Plastic injection molding is a technique of manufacturing where a fixed frame known as mold or matrix (also named as tool) is used for shaping liquefied by heat polymers or elastomers. The contemporary injection molding processes constitute plastic injection molding, insert molding, 2K molding, metal injection molding and over-molding. Injection molding occurs when plastic materials molten by heat are injected into the mold cavities, cooled, and solidified to attain molded products. It is an effective and most appropriate technique for large-scale manufacturing with intricate shapes and variety of materials.
What Is A Mold (A Tool)?
A mold appears like a metal box that is hollow from the inside (has a cavity), in which the molten plastic is injected with high pressure to take the desired shape of the plastic part produced. The cavity is a replica of the molded part. Mold is placed inside the molding machine which controls the injection stages: clamping, injecting, cooling, ejecting.
Injection molding tools also can be standard (classified by Plastics Industry Association (SPI) standards) and non-standard (e.g. micro molds used with Babyplast machines). The main difference between the two is the size of the molds and the complexity. Standard molds have many more components and thus are more expensive and difficult to make, however, in general, the mold (tool) mainly consists of:
Mold Cavity And Core Sides - Also known as injection side – plate A and ejector side plate B. Cavity and core sides are the negative replicas of the molded components. Cavity and core shape the chamber were the plastic is injected. It is important to distinguish them by cavity being the fixed side and core moving side, in other words – cavity is shaping the outer part and the core – inner of the part.
Heat Control System - Holes are drilled up in the block so that the temperature could be controlled with the help of circulating oil or water inside them. This cooling system helps to preheat the mold during the injection to prevent polymer clogging and cool down the mold to shorten the cycle of the molding.
Polymer Flow (channel) System:
The sprue - is the spot where plastic is injected through the nozzle.
The runners (channels) – are the the channels where molten plastic flows.
The gates – the entrance of the molten plastic to the empty chamber which is shaped by the cavity and the core inside the mold.
Mold venting channels – are necessary for qualitative parts not to form air pockets or cause material burning due to high temperature and pressure.
Cold slug wells – are the corners inside the runners to catch the cooled down plastic – the slug.
Demolding System - Ejector plate or individual ejectors push solidified part out of the mold (demolding happens) which falls straight into the packing box or for futher processing - sprue cutting, quality inspection, sterilization, etc.
Types of molds
Even though all molds has the same basic structure and are very similar they can also be divided in several groups because of some differences and ways of use:
3 plate molds – are the tools that has additional plate between cavity and core plates. This allows multiple injection gates for better flow and more flexibility of gate location.
Cold runner molds – as the name suggests these molds does not use hot runner nozzles and the plastic is injected through runners (channels) and gates.
Hot runner molds – are the direct injection molds, where every cavity has its own nozzle and the plastic is injected directly to the cavity.
Family molds are the ones which has multiple similar parts’ cavities located in a single mold.
High cavitation molds – are the molds which has high quantity of cavities and are used of high volume production.
Read more about types of molds here.
How Is Injection Molding Tool Made?
The main machining process for tooling is subtractive type of machining which are CNC machining, electrical discharge machining or even laser ablatios or selective laser etching for micron level precision machining. However, for inserts and in some cases additive manufacturing like 3D printing or electroforming also can be used. Also the technologies can be combined to achieved required result. It is important to consider the main parameters like size, shape, raw materials, product quantity, shrinkage of the plastic product, surface finishes, and cost restrictions before mold making. The process of mold making can be divided in few stages.
1. Design – CAD modeling
Input information of part drawing and specifications of material, molding machine specifications, and other tool specifications such as type of mold, runner system, gate, use of robotics, and estimated cycle time are necessary when designing the mold. Mold designers must be experienced enough to take all these considerations and mold-making capability to produce the designed mold.
Routine procedures can be automated, allowing conventional calculations on mold dimensions to be completed faster and with fewer errors and, at the same time, reducing modeling time. Mold-design software aims to free up the user's time to focus on the more challenging areas of mold planning while automating or easing typical or straightforward activities, which ultimately reduces modeling time, improves tool quality and efficiency, and lowers production costs.
All things considered, a typical mold-design CAD package today includes programs or modules for generating core and cavity from a part model, which helps optimize parting surfaces, select a mold base, and add shutoffs, cooling lines, runner systems, gates, slides, lifters, ejectors, columns, spacers, guides, nozzles, screws, and pins.
2. Mold Simulation
For achieving efficiency and decrease resetting time during mold testing, the simulation must be run while utilizing data from the injection molding machine's present state. In addition, the time it takes to design and manufacture a mold also determines the time for a product to reach the market.
Fundamentally, continuous data input of machines for the mold-making process aids mold designers in gathering up-to-date information on machine conditions and adjusting design accordingly with the functionality and availability of machining machines to avoid production delays caused by a tool or machine failure during the mold making.
The final mold design will serve as the mold's final model, virtually installed in an injection molding machine for future production planning and process simulation in real-time. The detailed drawing of the mold will be saved in the database once the mold design is completed, and mold making production facility will leverage the drawings for the mold-making process. Mold making primarily entails part machining, assembly, and testing.
3. Prototype Molds
Then comes the stage of creating prototype molds, typically used to make small batches of plastic injection parts, ranging from 200 to several thousand. A standard interchangeable metal mold base, customized aluminum or soft steel alloy core, and chamber inserts make up this type of mold. 3D printing or CNC machining can be used to create prototype molds.
3D-Printed Injection Molds
Previously, 3D printing was mainly utilized in the design and production process to build and test prototypes that would be injection molded later. However, 3D printers can now directly make molds, thanks to printer accuracy, surface polish, and materials advancements that can withstand high temperatures and forces.
Even though, as the name itself suggests, these molds has limits with only sma
ll parts (usually up to 20cc in volume) for small parts it is truly great way to prototype and even enter mid-range production. The simplicity of micro molds structure and the size of the mold and molding machine allows much faster and cheaper tooling process which becomes acceptable for low-volume production and prototyping.
Plastic Injection Mold (tool) Costs
While injection molding might give off an impression of being more costly than methods like 3D printing and CNC machining, its ability to scale and manufacture thousands of pieces makes it a cost-effective mass production alternative. There are several factors to which contribute to mold (tool) making cost:
Fixed time to start (setting up the CNC machine)
Raw material cost (steel or aluminum)
Hourly machine and operators rate
Machine costs per hour (depreciation or/and leasing)
CAD and CAM
Fixed time per cavity machined (empirical estimation)
Difficulty level (undercuts, threads, precision)
The CAD design is a critical factor of molding cost, and it indicates that the more complicated the part's geometry is, the greater the production costs will be. The most cost-effective parts will be those with no undercuts or less sophisticated surface finishes. Undercuts can make part ejection more difficult and glossy surface will require polishing. Although, many plastics are similar in strength and performance, some are intrinsically simpler to mold, eventually lowering part prices.
Read an extensive in-depth explanation about injection molding costs here.
How To Reduce Injection Molding Costs?
Besides the necessary steps included in the whole process, a few features can significantly increase the plastic mold cost. Here are a few things that must be avoided:
If possible, avoid the use of undercuts.
Remove any features that are not necessary.
Employ a core-cavity strategy.
Minimise the number of cosmetic finishes and appearances.
Create self-assembly components.
Reuse and modify molds.
Pay close attention to the DFM (design for manufacturing) evaluation.
Use a family or multi-cavity mold.
Select the option of on-demand production.
Experiment with overmolding