Injection molding technology is one of the most important points on the way of a product to the electronics market.
Injection molding is a relatively old technology and has been in use since the late 1800s. Injection molding machines have a huge screw (auger) that directs the molten plastic into the mold under high pressure. This screw drive method was invented in 1946 and is still in use today.
Injection molding machines are, of course, not the same as modern high-tech 3D printing machines. There is nothing innovative about them, but the use of injection molding is a prerequisite for the creation of most new “iron” products.
The injection mold consists of two halves (matrix and punch), which, when closed, form a cavity in the shape of the desired part. Hot liquid plastic is poured into it under high pressure.
High pressure is required for the viscous plastic to fill every corner in the mold cavity.
When the plastic cools down, the two halves of the mold are moved apart, and the finished body part is removed from them.
Developing the design and construction of a case for mass production is a rather difficult task, and the cost of the molds themselves is estimated at tens of thousands of dollars. At the same time, injection molding remains one of the most demanded technologies, because it alone allows the production of millions of identical parts at an incredibly low cost per piece.
Rigging is expensive. And most devices require multiple molds to manufacture, so the total cost can be significant. And the more parts are required to be produced using a particular mold, the more expensive it will be.
This is due to the fact that a “long-lived” mold has to work in incredibly tough conditions. Over and over again, it is exposed to high temperatures and pressures.
These two destructive forces work to wear out the mold until, at some point, the first casting defects appear.
Hard metals are used to create durable injection molds. The hardness of the metal depends on how many castings need to be made using that particular mold. The tooling for the manufacture of 10 thousand parts can be made from a softer metal compared to the one that is designed for 1 million parts.
For example, for the production of small series (up to 10 thousand pieces), aluminum is widely used. For larger production volumes, they switch to a harder metal such as steel.
However, the harder the metal, the more difficult it is to make the mold itself, and the higher its cost. In addition, it will take much longer to get the steel tooling. This is due to the fact that injection molds are created by milling, i.e. a harder mold will require an even harder milling tool.
Design for production (Design for manufcturing, DFM)
The high cost of molds is just one of the disadvantages of injection molding. The second drawback is the complexity and limitations at the design and construction stage of plastic parts.
With a perfect working prototype made on a 3D printer, it takes significantly more time and money to adapt it for injection molding.
The limitations of mass production should be taken into account already at the first stages of development. Some requirements for the shape of the castings, such as casting slopes, can be deferred until at least a second prototype is created.
Casting bias
The main challenge in working with injection molded parts is to properly remove them from the mold. As soon as the plastic has cooled, the two halves of the mold are opened and we have a new molded plastic part.
Any 3D injection molding design must include injection molding or process bias to fill the mold and retrieve the finished product smoothly. Casting bias is essentially a small slope that is added to any vertical surfaces that align with the direction of removal of the product from the mold. In most cases, 1-2 degrees is sufficient.
Ejector pins
Ejector pins or pushers are used to remove plastic parts from the mold. As the name suggests, these are small cylindrical pins that push the part out of the mold.
Pushers do not have a standard position, so you have to think about where they will be located. Ideally, they should be located in the strongest part of the casting to prevent deformation when removed from the mold.
It should be borne in mind that ejector pins usually leave small marks on the product. If you look closely at most of the plastic parts, you can see these tiny round marks that appear as the mold is pushed out.
This should be taken into account when developing a product. Try to make sure that the pushers touch the casting in areas that are not critical to the appearance of the product. You might even try to hide the pusher marks under a label or logo.
Double pushing stroke
Some plastic parts cannot be removed from a simple two-part mold in one go, in such cases, inclined pushers and a double ejection mechanism are used.
The tilting pusher is an integral part of the mold that is inserted before casting and then removed before the main parts of the mold are exposed. The tilting pusher moves perpendicular to the direction of travel of the two mold halves.
Every effort should be made not to use the double ejection mechanism as it greatly increases the complexity and cost of the mold.
One of the main tricks that allows you to avoid double ejection is to avoid using undercuts. An undercut is a protrusion or depression on the surface of the casting that prevents the product from being pushed out of the mold in one pushing stroke.
The situation with undercuts can often be corrected by adding a groove (notch) under the ledge and using a single push instead of a double push.
Uniform wall thickness
One of the important features of injection molding, which has a huge impact on device design, is the requirement for uniform wall thickness of the casting. It is due to the fact that the plastic poured into the mold must cool at the same rate over the entire surface of the part. Uneven cooling may cause the part to deform.
Therefore, ribs are used instead of thicker sections in the design of the injection molding body. Correctly designing a part with uniform wall thickness definitely takes experience.
Using double push strokes and uneven casting wall thicknesses are two of the most common mistakes made by 3D designers who are unfamiliar with the technical limitations of injection molding.
Radius / rounding of corners
Perfect corners and edges of parts are impractical for injection molding. The molten polymer will not be able to fill the entire sharp-edged mold evenly and completely, even under high pressure conditions. At least one shouldn’t hope for it with large production volumes.
Cold ducts versus hot ducts
Cold runner / hot runner plastics are variants of the gating system that directs molten polymer into cavities.
The wide gating channel allows the polymer to flow freely at lower pressures. However, wide channels take longer to cool the plastic and create more waste, both of which affect the cost of the part.
On the other hand, a narrow gating channel shortens cooling time and reduces material overruns, and ultimately minimizes casting costs. However, it has a disadvantage: the narrow channel requires a higher pressure to push the molten polymer into the mold.
There is a solution that allows the use of narrow channels at low pressure – the hot runner system.
Heating elements are installed directly into the mold along the channels, which maintain the polymer in a more liquid state, thanks to which the plastic fills the mold at a lower pressure.
Unfortunately, you have to pay for everything, and hot ducts also have their disadvantages: additional complexity in the manufacture of tooling, which always translates into additional costs.
In most cases, at least initially, it is better to use ducts without heating elements, i.e. cold channel gating system. It is always worth starting with the simplest and cheapest solution.
Form parting line
If you carefully examine any plastic part, you will see the so-called parting line. It will be located at the junction of the two parts of the mold.
This mating point of the two mold halves is never perfect, there is always a little polymer flowing along the contour. As the mold ages and wears out, this leakage becomes more noticeable.
It is very important to choose the optimal location for the parting line. Ideally, it should be placed on an invisible part of the device.
Single and Multiple Molds
At a certain stage of production, it becomes possible to reduce the casting time due to multi-cavity molds (they are also called multi-cavity). They are used to increase the production speed and reduce the cost of workpieces.
Multi-cavity molds, as the name suggests, allow you to create multiple copies of the same part with a single injection of resin. Just do not use these molds at the start, until the process is debugged and the ideal castings from single molds have not yet been created. It is advisable to produce at least several thousand units of products before switching to multi-seat forms.
Typically, entrepreneurs on a tight budget make the most of their single-seat molds, unless the manufacturer funds their molds themselves.
Family molds
In most cases, a separate mold is used for each individual plastic part in the device. For the case, you will need at least two parts: an upper and a lower one.
But most products will require more than two plastic parts. Molds are very expensive and buying multiple molds at once is a major financial hurdle, so you should aim for the minimum amount of plastic parts.
An alternative way to minimize the required molds is to use a special type of multi-seat molds, the so-called family molds. The family mold allows you to combine several different parts in one casting.
While the typical multi-seat (multi-pocket) shape creates multiple copies of the same part, the family shape creates different parts.
Sounds good, right? Unfortunately, not everything is so simple, you have to pay for everything. The main problem with family forms is that each piece must be roughly the same size.
