This article discusses the use of 3D printing to print molds for low run injection molding. Design considerations, materials, molds configurations and a comparative case study are all included
Injection molding is the most common method for producing plastic parts. While traditionally 3D printing was only used for verifying prototypes of parts that were later going to be injection molded, developments in printer accuracy and materials now allow 3D printers to print injection molds directly.
This article will discuss the benefits of 3D printing low-run injection molds and give advice on the best mold configuration, mold materials and how to design a 3D printed injection mold.
What is a low-run mold?
Injection molding is the process of injecting (under pressure) a thermoplastic or thermoset in a melted liquid form into a die. The plastic fills the empty cavities of the die and cools until it has solidified. The solid plastic part is then ejected from the die and the process is repeated again.
The high initial setup costs associated with injection molding do not make it cost effective at low volumes. The high level of design, engineering and machining required to produce an injection mold can result in the cost ranging from R50,000 to R500,000. Because of this injection molding is typically used to produce high volumes (sometimes in the millions) of the same part at a low cost. Low-run injection molding typically applies to runs of 10 - 1000.
An industrial injection molding die used for producing a large number of parts
Why use 3D printing?
Whether a mold is going to be used to make 20 part or 20,000 parts, historically molds needed to be machined to a very high tolerance from a solid block of metal (most commonly aluminium or steel). These materials provided good wear resistance to the repeated injecting, opening and closing and temperature gradients that they were exposed to during the injection molding process however do require large investments at the setup stage.
For low-run molding, wear resistance is no longer a critical factor. 3D printing technologies are able to produce parts to a high accuracy with excellent surface finish. This property coupled with temperature resistance and design freedom mean that 3D printed molds are now a viable method for low-run production injection molding. 3D printed molds also allow verification of injection mold designs before investing in expensive metal molds.
3D printed molds are best suited for:
3D printed injection molds are produced in 2 standard configurations.
1. Mold inserts for aluminium frames
This is the most common 3D printed mold configuration and generally produces more accurate parts. The mold is 3D printed and inserted into aluminium frames which provide support against the downward pressure and heat of the injection nozzle. Aluminium frames also help prevent the mold from warping after repeated usage.
A 3D printed injection mold inserted into an aluminium frame
2. Stand alone molds
This mold configuration does not require investment in an aluminium frame as the whole mold is printed. The disadvantage to this approach is that these molds use more material, which increases print cost and time, and may be more prone to warping.
Designing an injection mold for 3D printing
The 3 main material characteristics that will govern whether a 3D printed low run injection mold will work are:
A 3D printed injection mold made from ProdWays ceramic filled resin
Specific technical design of gates, runners, air vent etc. is out of the scope of this article. A quick internet search will reveal a large amount of information on mold design. This post by Seattle Robotics is a good starting point for those new to injection mold design.
Some general rules that can be followed when designing 3D printed injection molds include:
Designing parts for injection molding
As with conventional injection mold design consider:
Draft angle design for injection molding
Flash is the name given to the material that comes out between the halves of the mold during the injection process. This generally occurs when the two mold halves do not mate perfectly together, are not perfectly flush and flat or the mold is overfilled. Runners are used in mold design to help reduce the likelihood of flash occurring.
If designing for an aluminium frame, add 0.125mm of extra thickness to the back of the mold plates to account for compression forces and to ensure a complete seal. Increasing clamping force in the vise can also help mitigate flash, as can polishing the mold’s split plane to give it as flat a surface as possible.
Good mold design and a flat mold face reduce the likelihood of flash occuring
Due to the fragile nature of the materials used to 3D print injection molds when compared to traditional mold materials, struggling to remove a part from the mold can lead to rapid mold deterioration. Including a release compound on the mold cavity surfaced before the injection stage can assist with part removal.
Case study - plastic motor fitting
This case study will compare manufacturing a custom plastic fitting for a motor housing. The requirements of the design are:
Industrial FDM ABS part - Industrial FDM is a form of 3D printing that has high repeatability, produces part with a high accuracy and is able to print parts in batches. The cost of the ABS filament used in FDM printing is typically around R300 - R600 per kg. The main restriction for any part produced via FDM printing is anisotropic performance. Parts are significantly stronger in one direction meaning a designer is required to have a good grasp on the loads the part will be subjected and the orientation the model is printed at.
3D printed DLP MovingLight injection mold + ABS injection molded part - DLP High Temp resins are able to produce functional injection molds with a high level of accuracy. They are best suited for low level production runs. UV resins retail for around R1 500 - R2 500 per litre. A bench top injection molding machine has been used for this example with the 3D printed molds inserted into aluminium frames.
Traditional injection molded ABS part - Traditional injection molded parts have a very high level of accuracy, excellent surface finish and a very high level of repeatability. The main downsides to traditional injection molding is the high initial setup cost and the number of design conditions that must be implemented in the design of a part (draft angles, constant wall thickness etc). ABS pellets used in injection molding sell for approximately R20 - R30 per kg.
A summary of the prices (based on online quotes) to manufacture the ABS fitting using the technologies discussed above is summarised in the table below. All prices are excluding shipping.
Article sourced from 3DHubs.com
Written by Ben Redwood
***Article adapted for local conditions and availabilities
3D PRINTING OF INJECTION MOLD FOR SMALL SERIES
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