How to Reduce Reject Rate in Injection Molding

The reject rate directly affects production cost, delivery efficiency, and profitability in injection molding. Reducing rejects requires a systematic approach covering materials, molds, machines, processes, operators, and on-site management. This article provides practical methods to stabilize and lower the reject rate.

Raw Material Control

Many defects such as bubbles, black spots, brittleness, and color deviation originate from poor material management. Hygroscopic materials such as PA, PC, and PET must be properly dried. Inappropriate regrind ratio, contaminated materials, or incorrect grades cause consistent quality problems.

Material identification, separate storage, and thorough purging during color or material changes are essential. Contamination, especially with PVC, can corrode the machine and cause irreversible defects.

Mold Maintenance and Optimization

Mold quality determines the upper limit of product quality. Poorly maintained molds lead to persistent flash, sticking, drag marks, uneven cooling, and dimensional instability.

Key measures include regular cleaning of vents, maintenance of cooling channels to avoid blockage or scale, timely replacement of worn pins and inserts, and proper clamping force. Balanced cooling and effective venting significantly reduce warpage, sinks, and burning.

Machine Stability

Inconsistent machine performance causes fluctuations in weight, dimensions, and surface quality. Wear of check rings, inconsistent temperature control, unstable hydraulic pressure, and voltage fluctuations all increase rejects.

Preventive maintenance includes checking barrel and screw wear, calibrating temperature controllers, monitoring oil temperature, and ensuring stable clamping and injection performance. Machines running in good condition greatly improve process stability.

Standardized Process Parameters

Many factories rely on personal experience rather than standardized parameters, leading to high rejects after shift changes or mold changes. A complete process sheet should include temperature, pressure, speed, cooling time, and mold temperature.

Stable filling, proper holding pressure, sufficient cooling, and low internal stress are the core goals. Stepwise velocity control reduces jetting and flow marks. Sufficient holding pressure compensates for shrinkage. Standardized processes ensure repeatable quality.

Operator Training and Discipline

Human error is a major hidden cause of high rejects: wrong inserts, mishandling, unauthorized parameter changes, unreported abnormalities, and poor cleaning.

Strict training, clear operating procedures, first-piece inspection, and regular in-process checks help reduce avoidable losses. When operators understand quality requirements, rejects decrease noticeably.

On-Site Management and Continuous Improvement

A data-driven system is essential for long-term improvement. Record and classify rejects by cause: flash, sink marks, short shots, warpage, bubbles, scratches, etc. Focus on the top three defects each week.

5S management reduces contamination and damage. A rapid response system for abnormalities minimizes downtime and batch rejects. Through continuous optimization of molds, materials, processes, and management, the reject rate can be maintained at a low and stable level.