By Jason Vander Griendt

June 22, 2016 at 10:14 pm

This is a commercial mold, something DIY injection molders can look up too ūüôā

Injection molding is tricky business. You need a mold which is stable enough to handle large volumes of molten plastics and other materials, which means safety and quality concerns and possibilities of huge amounts of mess.¬†If you’re a serious hobbyist it’s possible to achieve on your own (although whether you’d want to when you can outsource your manufacturing to a company like us is another matter).

While the full process of diy injection molding is a bit too complicated to go into in a short blog post like this, from our experience of plastic injection techniques here are some of the possible obstacles and pitfalls you’ll need to watch out for.

Maybe these will inform your decision as whether or not to try your own injection molding in the first place.

Flow lines

Flow lines are the pesky patterns, streaks, or lines which sometimes show up when you’re using a prototype to create a mold. You’ll be able to identify a flow-line-issue because flow lines will leave¬†off-white or discolored patterns on the object you are molding. They are the result of an inaccurate cooling profile where some of the injection material finds its way to places it shouldn’t. The material¬†cools to form patterns or streaks on your object¬†that are not within the original design.

Flow line issues are usually caused by inexperience and the easiest way to fix them is to increase pressure and injection speeds. This can help in reducing the creation of flow lines to some extent. Alternatively you can block the cavities within the molds with thin walls in order to reduce or avoid any excess material flowing into them.

Sink marks

Sink marks are depressions or small craters which can sometimes occur in the thicker areas of the injection molded object, usually in the innermost sections of the mold. These sink holes are frequently caused when the type of cooling mechanism used or the time allocated for cooling of the molded object is insufficient. The solution to this issue is to let the molded object cool for longer before removing it from the mold.

Vacuum voids

Vacuum voids (sounds like some sort of awesome 90s grunge band..) are pockets of air which are trapped near to or in the surface of an injection molded prototype. They usually form due to uneven solidification between the inner sections and the surface of the molded prototype. If the pressure is too high in the mold they can become even worse as air from the injection molding process can become trapped in the mold creating a vacuum.

Surface delamination

Surface delamination occurs when thin layers appear on the surface of the injection molding prototype. This is usually due to contaminants existing in the mold before injecting plastic or another material into it. Surface delamination while being annoying, is less harmful than the other issues mentioned above, as extra surface layers can easily be peeled off.

In this short article we have just scratched the surface as to the challenges you might face when doing your own injection molding. Knowing what we do, there’s no way we’d leave the manufacturing process up to chance. Outsource it instead to companies and factories that have the skills, resources and know-how to manufacture your product right the first time!


Jason Vander Griendt

Jason Vander Griendt is a Mechanical Engineering Technician with years of experience working at major companies such as SNC Lavalin Inc, Hatch Ltd. Siemens and Gerdau Ameristeel. He is the CEO of JCAD ‚Äď Inc., a company he started in 2006 after seeing a gap in the market for businesses who could assist clients through the entire product design and manufacturing process.

Jason has been featured in Forbes, has had his businesses analyzed and discussed in multiple start-up books, was a previous winner of the Notable8 Digital Innovator of the year award, and is a regular guest on business panels and podcasts. Email Jason at or follow him on LinkedIn.

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