drum, causing the lead to solidify into a strip. The strip thickness is affected by: The condition of the drum surface (how long ago it was sandblasted), the height of the liquid lead within the tundish, and the drum’s rotating speed.

The drum speed affects how long it takes for the lead to solidify, which in turn affects the thickness of the strip. A slower speed results in a longer solidification time and a thicker strip. A faster speed results in a shorter solidification time and a thinner strip.

During the casting process, when the strips are run onto the winders, the master speed control is the slitter drive which controls the speed of the cooling drum. The drum has a clutch that allows it to be pulled along at a matching speed.

The manual measurement / operator factor

An operator running the continuous strip casting production process has many responsibilities: Starting and stopping the process, running the strip onto the winders, monitoring the state of the line, controlling the lead height by adjusting the tundish and the lead pump, and controlling the speed of the line either manually by turning a speed control knob, or using a touchscreen control. These operator-managed adjustments impact the thickness of strips moments after they’re made and can easily change the strip thickness by up to 10% - a significant amount!

How does an operator know when and how to make adjustments? Without inline thickness control, the operator must base their process control adjustments by referring to a manual micrometer reading or slug weight measurement taken at the end of the cast. This is problematic as it relies on a small amount of data from a specific snapshot in time. To effectively control strip thickness, the operator would need to take multiple measurements throughout the casting process, which isn’t possible with manual measurement. Note: Measuring the trim edges during the cast is not reliable because trimmed edges don’t represent the average thickness of the main strip.

The key issues

• Lack of skilled operators: Using manual measurement techniques requires skilled operators which are not always easy to find! Hiring a new operator and getting them to the required level of expertise takes a lot of time and training.
  
  
• Wasted material: To be on the safe side and ensure the coating is thick enough, manufacturers commonly run a bit on the heavy side and use up to 2% extra lead per battery. This may help avoid possible wire breakage from a too-thin lead coating but, over the course of a year, this “little bit” extra can easily amount to cost increases of $1M+ dollars.
  
  
• Increased downtime: When strips of the wrong thickness are fed into downstream processes, this can cause jams and other process interruptions, significantly impacting your OEE.
  
  
• Wasted time: Operators have to spend 1 - 3 minutes every hour taking manual measurements. This directly impacts production efficiency.
  
  
• Failure to meet customer requirements: Measurement variations caused by a lack of controls throughout the process can lead to out-of-spec product - customer dissatisfaction, loss of sales, damage to reputation.

The Mate Gauge Solution

The Mate Gauge MG-CLS system provides a continuous non-contact method of measuring across all strips made by the casting process. It’s designed to optimally measure the lead strip surface, and to work with alloys ranging from pure lead to other calcium-tin based alloys.

Ease of operation and access to real-time information: We design our systems to run smoothly and effortlessly without the need for extensive operator training. Automated measuring means your operators will be free to focus on what really matters - keeping the line running smoothly and efficiently. Our clear touch-panel display shows the thickness of each individual strip and the group average of all strips. Operators are informed about the line’s status and immediately alerted if something goes wrong. The MG-CLS is also easy to calibrate and maintain.

 

Automated caster speed control for continuous optimal thickness: Using automated caster speed control, the MG-CLS controls strip thickness via analog or PLC control to the speed of the casters slitter drive. The MG-CLS with closed loop operation simulates a near-perfect operator taking frequent high accuracy measurements and making the required adjustments to casting speed to lock the strip thickness as close as possible to the target.

Increased productivity: When strips of the wrong thickness are fed into downstream processes, this can cause jams and other process interruptions, significantly impacting your OEE. When operators have to spend 1 - 3 minutes every hour taking manual measurements, this directly impacts production efficiency. Automated measuring lets operators focus on all of the line operations, not only measuring.

Improved quality and reliability, reduced waste and cost: Produce a quality product that meets OEM requirements.The robust design of our MG-CLS helps your product stay in spec despite environmental impacts such as temperature fluctuations. Instead of only manually measuring the finished coil, reduce scrap and waste by measuring throughout the process. Control material cost by using just the right amount of lead - no more, no less.

During this cast, the voltage changed from 4.26V to 3.71V, corresponding to a thickness correction of 0.08 mm. What would have been  a 7.2% in-cast thickness variation that was eliminated by the automatic thickness control loop.

Why Mate Gauge?

Mate Gauge has been providing laser thickness measuring systems for cast lead strip applications since 2009 and currently has systems just like this in over 22 countries around the world. Manufacturers rely on our technology and our team to provide them with the best solution for their lead strip production.

Based on our field recorded data, our systems prevent approximately 20,000,000 pounds of lead from unnecessarily entering car batteries every year, something that not only benefits our customers but also the planet.