What Is The Downside To Injection Molding?

Because of the massive manufacturing scale usually involved in injection molding, small mistakes can have massive consequences - financially and logistically. While the margin of error is small, the implications are substantial.

Small Design Errors Have BIG Costs with Injection Molding

Upfront costs tend to be very high due to design, testing, and tooling requirements. If you are going to produce parts in high volumes you want to make sure you get the design right the first time. That is more complicated than you might think.

Getting the design right includes:

Designing and then prototyping the part itself to specification, usually on a 3D printer and prototyped in a different material

Designing an injection mold tool for an initial production round, to generate 300-1,000 prototypes

Refining every detail in the injection molded prototypes before mass production

If you have the right veteran voices at the table, this is still a fantastic option for the right projects.

Be Ready for High Tooling Costs for Injection Molding

Tooling is almost a project in and of itself and only one phase of the entire injection molding process.

Before you can produce an injection molded part you first have to design and prototype a part (probably via CNC or 3D printing).

Then you have to design and prototype a mold tool that can produce replicas of the part in volume.

Lastly, and typically after extensive testing in both of the aforementioned stages, you get to injection mold a part.

As you can imagine, all of the iteration required to get the tool correct prior to mass production requires both time and money. It is rare that you would prototype an injection molding tool. It does happen though, especially for parts that will be made in a multi-cavity tool.

When You Need to Adjust Your Injection Molding

Because tools are typically made out of steel (a very hard material) or aluminum it can be difficult to make changes. If you want to add plastic to the part you can always make the tool cavity larger by cutting away steel or aluminum.

If you are trying to take away plastic you need to decrease the size of the tool cavity by adding aluminum or metal to it. This is extremely difficult and in many cases might mean needing to scrap the tool (or part of the tool) entirely and start over. In other cases you might be able to weld metal into the cavity that is undesired.

Uniform Wall Thickness is Critical

Injection molding necessitates uniform wall thickness. If you were to cut a cross-section of the Panasonic mold above you would notice that the wall thickness is approximately 2-3mm thick throughout. Keeping walls from being too thick is important to prevent inconsistencies in the cooling process resulting in defects like sink marks.

A good rule of thumb is to keep walls less than or equal to 4mm thick. The thicker the walls the more material you will use, the longer the cycle time will be and the higher your cost per part will be.

Conversely, if wall thickness is any thinner than 1mm or so you might experience trouble filling the mold tool (resulting in gaps or short shots). Designers can compensate for this potentiality by using a material with a higher melt flow index like Nylon which is often suitable for walls as thin as 0.5mm.

Different manufacturing techniques like CNC don’t require uniform wall thickness at all.

Restrictions for Larger Injection Molds

Oftentimes large parts cannot be produced via injection molding as a single piece. This is due to the size limitations of injection mold machines and the mold tools themselves.

For an example of a large injection molded part, consider the shopping carts at Target. Although the machinery exists to mold very large pieces (e.g. 1000 ton presses roughly the size of a train’s caboose), using it is very expensive.

For this reason, objects that are larger than a typical injection molding machine’s capability are most often created in multiple pieces.

CNC machines have similar limitations regarding product size while 3D printing has even more limitations. CNC is limited to the travel and size of the bed in the milling machine while large 3D printed parts often need to be printed in multiple pieces and then bonded together.

Large undercuts require experienced design to be avoided and can often add costs to the project.