Common Plastic Part Defects

Common Plastic Part Defects

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Plastic injection molding is a reliable manufacturing platform for efficiently producing high performance lightweight, durable, and chemically resistant components.

Although a common manufacturing technology Injection Molding can still produce defective or substandard quality parts and it’s critical as a vendor,  or customer to understand the identification, causes and solutions to common quality defects.

Fundamentally there are 3 potential places to look for problems with molded parts.

1. The Mold
2. The Plastic
3. The Process


1. The Mold

Here are a list of common defects which are associated to mold design, mold maintenance, or mold build quality.

Flash: Typically flash can be identified as a sharp edge which is often seen on shutoff areas or the parting line of the part.
Scuffs: Marks which are often found on poorly drafted faces or caused by distortion during part ejection.
Short Shot: When a mold cavity is not completely or properly filled with plastic leaving voids, or an incomplete part.

It is important to remember that mold design and processing are interconnected.  It is possible for a good mold to be processed improperly to cause flash, short shots, or scuffs.  At Three R Plastics injection Mold Setup Technicians work directly with our Mold Designers, and Mold Makers to ensure the most efficient process start to finish.


2. The Plastic

Plastic quality and preparation are critical factors in delivering in both strength and appearance.

Splay: Splay is often seen as a white swirl or spray pattern on the surface of plastics indicating improper drying or preperation to the thermoplastic prior to the injection process.
Contaminates: Foreign objects from a improperly purged barrel or unclean work environment can cause defects in a parts finish.
Color: Occasional variances in color or impinging swirls can be the sign of contaminates from improper purging of the barrel or an improperly blended material.

Reliable and certifiable materials suppliers can be critical to achieving the best results especially when quality finishes, and the high performance is required.


2. The Process

The processing which refers to the temperatures, pressures and timing of the mold and plastic injection can have a drastic effect on part performance, part finish, and ultimately part cost.

Flow Marks: Flow marks usually appear as a wavy discoloration caused by a slow or improperly flowing part.
Burn Marks: Brown or black areas which indicate burning which is often an indicator of improper venting or improper injection or temperature processing.
Cold Slug:  Improperly processed and un-melted plastic pellets or slugs tumble through the mold during injection causing distortions to surface finish, and often negative effects to the parts structural performance.
Sink Marks: The inconsistent heating or cooling of plastic during the molding process often caused by varying differences or changes in wall thickness, or due to improper injection pressure or dosage.
Knit Lines: These lines occur when internal mold flow cools before contacting an opposing flow.  Often the result of processing some knit line problems start as early as part design, and most are a result of poorly engineered mold.

The best process starts at product design and ends with quality control.

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