What you should know about injection molding.
Things You Need To Know About Injection Molding.
What is injection moulding?
Injection moulding is a process for producing parts in large volumes. This process is employed when the same part needs to be made thousands or even millions of times. The process is very repeatable and consistent. Injection moulding can use many different types of material. The main advantage of injection moulding is that it can produce consistent and repeatable parts on a large scale. While the initial setup cost can be extended the unit price per piece is relatively low. It is also essential to understand as the order quantity increases the unit price cost decreases. This is mainly due to the injection moulder being able to buy larger quantities of material and being able to get a discount for large amounts. The transportation costs involved in transporting materials can be quite expensive as it is heavy and bulky. Also, there is a production benefit in ordering larger batch sizes in that the molding machine can run for a longer time, which reduces set up and downtime for the molder.
Other advantages of injection moulding :
Injection moulding generally produces low levels of scrap, unlike other processes such as CNC machining that use a subtractive manufacturing method. With injection molding, there is usually scrap from the sprue and runner system if we are using a cold runner mold however, this scrap can be eliminated by using a hot runner system within the mold. The 3D printing process uses an additive manufacturing approach which produces less waste, however, the time it takes to make one part is significantly longer than an injection moulding cycle and is not suitable for mass production. That’s said, scrapped levels from injection moulding can build up very quickly due to the quick automatic cycling of the machine, so if the part quality is not acceptable, the machine will produce a lot of scrap parts. This is why quality control and efficient manufacturing procedures are so necessary for injection moulding.
Parts of an injection mold.
The plastic is injected from the barrel of the injection moulding machine through a nozzle. The nozzle is forced against the mould where the plastic is injected into the mould, through what is known as a sprue bush. The sprue bush is typically a circular metal part that is inserted in the mold. The sprure bushing has a hole in the middle, which is usually a little bit smaller than the injection moulding machines nozzle opening. The sprure bush usually has a tapered angle of five degrees or so and connects to the runner system within the mould. The runner system within the mold is generally circular in shape; however, they can also be trapezoidal or semi-circular however the fully circular runners are the most efficient for plastic flow. The runner system enables the material to feed different parts within the mould.
Types of runner systems.
There are two types of runner system, a cold runner and a hot runner system. The hot runner system uses electric heater bands around the enclosed channels so the material within the system can be heated up. Within hot-runner manifold and drops, there must be good temperature control in the hot runner system. For a hot-runner system, it is essential that the temperature of the material must is maintained at a consistent, controlled temperature. A hot runner mould is generally more expensive than a cold runner mould, however over when producing a large number of parts it usually make enconimal since to investing in a hot runner for your mold.
Gating for your mold.
The runner can divide into sub runners depending on the number of parts within the mould. At the end of the runner system, the gates connect the runner to the part. The part has a small opening, usually a millimetre or so in diameter, which allows the molten material to flow into the cavity. The injection cycle usually only takes one or two seconds, then there is a holding phase which is when the plastic is pushed forward under pressure to pack out the mould and reduce shrinkage. All material change volume when they cooled down and plastics are no different. The amount plastic shrinks depending on the type of plastic. The shrinkage rate for most polymers is somewhere between 0.5 and 3% typically. The part and runner in the mold freezeout very quickly, usually within 30 seconds so that the mould can be opened and parts ejected from the mould. A typical moulding cycle may only last 5 seconds for high-speed cycling molds or up to a minute for thicker larger parts.
For plastic injection molding there are two types of material, a thermoset material and a thermoplastic material. The thermoset material like epoxy resin or liquid silicone cures or sets once the material is exposed to elevated temperatures or oxygen. The material crosses links, that is the molecules within the material will cross-link to each other. They thus creating a very strong intermolecular network and where the application of heat to try and re-melt the material will just result in the material burning and degrading.
The thermoplastic material, like polypropylene, polyethene, polycarbonate, nylon can be re-melted over and over again. |However, the re-processing the same polymer will result in a reduction in the molecular strength, shortening of molecular chains and an overall reduction of mechanical strength of the material. Thermoplastic materials can be recycled and reused this can happen beside the moulding press, where runners and scrap parts can be reground and recycled directly back into the process right beside the press. In other situations, the runners and scrap material will be reground off the factory floor or off-site. This reground material can then be reused in a controlled way on the factory floor or can be resold to a secondary operation that may require a material that does not require such high mechanical properties.
Repeatability of injection molding .
Over the years injection moulding machines have got more reliable and repeatable with the improvement in hydraulic, electric systems and control systems. The newer electrical machines these days have very accurate and repeatable injection strokes that can inject plastic into the mold at a consistent speed and pressure shot after shot. This is very important, especially when producing large volumes in a multi-cavity tool and where dimensional and quality requirements are high.
Major drawbacks for injection molding .
One of the major drawbacks for injection molding is the large initial upfront costs involved in injection moulding. The cost of having an injection mould designed and made can cost anything from 10,000 euro’s for a very simple tool up to 100,000 for a more complex tool. Likewise, the equipment costs for an injection moulding machine can be anything from 10,000 for an older second-hand moulding machine up to 300,000 for a new large moulding machine. So the initial upfront costs are quite large even if you’re just getting a mould made and having a contract moulder to mould your parts. I provide advice and consultation on sourcing suitable mold makers and molders, if you require any assistance you can get in contact.
A second major drawback to injection moulding is the time it takes to go from initial design concept into production. There is quite a lot of engineering work and design work that needs to be done before the tool can be made. Typically a part design may take anything from four weeks to 12 weeks, then tool design can take up to two weeks, with a DFM, design for manufacturing study done at this stage. Then once the tool has been designed it can take a further 8 weeks for the tool to be built. These would be fairly typical timelines however it really depends on the part and how much experience the product designer and toolmaker have with this kind of design.
For a new product design a series of iterations of design prototyping would usually be done. Where different designs and iterations would be prototyped until a final decision was made on the best design. An industrial designer would then review their design with a toolmaker to come up with their tool design, also a series of simulations in maybe ran at this stage to ensure troublefree moulding.
A common method of new mould building.
For a new part design, a common approach would be to make pre-production tool, which is a low cavitation soft tool that can make a few thousand parts. Any issues or problems can be reviewed and the next mould that is made can incorporate with the improvements. If the client wants to go to a larger production volume they can use the single cavity tool design and incorporate that design into a multi-cavity tool. This means the issues or problems found within the pre-production tool can be fixed before large expensive multi-cavity tools are built, most companies would take this approach where they would start with a small simple tool, for example, a single double or four cavity tool design and then just build on that design for a larger multi-cavity tool. A lot of time and effort goes into designing and testing injection moulding tools due to the cost involved and the large volumes of parts that are generally required. Also the quality control and requirements for injection moulding parts are becoming more stringent with medical and automotive quality standards.
Because tools are generally made from a hard tool steel P20 or H13, it can be difficult to make changes to the tool even for prototype tools made from aluminium small changes can take time and cost quite a bit of money. If you are trying to add plastic to the part this is a metal off modification which in most cases is reasonably straight forward. However, if you are trying to take plastic away from the part this will be “a metal” on modification, at best which would typically mean the mould would have to be welded and re-machined. In worst cases, the mould would have to be scrapped and a new mould made. This is why the design process is so critical and can take so long to do. Ensuring the tool is made right the first time is a skill, as any modifications can be expensive and time-consuming.
Plastic part design considerations.
Plastic part design is very important so as to ensure that the parts be ejected from the injection mould easily without warping or deform after the moulding process. There are many design guidelines for injection moulded parts. One of the major design guidelines is that the part should have a uniformed wall thickness. For most thermoplastic materials the wall thickness recommendation is between one and three millimetres depending on the material. By keeping a uniform wall thickness the volume of material and the cooling time can be reduced, and in so doing reducing the unit part cost. By increasing the wall thickness the part may be stronger however it will require more material and it will take longer to cool in the mould. There is also the risk that the part will deform and shrink causing sinks in the part.
Conversely, if the part wall thickness is under 1 millimetre it will be more difficult to inject the plastic into the mould cavity and will result in higher pressures being used the fill the cavity. Higher cavity pressures can deformed the part due to higher moulded in stresses. The type and grade of material being used will to a large extent determine what the minimum wall thickness can be. As a material with a high melt flow index like nylon can be used to fill thin-walled parts as thin as 0.5 millimetres. If it is not possible to maintain a uniformed wall thickness throughout the part it is recommended that there be a gradual transition between thick and thin wall sections. Other processors like CNC do not require a minimum wall thickness in part design.
Plastic part size for injection molding.
Another limitation of injection moulding is the size of parts that can be produced. most injection molding factories would have mid-size range machines from 30 ton to 350 tonnes. Larger tonnage machines up to 550 tons are also quite common however machines larger than this up to 3000 tonnes are quite rare as they are expensive. This is why most injection moulded parts are designed for small to mid-size molding machines and why larger parts are usually split up into smaller sections or alternative processes are used like rotational moulding or composite inlay moulding. However, this is not a unique problem for injection moulding order processes like CNC are also limited by the bed size of the machine as well as 3D printing which is even more limiting.
Things to consider when designing plastic parts.
If you know you are going to have to use injection moulding as a means of manufacturing your parts you need to design the parts for injection moulding. If you want to first make prototypes with 3D printing or resin casting you will have to design the part for that process, separately. When it comes to part design for injection moulding best to keep it simple and not get to complex with the part design.
For tool design consider gating at the thicker areas and running flow simulations so as to design out any potential moulding defects. Parts can be moulded with many sorts of defects including flash, shorts, weld lines, flow marks, sink marks, warpage and many other types of defects. These moulding defects usually originate from poor part design but can also be caused by poor tool design or incorrect process set up.
Design Considerations for production.
Minimalizing the cycle time in injection moulding has obviously an effect on the production quantity’s which should also be translated into a reduction in unit price. For the injection moulding cycle, the longest part of the cycle is usually the cooling phase. by designing the tool with optimal cooling the cycle time can be significantly reduced using conformal cooling within the tool, which can also have a significant effect on the part quality and cooling efficiency of the mold.
You should also consider secondary operations if your part needs to be assembled, designing your part for minimal assembly can have a significant saving on your finished part cost. A big reason a lot of injection moulding is done in Asia is due to the lower labour costs and for parts that need simple assembly, it makes economical sense to mold an assembled parts in Asia. However, if you can design a part to be assembled more simply there is significant money and cost savings in that.
An example of designing for injection molding .
You’re were probably sitting in front of a keyboard at some stage today. A computer keyboard even 20 years ago was quite a bit more expensive than it is today, the buttons were quite a bit bigger and there was probably fewer buttons on the keyboard. Nowadays the keyboard is quite a small lightweight and relatively inexpensive item. This is an example of how the moulding process has developed over the years, mainly in making parts lighter and more affordable. With our keyboard examples, the keyboard has many small buttons that have thin walls, and can be easily assembled onto the keyboard. So by designing the keyboard with thin walls the overall item thickness and with of the keyboard has been reduced. Also due to the buttons been eastly assembled it is then quicker to manufacture.
Injection moulding is a great process for mass production, and with good design of the plastic part as well as good tool design parts can be made relatively cheaply. However, a major drawback for injection moulding is the cost of tooling and the time it takes to design and to get into production. There are alternative ways of prototyping and less expensive ways of manufacturing tools. I provide a part design and tool design review service. this is where I can review your initial part design or your toolmaker’s tool design and help you modify and optimise your design. please get in contact if I can be of any assistance