Two of the main disadvantages of injection molding are the high cost of tools and the long lead times required. The tooling is almost a project in itself and only one phase of the whole injection molding process. Before you can produce an injection molded part, you must first design and prototype a part (probably using CNC, additive manufacturing, or 3D printing), then you must design and prototype a mold tool that can produce replicas bulk part. Finally, and usually after extensive testing in the above two steps, you can injection mold apart. As you can imagine, all the iteration required to get the right tool before mass production takes time and money.
It is rare for prototypes of an injection molding tool. However, this happens, especially for parts to be made on a multi-cavity tool. For example, suppose we are going to injection mold a new shampoo bottle cap. This cap is likely to have threads for fixing it to the bottle, an active hinge, a snap closure and possibly also an excess of molding. Qmolding company can choose to make a single fingerprint tool from this part to ensure that all functions are molded as desired. Once approved, they will manufacture a new tool, capable of molding, for example, 16 plugs at a time. They first make the single cavity tool, so if there is a problem, they don’t have to pay and wait for it to be repaired 16 times for each cavity.
Because the tools are usually made 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 enlarge the cavity of the tool by cutting steel or aluminum. But if you are trying to remove the plastic, you need to reduce the size of the tool cavity by adding aluminum or metal. This is extremely difficult and in many cases it may be necessary to completely throw the tool (or part of the tool) out and start again. In other cases, you could weld metal into the unwanted cavity.
Injection molding requires a uniform wall thickness. If you cut a cross-section of the above Panasonic mold, you will notice that the wall thickness is around 2-3mm. It is important to prevent the walls from being too thick to avoid inconsistencies in the cooling process which leads to defects such as crankcase marks. A good rule of thumb is to keep the walls less than or equal to 4 mm thick.
The thicker the walls, the more materials you will use, the longer the cycle time, and the higher the cost per room. Conversely, if the wall thickness is less than 1mm or less, you may experience problems filling the molding tool (resulting in gaps or short strokes). Designers can offset this potential by using a material with a higher melt flow index such as nylon, which is often suitable for walls as thin as 0.5mm. Different manufacturing techniques like CNC do not require a uniform wall thickness.
Often, large parts cannot be produced by one-piece injection molding. This is due to the size limitations of the injection molding services machines and the molding tools themselves. For example, for a large injection molded part, consider shopping baskets at Target. Although there are machines for molding very large parts (for example, presses about 1,000 tones the size of a train cabin), their use is very expensive. For this reason, objects that are larger than the typical capacity of an injection molding machine are most often created in several parts. CNC machines have similar limitations when it comes to product size, while 3D printing has even more limitations. The CNC is limited to the displacements and the size of the bed on the crusher, while the parts printed in 3D must often be printed on several parts, then assembled.
Large undercuts require an experienced design to avoid and can often add costs to the project.
Some of the other reasons why injection molding is well suited to this example include the fact that the drill is produced in high volumes. In other words, Panasonic creates a large number of copies of the same drill handle. Injection molding is wonderful for this type of high volume production because the high initial costs pay the manufacturer over time with low unit costs. For the same reason, injection molding may be a poor choice for low volume production. In addition to the note, there are certain design restrictions if injection molding is used.