Acceptance Sampling

Statistical Logic

Evaluation of a shipment begins with the selection of an inspection level that determines the relationship between the lot size and the sample size. The use of acceptance sampling requires a clear definition of the acceptable quality limit, which represents the worst tolerable process average. When a technician pulls samples from a container of polyethylene, the samples must be random to ensure they represent the entire lot.

If the number of defects found in the sample is below a certain number, the entire batch moves to production. If the count exceeds the threshold, the material is quarantined or returned to the supplier. This binary outcome forces suppliers to maintain high standards or face the costs of rejected material.

Risk Allocation

Manufacturers must account for the possibility of a type one error, where a high quality lot is rejected due to a poor sample. Conversely, a type two error occurs when a low quality lot is accepted, potentially leading to part failures or machine damage. In the context of injection molding, acceptance sampling protects the molder from using resin with inconsistent melt flow or moisture content.

These variations can cause flashing and short shots. By setting strict sampling plans, the buyer shifts the burden of quality onto the producer. The producer must then ensure that their internal controls prevent the shipment of substandard resin.

This relationship is governed by the operating characteristic curve, which shows the probability of acceptance for various levels of lot quality. The shape of this curve determines how well the plan discriminates between good and bad lots. A steep curve indicates a plan that is very effective at protecting the buyer from receiving poor material while minimizing the risk to the supplier.

Lot Disposition

Final decisions regarding the material depend on the severity of the defects found during the inspection process. Major defects that impact the function of the part lead to immediate rejection, while minor aesthetic issues might be tolerated depending on the application. When acceptance sampling results in a rejection, the cost of sorting the remaining parts or returning the resin is significant.

This process ensures that only materials meeting the specification are used in the mold. It also provides a record of supplier performance over time. Consistency in these results allows for a reduction in sampling frequency if the supplier demonstrates a history of high quality.

Acceptance sampling remains a standard practice in global polymer procurement.