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Manufacturers across the country benefit from sourcing plastic injection-molded parts from a high-quality turnkey manufacturer. On this page, we discuss a few of the best practices regarding the plastic part acquisition process, as well as some of the design possibilities you can achieve when working with a trusted turnkey plastic injection molder.

Designing the Perfect Plastic Part   

Creating a high-quality, reliable precision injection molded plastic part involves careful planning, prototyping, and testing. Although most part issues originate during the first steps of manufacture, they can be easily avoided from the outset by working with a trusted turnkey molding manufacturer, rather than a company that outsources their work.

These types of molding companies work with outside designers and then contract another company — often an offshore operation — to build the mold, essentially relinquishing control over operations in the process. Turnkey manufacturers, on the other hand, handle all steps under one roof, allowing for full oversight and ensuring accountability and quality.

Designing the Part

Plastic injection molding allows for the production of precision parts at low costs, but high-volume molding processes require specialized part design. With the right design, consistently high-quality parts can be produced in bulk, avoiding expensive processing errors and delays.

Before beginning the injection molding design process, the following factors should be taken into account:

Wall Thickness    

Wall Thickness    

To prevent the most common part defects that occur during manufacturing, it’s crucial to maintain a uniform wall thickness. When melted, plastic will naturally flow into the areas of least resistance, so if your part features both thin- and thick-walled sections, the melt will flow into thick-walled sections first, depending on the gate placement. Also, thicker sections typically cool more slowly, making them vulnerable to sinking defects and voids.

However, if you’re dealing with a part that has inconsistent wall thickness, you can take preventative steps to avoid improper fill. These can include designing the cavity in a way that allows thicker sections to gradually transition into the thinner areas and properly placing gates to ensure optimal cavity fill.

Although these measures can help reduce the risk of part defects, starting out with uniform wall thickness will ensure the mold fills and cools properly. Thicker areas can also be cored or hollowed as needed to maintain uniformity while enhancing overall part quality and aesthetics.

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Rib Design    

Rib Design    

Reinforcing the strength of injection molded parts, ribs support the walls and other dimensional components of a part — akin to flying buttresses. Depending on the material of construction, ribs should account for 50% to 70% of the wall thickness; anything greater can cause sinking.

To further ensure sinking doesn’t occur, designers will often core out some material in order to reduce the amount of shrinking. Ribs must also not be too tall or too thin; generally, it’s recommended that the height be no more than three times the wall thickness. The corners should include radii, and the height should include a draft of 0.5° to 1.5°, which will allow the part to be easily ejected from the mold.

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Boss Design    

Boss Design    

Incorporating a boss into part design accommodates part assembly through the use of screws and pins. Bosses should have the same thickness as ribs, as thicker bosses will cause sink defects during part cooling. Rather than using a thick-walled boss, designers may use ribs to support the boss cylinder. And, as with ribs, bosses must have some draft to allow for ease of ejection.

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Corner Transitions    

Corner Transitions    

As discussed earlier, plastic melt flows into areas of least resistance. Therefore, sharp corners or angles changes can impede this flow; such geometries can cause improper cavity filling and packing, resulting in a defective, unreliable part.

To avoid such scenarios, smooth transitions are critical; all corners should feature curves, rather than angles, and the radius should be consistent on the outside and inside of the wall. This allows for a uniform thickness and enables proper material flow through the cavity.

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Weld Lines    

Weld Lines    

All plastic injection molded parts will inevitably have weld lines — formed when two material flows meet — but proper tool design will ensure they do not compromise part integrity. Both the tool and the part can be made with various wall thicknesses, geometries, and strategic gating in order to position the weld in a way that won’t interfere with part performance.

One of the most common injection molding issues, weld lines cause more than just performance problems; they can also present serious structural and cosmetic problems. But with the assistance of computer-aided flow simulators, such as SOLIDWORKS Flow Simulation, designers can easily determine where weld lines will occur and take necessary action.

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Gate Placement    

Gate Placement    

Optimal gate placement is imperative for proper mold filling, as gates serve to direct plastic flow from runners for efficient distribution throughout the part. The specific type of gate — and its specific placement — will significantly affect overall part integrity and quality, so it’s essential to take care with the selection process. Gate size and placement will depend on the part’s wall thickness and geometry.

Gates should be placed in a way that will effectively minimize flow length in order to prevent flow marks. Gates that flow from thick to thin walls will pack and fill better than those that flow from thin to thick. And, in some scenarios, additional gates may be used to avoid shrinkage and cooling issues.

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Vent Placement    

Vent Placement    

Allowing the gas (air) in the mold cavity to escape without causing superheating or resin burning, vents are critical for ensuring reliable part performance. And, if properly placed, vents can also help to minimize weld lines.

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More Information    

More Information    

Manufacturing the perfect injection molded plastic part requires close attention to every aspect of part design, but this is just the first step. While this can seem daunting, working with a knowledgeable turnkey manufacturer will help ensure the rest of the process goes as quickly and smoothly as possible.

With more than 60 years of experience in turnkey part manufacturing, the expert team at The Rodon Group understands the importance of fully understanding every step of large-volume injection molding.

To assist engineers and product designers in creating the ideal plastic part, we’ve put together a comprehensive eBook that delves deeper into part design and other key aspects of plastic injection molding, including:

  • Tool design and building
  • Material selection
  • Manufacturing processes

To download your free copy, click here.

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