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中文简体I. Injection Molding Basics and Defect Tracing
Plastic soup spoons, as high-volume, low-cost disposable or durable tableware, are primarily manufactured through the injection molding process. Injection molding involves injecting molten polymer into a mold cavity under high pressure, allowing it to solidify and form the desired shape upon cooling. Despite the maturity of this technology, the high cycle speeds and extremely thin wall thicknesses required make soup spoons highly susceptible to various molding defects.
These defects not only affect the spoon's aesthetic appearance but, more importantly, impair its mechanical strength, food contact safety, and user comfort, directly impacting the end-user experience and brand reputation. Professionally identifying, describing, and controlling these defects is a core criterion for measuring the quality control (QC) level of an injection molding plant.
II. Major Aesthetic and Surface Defects
Aesthetic defects are the most easily observed quality issues by customers and have the greatest impact on website bounce rates.
1. Flow Marks / Flow Lines
Definition: Wavy, flowing, or serpentine patterns appearing on the surface of a soup spoon. Cause: This is usually caused by uneven melt flow within the mold cavity, low melt temperature, or slow injection speed. The melt front rapidly cools upon encountering the cavity wall, resulting in the superposition of melt layers with different temperatures and viscosities. Flow marks are particularly common at the junction of the spoon's head and handle due to varying wall thicknesses.
2. Weld Lines / Knit Lines
Definition: Linear marks left behind when molten plastic fails to fully fuse within the mold cavity.
Cause: Weld lines occur when the melt bypasses an obstruction within the mold (such as an insert or runner splitter), or when two or more melt streams merge in multi-gated injection molding. In spoon designs, weld lines often appear at the bottom or sides of the spoon head, representing the weakest area and potentially leading to breakage.
3. Flashing / Burr
Definition: Excess, thin flakes of plastic that overflow from the parting surface or other joints of a product. Cause: Usually due to insufficient clamping force, mold wear, or excessive injection pressure. Flash affects the feel and mouth-safety of the spoon and must be removed through post-processing.
4. Jetting
Definition: The melt is ejected from the gate into the mold cavity at excessive speed, forming a "jet"-like curly snake within the cavity that is then surrounded by the remaining melt.
Cause: Excessive injection speed or improper gate size design. Jetting can reduce local strength and leave a noticeable spiral defect on the spoon surface.
5. Sinks / Air Traps
Definition: Sinks are silvery-white or gray streaks on the surface of the part that resemble fine hair or water vapor trails. Air traps are voids within the part.
Cause:
Sinks: Primarily caused by excessive moisture content in the raw materials (inadequate drying) or by the degradation of the melt within the barrel, which produces volatiles.
Bubbles: These are usually caused by poor venting, air trapped in the mold cavity, or uneven volumetric shrinkage during melt cooling.
III. Structural and Performance Defects
These defects often affect the functional lifespan and safety of a spoon.
6. Sink Marks
Definition: A depression that appears on the surface of a thick-walled area of a spoon.
Cause: The plastic in thicker areas shrinks unevenly during cooling. While the exterior solidifies, the interior continues to shrink, pulling the surface inward. These are common in thicker areas of a spoon, such as the ribs or handle base. Sink marks not only affect appearance but can also become the starting point for fractures due to stress concentration.
7. Warpage
Definition: A distorted shape of a spoon after demolding, not conforming to the designed geometry.
Cause: Primarily caused by internal stresses, including uneven cooling rates, uneven mold temperatures, or anisotropic shrinkage of the plastic itself. The long handle of a soup spoon is particularly prone to warping, affecting its flatness and stackability.
8. Short Shot
Definition: The melt does not completely fill the mold cavity, resulting in an incomplete part.
Cause: Insufficient injection volume, low injection pressure, or excessive melt viscosity. This condition is commonly seen at the end of the handle or the thinnest edge of a soup spoon, leading to product rejection.
9. Crazing / Stress Whitening
Definition: Small cracks appear on the surface of the part, or whitening occurs after being subjected to stress or contact with chemicals.
Cause: Excessive internal stress, which may be caused by improper demolding, excessive holding time, or uneven cooling. Stress whitening is a defect that often occurs when plastic soup spoons are used in hot soup environments due to concentrated thermal stress.
10. Black Specks / Contamination
Definition: Small particles or foreign matter that is not the color of the original plastic have been introduced into the part. Cause: Primarily due to contaminated raw materials or the release of carbonized or degraded char from the injection molding barrel. For food contact products, any impurities are a serious quality issue.
