Plastic Injection Molding Defects: Sink Marks and Their Remediation

1. Phenomenon of the Defect**
During the injection molding process, certain regions of the mold cavity may not experience sufficient pressure. As the molten plastic begins to cool, areas with larger wall thicknesses shrink slower, generating tensile stress. If the surface rigidity of the molded product is insufficient and is not supplemented with adequate molten material, surface sink marks appear. This phenomenon is termed “sink marks.” These typically manifest at regions where the molten plastic accumulates in the mold cavity and at the thicker sections of the product, such as at reinforcing ribs, supporting columns, and their intersections with the product surface.

2. Causes and Solutions for Sink Marks

The appearance of sink marks on injection-molded parts not only deteriorates the aesthetic appeal but also compromises their mechanical strength. This phenomenon is intimately associated with the plastic material used, the injection molding process, and the design of both the product and the mold.

(i) Concerning Plastic Material
Different plastics have varying shrinkage rates. Crystalline plastics, such as nylon and polypropylene, are particularly susceptible to sink marks. In the molding process, these plastics, when heated, transition to a flowing state with randomly arranged molecules. Upon being injected into a colder mold cavity, these molecules progressively align to form crystals, leading to a significant reduction in volume. This results in dimensions smaller than prescribed, thus causing “sink marks.”

(ii) From an Injection Molding Process Perspective
In terms of the injection molding process, causes for sink marks include insufficient holding pressure, slow injection speed, too low mold or material temperature, and inadequate holding time. Therefore, when setting molding process parameters, it’s crucial to ensure proper molding conditions and adequate holding pressure to mitigate sink marks. Generally, prolonging the holding time ensures that the product has ample time for cooling and molten material supplementation.

(iii) Related to Product and Mold Design
The fundamental cause of sink marks is the uneven wall thickness of the plastic product. Classic examples include the formation of sink marks around reinforcing ribs and supporting columns. Moreover, mold design factors like runner system design, gate size, and cooling efficacy significantly impact the product. Owing to the low thermal conductivity of plastics, regions farther from the mold walls cool slower. Hence, there should be sufficient molten material to fill these regions, requiring the injection molding machine’s screw to maintain pressure during injection or holding, preventing backflow. Conversely, if the mold’s runners are too thin, too long, or if the gate is too small and cools too swiftly, semi-solidified plastic may obstruct the runner or gate, leading to a pressure drop in the mold cavity, culminating in product sink marks.

In summary, causes for sink marks include inadequate mold filling, insufficient molten plastic, inadequate injection pressure, inadequate holding, premature transition to holding pressure, too short injection time, too slow or fast injection speed (leading to trapped air), undersized or unbalanced gates (in multi-cavity molds), nozzle obstructions or malfunctioning heater bands, inappropriate melt temperature, suboptimal mold temperature (leading to deformation at ribs or columns), poor venting at the sink mark regions, thick walls at ribs or columns, worn non-return valves leading to excessive backflow, improper gate positioning or excessively long flow paths, and overly thin or long runners.

To alleviate sink marks, the following remedies can be adopted: increasing the melt injection volume, increasing the melt metering stroke, amplifying injection pressure, elevating holding pressure or prolonging its duration, extending injection time (employing a pre-ejection function), adjusting injection speed, enlarging the gate size or ensuring balanced flow in multi-cavity molds, cleaning the nozzle of any foreign objects or replacing malfunctioning heater bands, adjusting the nozzle and securing it properly or reducing the backpressure, optimizing melt temperature, adjusting mold temperature, considering extended cooling times, introducing venting channels at sink mark regions, ensuring even wall thickness (using gas-assisted injection molding if necessary), replacing worn non-return valves, positioning the gate at thicker regions or increasing the number of gates, and adjusting runner dimensions and lengths.

Location: Ningbo Chenshen Plastic Industry, Yuyao, Ningbo, Zhejiang Province, China
Date: 24/10/2023

Post time: Oct-30-2023