Injection Mold Design Guide


The plastic molding process is quite involved, and one of the basic elements is to use an injection mold design guide. In the rush to get the mold design into manufacturing it is easy to overlook basic guidelines, and a design guide can help with this.


Few things in injection mold making are as frustrating as getting a flawed mold design. Once errors are detected, it is natural to lose a degree of confidence in the entire design. This is especially true when the mistake is significant and appears to be due to a lack of concentration.

Here are ten top tips for an injection mold design guide

  • Check the design. Make sure the design is checked. All blueprints have a place for a checker to initial, yet, very often, nobody ever actually checks the print. With such powerful CAD design systems in universal use, it might seem redundant to have a human check the design, but this is not the case. A good injection mold design guide can make this process much easier.
  • Draft angles. Are all surfaces drafted? Some companies do not even use drafted walls when tolerances are tight, and very often this makes the molders job very difficult. Stuck parts can be difficult to extract, and damage can occur when prying the hardened plastic part out of a rib area, for example.
  • Wall thickness. Ensure that wall thickness is equal over the entire plastic part. Obviously, some areas will require thicker walls, such as a boss with an attached rib, but try to maintain constant wall thickness.
  • Corner radii. Use radii at corners to maintain even wall thickness. Larger radii are preferred due to the fact that larger radii helps to reduce stress, thus preventing conditions such as cracking. This should be part of your injection mold design guide.
  • Ribs. Make ribs 1/2 to 2/3 of the desired wall thickness and less than 3X thickness in height. Avoid deep, narrow ribs, as they are very difficult to polish and mold. Molding cycle times can be greatly increased while trying to fill deep, narrow ribs. Provide as much draft as is permissible.
  • Weld lines. Weld lines are generally the weakest area of the plastic part. The unsightly lines can also be cosmetically unacceptable. Consideration should be taken to ensure that weld lines are acceptable.
  • The mold maker. Consider the plastic mold maker and ease of manufacturing. For those mold designers who have no machining experience, it can be difficult to understand how the various mold components are made. Make sure the details can be manufactured with a minimum of processes and set-ups.
  • Gates. Determine which type and size of gate is optimum for the mold. Whether it is tunnel, cashew, banana, edge, fan gate or a hot runner mold, make sure the type, size and placement are correct.
  • Runners. Runner size and placement is sometimes overlooked because they are not actually part of the molded product. Nevertheless, with the wrong size or an imbalanced runner system, the parts will always suffer from things such as uneven fill, uneven cooling and shrinking.
  • Surface finish. Specify the correct surface finish for each component. Many hours can be wasted when the design specifies an A-2 finish, when all it needs is a B-3 finish. If your design template has a surface finish default, take the time to correct the requirements when they are different from the default.

Injection mold design guide

A good injection mold design guide is only as good as it gets used. These tips are just the tip of the iceberg, but can offer a good starting point. Take the time to develop your own personal injection mold design guide and you will be glad you did. So will the plastic mold maker and molder.

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