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Injection molding is a widely used manufacturing process for creating a variety of metal or plastic parts. From micro-components to large panels, a single molding operation can create detailed parts complete with threads, holes, or complex curves.

Injection molding is the most common process for producing plastic parts and involves forcing the heated molten polymer into a metallic mold. The mold is the inverse of the actual part to be manufactured and is usually precisely machined from steel alloys, stainless steel, or aluminum. Once the molten polymer has been injected through the mold gates, cooled, and solidified within the mold, the product is ejected and the cycle is repeated.

There exists many process parameters involved in the injection molding process, in order to guarantee consistency and quality. Plant temperature and humidity, polymer quality or resin grades, and molding machine accuracy all play a crucial role in maintaining the dimensional, visible, and material quality standards of the product.

Utilizing a quality injection molding facility, that is knowledgeable in minimizing process variations in the molding process, will allow you to create precision molded parts and reduce scrap. Some defects that can occur with lower-quality production runs include:

  • Flash—unwanted material located at the edge of the part
  • Flow lines that show the physical path as the molten polymer cooled.
  • Shrinkage and sink marks—small depressions that develop due to inconsistent environmental and process temperatures
  • Weld lines resulting from molten substances meeting each other
  • Vacuum voids or pockets of air trapped with the molded product
  • Jetting, which occurs when the molten substance fails to adhere to the mold surface

The key to overcoming injection molding defects is minimizing production line variations and maintaining a controlled operating environment to regulate temperature and humidity. Incorporating a highly efficient plant HVAC system, that can ensure the ambient dew point and temperature of the plant air does not exceed the temperature of the mold, can prevent up to 20% of shrinkage that occurs during the opening and closing of the mold.

A quality mold design, created with the proper alloy materials, will mitigate many process defects. Weld lines and flow lines can be virtually eliminated with a mold design that incorporates rounded corners to minimize the sudden changes in polymer flow direction or wall thickness. Additionally, the proper location of polymer inlet flow gates and the best source flow pattern should be determined during the mold’s prototype testing to ensure the visual and structural quality of the product.

Jetting can be caused by a number of factors, including the incorrect location of the injection gates. Yet, decreasing the injection speed and increasing the melt temperature will lower the viscosity (or thickness and stickiness) of the polymer, in order to allow the product to adhere to the walls of the mold. Jetting is not only an appearance issue for the product, but it also damages its structural integrity.

Flashing of molded material at the edges of the product can be caused by a number of in-process issues, including insufficient clamping pressure of the press or a shot size that is too high. A shot refers to the polymer pellets that are melted and eventually pass into the heated mold. For good quality parts to be manufactured, the correct shot size needs to calculated, tested, and adjusted for accuracy.

Only through a combination of precise machinery, expert mold making, continuous optimization of process parameters, and minimization of production variations can precise injection molded products be manufactured. Rely on a reputable manufacturer with years of injection molding experience for the production of your closely toleranced, plastic product.