Why 3D Printing is the answer for short-series production

Everyone is talking about how 3D printing is revolutionizing the way companies prototype and iterate designs faster and with more freedom than ever before. One thing that doesn’t get the attention it deserves is 3D printing for injection molding; not the most glamorous of applications but one which is making meaningful changes in the manufacturing industry. This is an area which is beginning to see even more innovation as molding goes from the machine shop, to the industrial 3D printer, to the desktop 3D printer.

This post will breakdown what happened before, what’s happening now, and what‘s to come.

Traditional Injection Molding

A mammoth 2700kn Plastic Injection Molding Machine (FL270)

Injection molding was first patented back in 1872 by two business savvy brothers (John & Isaiah Hyatt) who built a basic machine that worked like a hypodermic needle: pushing plastic through a hole into a heated cylinder. It was created to make billiard balls for a manufacturer. James Hendry, in the the 40’s, went a step further building the first screw injection molding machine. Instead of a plunger pushing the plastic through a hole, an auger is placed in the cylinder and mixes the injection material before pushing forward and injecting the material into the mold.

This is the process most commonly that is used today and has led to higher accuracy thanks to the pre-mixing and increased speed due improvements in automation. Injection molding is now used in all manner of industries and to create a large range of high volume products. Why is it used for the purposes of mass production? Well you can create a mould in which you can inject low cost plastic into repeatedly creating the same shape making it cheapest way to create 1,000’s of the same product. The more products you produce the cheaper the products become as you begin offsetting the great expense of the mold.

How it works: Material granules are fed via a hopper into a heated barrel , melted using heater bands and the frictional action of a reciprocating screw barrel (auger). The plastic is then injected through a nozzle into a mold cavity, where it cools and hardens to the configuration of the cavity. The mold tool is mounted on a moveable platen — when the part has solidified, the platen opens and the part is ejected out using ejector pins. AV Plastics
4 Stages of Screw Injection Molding

You cannot create any geometry with injection molding and there are many design limitations to consider when creating a mold. These include part and mold material, component geometry and mold design requirements (draft angles, ribs and gussets, wall thickness and split lines). The geometries especially for injection molding are a challenge as you have to make sure you can get the part out of the mold. Ejector pins can be found when looking at most mass manufactured goods, this is one of the considerations you need to make when creating your mold design. Injection molding is best used when you need to create the same part 100’s or 1,000’s or even 10,000’s of times as you can take advantage of the economies of scale.

Injection Pins

Traditionally, molds are made by a mold making company that machines p-20 steel bars which are then placed into a block, known as a base. This base is then milled to the required mold geometry. Both halves of the mold are then drilled for the bushings and guide pins, which hold both parts together during the injection process. Grinding is then performed to produce a smooth and accurate surface, after which the CNC’ing begins to create the final outline of the mold.

This is a long process, with the CNC machining taking up to 20 hours alone, and involves a variety of manufacturing techniques, often resulting in the need to outsource the process to a company specifically equipped to deal with the stringent requirements of mold production. Going from block, to outline, to detail, to fine detail while changing manufacturing methods creates long lead times for engineers to deal with when waiting to get their parts.

Water CNC cutting machine adding details to the mold (How it’s made)

Injection molding is and will continue to be a manufacturing method regularly employed by industry, thanks to its ability to mass produce parts at a high speed for a low cost. Evidently, it’s not as simple and straightforward as producing a metal mold and popping it into the machine, as strict design considerations are needed (material, design, process) and a wide variety of manufacturing steps are required. So how does 3D printing fit into this complex world and how can on-demand manufacturing play a role in mass production?

PolyJet

The first 3D printing technology to step into the world of mold-making was material jetting, via Stratasys Connex PolyJet machines back in 2011. Material Jetting (Stratasys PolyJet) technologies are similar to inkjet printing, but instead of jetting drops of ink onto paper, these 3D printers jet layers of liquid photopolymer onto a build tray and cure them instantly using UV light. It uses a support material which easily breaks away to create complex geometries that would otherwise be difficult with traditional tooling.

PolyJet Schematic

The biggest hurdle for 3D printing to be applied in the molding industry was the materials. Traditionally, 3D printing materials lacked the properties to withstand the forces and heat continuously exerted throughout the injection molding process. From these material limitations, Stratasys launched a material to in the form of Digital ABS (Simulated ABS), a photo-reactive resin capable of withstanding high impact designed specifically for short injection molding runs.

Digital ABS Blow Mold, Digital ABS Injection Mold (Stratasys)

It’s important to highlight that injection molds can withstand 1,000's of shots (cycles) and Digital ABS isn’t intended to replace molding for mass production but instead for short production runs due to the lack of material durability. Most Digital ABS molds will last from 10–100 shots, giving companies the ability to run test productions with the goal of finding expensive design flaws before they move to full production.

Stratasys’ highlight:

PolyJet 3D printed molds are not production tools. But should be used for low quantities, when design changes are likely, and for complex geometries that would make traditional tooling difficult.

The creation of a PolyJet mold is where you see the benefits of 3D printing for short production runs versus traditional mold making. You don’t need huge steel blocks (cost savings), you don’t need an incredible amount of manufacturing techniques (time savings), and most importantly, if you have the printer, you don’t need to outsource it (time and cost savings). When you want to create 100 of the same product or less then it makes sense to 3D print the mold in Digital ABS instead of the vast expenses incurred with traditional mold making, the further over a 100 you go the more sense traditional injection molding makes.

Analysis done with basic bracket (Marcus Morrissette)

The process goes like this:

1. Design the mold with draft angles and gate size in mind. Using sprue or edge gate rather than tunnel or point gate will increase part quality and mold life
2. Print the mold — make sure the layer lines are oriented in the same direction in which polymer will eventually flow into the mold.
3. Optional finishing — most molds do not require post processing
4. Mount the mold — place the mold into the injection molder
5. Produce your prototype — begin churning out your parts
Simulated ABS Mold being removed from the Injection Molder

Formlabs

This year, another exciting step was taken into the world of molding by a 3D printing company. This time, it was Formlabs looking to disrupt conventional mold manufacturing with their desktop machine, the Form 2, and anew High Temp resin material.

Formlabs’ Form 2 printer uses stereolithography (SLA) technology, which uses liquid photopolymer resin just as PolyJet, but instead of jetting it from a nozzle, it solidifies the material using a UV laser through the bottom of a resin filled vat. SLA parts are chemically bonded resulting in parts that are fully dense and isotropic.

SLA technology schematic

The new High Temp material was designed specifically for an application where withstanding continuous heat is required. The material has a heat deflection temperature (HDT) of 289° C at 0.45 MPa — the highest of any 3D printing material on the market.

High Temp molds have been tested with common injection molding plastics such as LDPE, PP, TPE, PLA, ABS, HDPE, EVA and PS, with the molds showing no temperature degradation on the surface after 25 shots.

High Temp Resin showing off it’s material properties used as a blowtorch nozzle (Formlabs)

The main goal of Formlabs’ new material is to bring in-house injection molding for low-volume production of functional parts within the reach of many companies. With the combination of a Form 2 SLA 3D printer and an affordable bench-top injection molding machine like the Galomb Model-B100, which costs less than $10K, you can get set up. The combination of both pieces of equipment is roughly 15 times lower than the cheapest PolyJet printer capable of printing with Digital ABS.

High Temp Mold (Formlabs)

Important to consider is that High Temp resin must be post-cured (UV) in order to exhibit its high-temperature properties. Curing is necessary to improve over material properties. Some final food for thought is the size of prints the Form 2 can create. With its build volume of 14.5 x 14.5 x 17.5 cm, there are some limitation on the size of the molds.

The core benefit Formlabs promote with the material is that it offers an agile manufacturing approach. This allows engineers and designers to easily modify molds and continue to iterate on their designs with low lead times and cost, similar to the benefits shared by Stratasys when using their Digital ABS material. The best time to use High-Temp Resin is when your looking to test the market or gain feedback from your customer base with a short-series run of production. Anything less than 25 replicated parts makes High-Temp resin the choice, and potentially more with the cost being so low of creating another mold.

So what?

So we’ve seen that molding, and in particular injection molding, isn’t going anywhere as a popular tool in mass production. What is important to note though is that many companies still spend a great deal of money on short production runs - something we’ve seen 3D printing can do whilst greatly reducing the time and cost associated. Manufacturing companies like Robert Seuffer GmbH & Co (White Goods, Commercial Vehicles) are using PolyJet to make molds in just days with cost savings of 96% compared to traditional metal mold creation. Now Formlabs is aiming to provide the same benefits, but at a far more accessible price point, it will be interesting to see how short-series production runs evolve in terms of the technology applied.

The real benefits of 3D printing for prototyping (saving of time and money) are slowly coming to the mold-making industry, making 3D printing a viable options for doing short-series runs of production. We’ve seen PolyJet perfect for large molds of up to 100 shots and the Form 2 with it’s high temp resin perfect for slightly smaller molds with smaller runs of production at an even greater reduced cost. Materials will continue to evolve in this area, and as they do, so will the possibilities for 3D printing.

Find out more about 3D printing materials with our guide and Material Wizard found here: https://www.3dhubs.com/materials
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