A checkweigher system is normally used in an industrial or retail operation where it is necessary to weigh items while they move through a production line and then to classify and separate these goods on weight. It is also used to reject items that are too heavy or light. These checkweigher systems will typically consist of an infeed section, the scale, a discharge section, and relevant computerised controls.
There are many types of checkweighers and these are applied to a wide range of different industries and environments. The components of a checkweigher will usually also vary from one operation to another and it is important that the correct system is chosen for each application.
Typical Uses
- Ensure compliance of product to certain weight standards and content laws
- Check for items that are above or under set weight standards
- Check for items that are missing such as labels, lids or information leaflets
- Verification of the count of product during a check for missing product
- Weighing components of package mixes to ensure accurate measures of product
- Reduce excess product loss by determining the sizes of batches (by weight)
- Classification of products into different classes by weight
- Provide production reporting
- Determine weight of a product before and after a process to ensure optimal process performance
- Measurement and reporting on the effectiveness of the product line
The checkweigher system is usually seen as the “policeman” on the production or packaging line. It protects the business against product or packages of unacceptable weight and prevents packages that are too heavy or too light from reaching the customer. There are many laws and legislative requirements and having such a system in place will ensure that compliance rates remain acceptable. Legal or industrial compliance is normally calculated by determining the mean and standard deviation rates, and these have to fall within applicable standards.
Accuracy of Scale
The accuracy of the scale is crucial to the success of the process. The two most crucial factors that help to measure the accuracy of a scale are linearity and repeatability.
- Linearity: A test package is weighted (of which the weight is known). The closer the indicated weight on the scale is to the actual weight of the test package, the smaller the error margin. Good linearity means that a very small or no error is present. Over the course of time the average error margin of a machine is measured to give an indication of the accuracy of the scale.
- Repeatability: This can be calculated by using a standard deviation, which means that one test item is weighed several times in succession and the differences are measured. The lower the error margin or the difference in indicated vs. actual weight, the better the repeatability of the scale.
It is normal to variance to occur as there are various factors that could influence the accuracy of the reading. Some scales tend to settle slower than others and as some scales weighs hundreds of items per minute, the settlement times may vary and influence the reading. The accuracy is usually the greatest determinant of the effectiveness of the scale, but some deviations are totally normal. If these deviations remain within acceptable limits they do not tend to pose a problem.
Machines with very high accuracy levels can sometimes not be applied to all operations and some compromises on accuracy or machine use may have to be made. Systems with great accuracy levels tend to cost a lot more than those who don’t, but in the long term they tend to save money in scrap product and reruns.
Environmental Factors
Accuracy can be influenced by various factors in the immediate environment such as temperature, vibration levels, speed of the production line, debris or dust, air flow, electrical noise and caustic surroundings.
In order to protect your checkweigher system from environmental influences, you can do the following:
- Keep the area around the scales very clean and eliminate airborne particles
- Avoid contact of scale with any other mechanical appliance
- Ensure that your checkweigher is fastened securely to the ground
- Choose a system that will operate efficiently in the physical conditions of the operation
- Keep scale away from any vibration
- Effective training of all checkweigher operators, servicing crew and maintenance staff.
If you need to find an accurate checkweigher system for your production or retail operation, you will have to know which system can function accurately in your particular operation. Our helpful and knowledgeable consultants know our products well and will be able to help you find the perfect scale for your operation.