Two of the major disadvantages to Injection plastic mold are the high tooling costs and large required lead times. 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. 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 tool of that part to make sure all of the features will mold as desired. Upon approval, they will make a new tool, that is capable of molding, for example, 16 caps at a time. They do the single cavity tool 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 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. But 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.
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.
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 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.
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