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Considerations of building an injection mold

As a custom injection molder in China, we consider first, the requirements of and the complexity of the plastic part that the customer needs. Second, the quality class of the injection mold the customer is paying to have built, quite often the customer will balk at the price paid to get the part that they need, so sometimes there needs to be added language to the quote indemnifying us against the customer rejecting our professional assessment. Third, the type of material that will be run. Fourth, the "production irritation" factor caused by anticipated fill problems and the attendant loss of productivity owing troubleshooting and scrap.

After consideration of the above, we generally use resin makers guidelines for mold vent depths and styles, or our own internally documented design guide based upon experience.

Adding vents during the build is more controlled and costs are accommodated in the price of the plastic injection mold. Adding after the fact comes off the bottom line for the entire tooling project, usually comes under a crisis management situation where every delay hurts. If planned properly, some of this (adding after the fact) can be covered in the process development cycle and the injection mold can have the minimal amount of vents added to get the desired end result.

That said, we have never seen an injection mold that had too many properly configured vents, but have seen way too many injection molds without enough vents.

I often find that injection mold makers like to make the vents as small as possible to avoid any flash at mold making. I find that this is great for the initial mold qualification but becomes an issue not long after the injection mold starts running production. I prefer to start with a little heavier vent and let the injection mold "work itself in". I like a little flash at mold qualification that way I know I am at the limit. We all know they will only get smaller as the plastic injection mold runs. The other factor that I find interesting is that I have had injection molds that I liked to call "self-venting". This is where the injection is actually blowing the mold open just enough to blow the mold open and vented itself for 12 million cycles. These are all injection molds for packaging so I do not know how they would apply for other areas.

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