PLASTIC INJECTION MOLDING: COMMON DEFECTS AND THEIR SOLUTIONS

With plastic injection molding, production is quick, efficient, and accurate, with the ability to craft products in any shape or size. Injection molding also allows for consistent product performance — a huge plus in modern manufacturing techniques.

However, manufacturing defects can occur, often impacting the quality and reliability of a product. Severe quality-related issues may also negatively affect a product's performance, function, and safety. Most problems develop during the molding process and are preventable by focusing on the quality of work. Discover the three most common defects of the plastic fabrication technique.

Sink Marks

You might notice sink marks on your prototype as tiny but visible dips on the flat molded sections. Sink marks mainly result from inconsistencies in the manufacturing process, especially where the plastic doesn't have enough time to cool properly.

Some sections of your prototype design will have different thermal masses, often around rigid structural supports. The areas with lower thermal mass can lose heat faster and shrink more, creating depressions. Uneven pressure might also create regions of unequal thickness, where thinner patches cool faster than thicker ones.

A qualified injection molding expert can eliminate sink marks by first reducing the thickness of mold material, making it easier to control the overall wall thickness. They could also increase both the cooling time and holding pressure to create an improved prototype before proceeding with the mass fabrication of your product.

Flow Lines

Flow lines might not affect your product's structural integrity but could greatly affect its appearance. The lines form wavy patterns that are more pronounced on the narrow points of the mold. Flow lines can also be ring-shaped if they form around the molten entry points.

The defect develops from non-uniform cooling of the molten plastic mold, with specific sections cooling more rapidly than others. Uneven wall thickness is the main reason for non-uniform cooling during the injection procedure. Slow injection speeds may also encourage flow marks on the prototype, usually visible after cooling.

To prevent flow lines from showing up, your injection molding expert might increase the operating temperature, injection speed, or both. Also, they may round off the sections where the mold material is thicker for more uniform cooling. The molten material also cools slower if there is a reasonable distance between the coolant and mold gates.

Air Pockets

Air pockets, or vacuum voids, form when air is trapped inside the mold. They often appear as bulges on the surface of the final finish. Larger or multiple air pockets are quite severe and can heavily affect a finished product's structural integrity. Sections with several air pockets are weaker because there's air instead of solid molded material.

Insufficient pressure during the injection molding process could cause air pockets to form. With low pressure, forcing out the trapped air becomes difficult, forming bubbles after the mold material cools. Also, if the mold material closest to the surface cools too rapidly, it hardens and bulges outward. Misalignment of co-joined mold parts could also contribute to void formation on the final product.

Air pockets are sometimes not easily visible, making them a dangerous structural defect to ignore. Your injection molding specialist may incorporate crucial design practices, especially in gate positioning and joint-section alignment.

Here, they could have the gates in the thicker mold sections to ensure that the mold material cools slowly. They might also suggest that you use less viscous plastic for your product to prevent air pockets from forming.

Be sure to watch out for these defects on your prototype to ensure that you get the quality you pay for. For more information on quality control and injection molding, contact Seiler Plastics today.