Two of the major disadvantages to injection molding are the high mold costs and large required lead times. Mold 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 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 mold correct prior to mass production requires both time and money. It is rare that you would prototype an injection molding mold. It does happen though, especially for parts that will be made in a multi-cavity mold. For example, let’s say we were going to injection mold a new shampoo bottle cap. That cap would likely have threads to attach it to the bottle, a living hinge, a snap closure, and potentially some overmolding too. A company may choose to make a single cavity mold of that part to make sure all of the features will mold as desired. Upon approval, they will make a new mold, that is capable of molding, for example, 16 caps at a time. They do the single cavity mold first so if there are any issues, they don’t have to pay and wait for it to be fixed 16 times for each cavity.
Because molds 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 mold cavity larger by cutting away steel or aluminum. But if you are trying to take away plastic you need to decrease the size of the mold 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.
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 (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.
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 molds themselves. For 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 avoid and can often add costs to the project.
One of the few disadvantages for injection moulding is that there may be a high initial tooling and machinery cost. If you want to produce parts at a low volume then small runs of parts can be less cost effective.
It’s important to note that there are a few design restrictions for plastic injection moulding, however by working alongside our experienced design team you should be able to overcome initial issues.
Disadvantages of injection moulding
1) High tooling costs and long set up lead times. Up-front costs are high due to the design, testing, and tooling required. There is the initial design and prototyping (probably via CNC or 3D printing), then the design of a prototype mould tool to produce replicas of the part in volume. Lastly, and only after extensive testing during both stages, you can finally injection mould a part.
2) Part design restrictions. Plastic parts must be designed with injection moulding consideration and must follow the basic rules of injection moulding, for example:
Avoid under cuts and sharp edges as much as possible
Use uniform wall thicknesses to prevent inconsistencies in the cooling process resulting in defects like sink marks.
Draft angles are encouraged for better de-moulding.
Don’t forget, because tools are typically made from steel or aluminium, it can be difficult to make design changes. If you need to add plastic to the part, you can make the tool cavity larger by cutting away steel or aluminium. But in order to take away plastic, you need to decrease the size of the tool cavity by adding aluminium or metal to it. This is extremely difficult and in many cases might mean scrapping the tool (or part of it) and starting over.
Also, the weight and size of the part will determine the tool size and necessary press size. The larger the part, the more difficult and expensive it will be.
3) Small runs of parts can be costly. Due to the complexity of tooling, and the necessity to rid the machine of all previous material before the next product can be made, the setup time can be quite lengthy. Therefore small runs of parts have traditionally always been thought of as too expensive to injection mould.
One of the few disadvantages for injection moulding is that there may be a high initial tooling and machinery cost.
If you want to produce parts at a low volume then small runs of parts can be less cost effective.
It’s important to note that there are a few design restrictions for plastic injection moulding, however by working alongside the experienced design team you should be able to overcome initial issues.
The process of injection moulding means there are some restriction with regards to part design. You might need to make a few changes to your parts so they’re able to be produced for you or you may have to decide on a different manufacturing technique.