Injection molding is a process by which plastic pellets are melted using heat and shear in an extruder and injected, at high pressure and flow, into an injection mold to form the part. It is one of the most productive and cost-effective means of producing a high-quality product.
While this process seems straightforward, it is full of engineering challenges and complexities. An injection molding work cell can consist contain 4 to 16 separate elements (dryer, hopper loader, machine, hot runner, mold, robot, conveyer, etc.).
The mold is the heart of the system. Due to the importance of the mold design, it is critical to understand the basics of good mold design, and the techniques used to create a mold that will allow it to meet and exceed its intended purpose. It would lead to better quality, higher productivity, simpler molds, and savings in the cost of the molded products.
The mold designer must specify the number of cavities that will be required for the expected output, but also the selection of mold materials and the degree of sophistication of the mold.
What Is an Injection Mold?
An injection mold is a permanent tool, i.e., a tool that, if properly designed, constructed, and maintained, will have a life expectancy (useful life) well beyond the time when the product itself becomes obsolete. A mold can be used to make products in a virtually infinite variety of shapes made from injectable plastics.
A basic mold consists of two mold halves, with at least one cavity in one mold half, and a matching core in the other mold half. These two halves meet at a parting plane (parting line). Once the injected plastic is sufficiently cooled, the mold opens, and the product can be removed by hand or be automatically ejected.
Because injection molding machines are mostly built with the injection on the stationary platen side, there is typically no built-in ejection mechanism on this side. If ejection from the injection side should be required – which is always the case in stack molds, and occasionally so in single-level molds – any required mechanism must be added to the mold, and occasionally to the machine; in either case, this adds complexity and increases costs. Only molds designed for using only air ejection do not require any external ejection mechanism.
Most products are removed (ejected) from the core. There are also many molds that need special provisions to allow the products to be removed from either the cavity or the core. This is the case for products with severe undercuts or recesses on the inside and/or the outside of the product, such as screw threads, holes, ribs or openings in the sides of the product, etc., or molds for insert molding.
Some of these design features of the product may require moving side cores, which are either inserts or whole sections of the cavity that move at an angle that is 90° to the “natural opening path” of the mold. Others may require special unscrewing mechanisms, either in the core or in the cavity side. The mold may require split cavities (or “splits”), i. e., the cavity consists of two or more sections, which are mechanically or hydraulically moved in and out of position, and then clamped together during injection. In some cases, the mold may equire collapsible cores or retractable inserts, which are all quite complicated (and expensive) methods.
Any of the above special features can add considerably to the mold cost when compared to a simple “up and down” mold where the products can be readily ejected with the machine ejectors during the mold opening stroke or after the mold is open, without the need for any of these complicated mold features.
Elements of an Injection Mold
Every injection mold consists of the following basic elements:
1. One or more matching cavities and cores, defining the cavity space(s) (there are molds with anywhere between one and 256 cavities).
2. A method, or element, to duct the (hot) plastic from the machine nozzle to the cavity spaces. There is a choice between:
Cold runners (two-plate or three-plate systems)
Hot runners (various systems)
Sprue gating (cold or hot).
3. Provision to evacuate air from the mold (venting). There is a choice between:
4. Provision to cool the injected hot plastic sufficiently to allow ejection of the molded product.
5. Provision to eject the molded product. There is a choice between:
Manual product removal
Ejector pins and sleeves
Strippers (stripper rings or bars)
Free-drop ejection onto a conveyer
Various methods for in-mold product removal
Robotic product removal.
6. Provision to attach (interface) the mold to the molding machine. There are several methods to consider:
The mold is for one machine only. In this case, the mold may be mounted with bolts to the platen
The mold is to be used on several different machines. In this case, clamps and clamp slots on the mold may be used to bolt the mold to the platen
Quick mold-change methods (various designs). This could involve magnetic mounting.
7. Method of alignments of cavities and cores. There are several methods to consider:
No alignment feature provided in the mold. Also called flat parting line
Leader pins and bushings
Leader pins and bushings between individual cavities and cores
Taper fits between individual cavities and cores
Taper fits between plates. These are also called side locks
Any combination of the above.
8. Any number of (mold) plates to provide the necessary means for carrying and providing rigid backup for the above elements.
In addition to the above parts, molds can have additional features. Each of these features can add (often considerable) costs to the mold, but in many cases, they increase the productivity of the mold and reduce the cost of the product.
Defining what is really required considering the shape and complexity of the product and the required production quantities will enhance mold productivity.
The below figure shows a schematic of a basic injection mold with the key elements of the mold.
Classification of Molds
SPI has developed a set of standards to classify molds by their design and intended usage.
|Built for extremely high production. This is the highest-priced mold and is made with only the highest-quality materials.
|Pre-hardened 28Rc steel
Stainless steel plates
|All hardened > 48Rc steel
|Guided ejection Wear plates on slides
|Medium to high production mold, good for abrasive materials and/or parts requiring close tolerances.
|Some guided components
Some corrosion protection
|Medium production mold. This is a very popular mold for low to medium production needs.
|> 28Rc steel
|Low production mold. Used only for limited production preferably with non-abrasive materials.
|Mild steel or Al
|Mild steel or Al
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