There are several “rules of the road” when it comes to design a part for the injection molding process. These are not only meant to be beneficial for the mold designer, but to ensure that the mold runs in an optimum way. This will result in greater productivity and efficiencies, and better quality with less scrap/non-conforming parts, which will give you better return on your mold investment and provide high-quality parts to maintain your products’ brand integrity.
- One of the first rules of the road is creating a part with wall thickness consistency, commonly referred to as uniform wall thickness. Parts with variable wall thicknesses can cause problems in molding such as slower cycle times and quality problems, especially when going from a thicker wall section to a thinner one as pressures and flow rates will vary. Quality will also be impacted, as will cost due to higher scrap rates.
- There is an optimum gate location that is critical. The gate is the opening where the molten plastics enters the mold. Don’t design a part that will have a gate in an area that will create molding problems such as in a very thin part of the wall or in a location on a part in which cosmetic appearance is critical. There are many different types of gates to accommodate all types of part designs, but don’t ask for the impossible such as “I don’t want any gate vestiges anywhere.” The material has to get into the mold through a gate, but the mold designer, along with a Moldflow analysis can provide an optimum gate location that also meets the appearance requirements of the part. Experienced designers have a lot of tricks but they can’t defy the rules of physics.
- Allow for reasonable tolerances. Most mold makers can produce molds with very tight and specific tolerances, but there must be a range – not plus or minus zero. Remember, you are dealing with the injection molding process where there are many variables: heat, pressure, flow rate depending on the viscosity of the material, and part design. Talk with your injection molder/mold manufacturer at the front-end of the part design process about reasonable and viable tolerances for the part you need molded.
- Avoid sharp corners. Rounded corners and good radii are your friend in injection molding. When the molten plastic flows into the mold and hits a sharp corner it can cause quality problems as well as cycle time issues. Slowing the cycle to try and accommodate sharp corners to prevent quality issues results in higher costs as well.
- Don’t forget to design in draft! Even a small amount of draft, especially on a deep part, will make a big difference in cycle time and quality. “I don’t want any draft” is not a good answer when the mold designer asks you how much is allowable.
- Watch your bosses and ribs! Bosses are typically used as a mounting location feature. Ribs are used to strength a part without increasing wall thickness. If not designed into the part properly, bosses (because they generally have thicker wall sections to accommodate inserts for assembly) and ribs (typically to support the functionality of a large part), can cause quality problems on the part’s outer surface, particularly sinks, in spite of the fact they are on the underside of the part.
- Resin type. Sometimes the material required is a forgotten issue, but it’s critical to the part and mold design. There are recommendations provided by material producers as to proper wall thicknesses, part design considerations and other advice that you’ll need to improve part design and create optimum injection molding conditions.
At Anderson Technologies, our engineering team can help you through this process so that no technical detail is left behind. We know molds and we know injection molding. At Anderson, we help you achieve new levels of success!