Injection molding small series is a type of injection molding primarily used for small runs of products. This differs from typical injection molding, which focuses on mass production at a higher cost. Manufacturers often use injection molding small series for runs of 100,000 or fewer products. Though this may seem like a high number, it’s small in comparison to the millions of products a manufacturer may produce for high-volume parts.
The Step-by-Step Workflow for Low Volume Injection Molding
The key difference between injection molding small series and traditional injection molding lies in the rapid tooling methods used for small-scale manufacturing. Here are the three key steps of the process.
Step No. 1 – Mold Design
The manufacturer uses computer-aided design (CAD) software to create the design for the mold. The specific design methods differ depending on the tool required for the part. For example, molds intended to be 3D printed require different design techniques than aluminum molds.
Step No. 2 – Select a Rapid Tooling Method
With the design in place, the manufacturer moves on to choose a rapid tooling method. There are several available, each offering benefits and drawbacks.
3D-Printed Molds
3D printing has become increasingly popular for low-volume production runs because it offers the most cost-effective alternative to aluminum molds. The printed parts are both solid and isotropic, allowing manufacturers to maintain consistency during production runs. The main drawback of 3D printing is that molds have short shelf lives. Certain techniques, such as injecting the mold with polylactic acid, may help improve a 3D-printed mold's lifespan and quality.
3D-Printed and Soluble Inserts
Inserts improve the structure and strength of molds made using thermoplastic, such as those made using 3D printing. For example, a manufacturer may use a heat-set insert to soften the material used to create the mold as it’s being installed. Following installation, the manufacturer removes the heat source, leading to the thermoplastic solidifying around the supportive insert.
Aluminum Molds
Though aluminum’s perceived lack of strength often means manufacturers prefer steel for large-scale injection molding, the material has several qualities that make it suitable for injection molding small series.
The material can usually withstand the creation of up to 100,000 replicas, making it suitable for prototyping and mid-range production. Aluminum’s more pliable nature also allows manufacturers to make slight adjustments to their molds if they discover errors during the production process.
Micro molds
Micro molding becomes relevant if a manufacturer needs to create parts weighing a fraction of a gram or maximum up to 20 grams. Such precision parts are often used in the medical and dental industries, though other sectors can benefit from micro molding too (E.g. electronics). The tooling used for micro molding is incomparably simpler and thus cheaper than traditional molding as well as the molding machine used for molding. Mainly the molds are many times smaller as the machine too. All this reduces NRE tooling costs and machine operating costs which enables rapid tooling and low-volume injection molding. However, this comes with a limitation of producing only small parts – not weighing more than 20 grams.
How Low-volume Injection Molding Differs from Traditional Injection Molding
From the technology point of view those two are the same. However, small volume injection molding differs from traditional injection molding processes in a variety of other ways.
Lower Cost
Compared to making a mold using CNC-machined metal, injection molding small series is cost-effective. Manufacturers may spend between $10,000 and $100,000 on a CNC-machined mold made from steel, with the price varying depending on the size of the mold. Though these more expensive molds last much longer, their cost makes them prohibitive for small production runs. By contrast, a 3D-printed mold can cost less than $500, but it will likely last for less than 100 products before failing. The golden middle here seems to be an aluminum micro molds, which on average cost €2,500 to make and can withstand up to 200,000 cycles depending on part complexity.
Also, in low-volume production, simpler and thus cheaper molds (molds without automatic sliders or hydraulic actuators) can be used whenever undercuts are present. By using manual work, the inserts that shape undercuts can be taken in and out. This prolongs the cycle time but at low-volumes it may pay-off considerably on overall project cost.
Faster Mold Production Times
A steel CNC-machined mold can take between four and eight weeks to produce, depending on the material. This isn’t ideal in time-sensitive situations. Molds made using 3D printing techniques offer a much shorter lead time of one-to-three days. Aluminum molds also offer shorter lead times than steel molds, clocking in at between two and three weeks.
The Limitations of Small Batch Injection Molding
The lower costs and lead times for most injection molding small series techniques result in molds that aren’t as durable as their CNC-machined traditional steel equivalents. Manufacturers must consider more traditional mold-creating techniques for large production runs.
However, some emerging techniques may make small series molds more durable in the future. Freeform Injection Molding combines the short lead times of small series molds with the greater scalability of injection molding, though it is more time-consuming and has compatibility issues with some polymers. Additive manufacturing (AM) processes may also lead to the production of strong molds. Unfortunately, AM processes come with surface finish issues and higher fabrication costs.
The AM approach also has drawbacks in its nature. Additive manufacturing used for mold making where subtractive manufacturing is more reasonable is not always the most efficient way to go. The huge potential of creating intricate shapes by AM cannot be fully used since undercuts are unavoidable in molding technology.
The Benefits of Low Volume Injection Moulding
If looked at the product life cycle it's clear that on-demand injection molding is relevant at the beginning stages. So why might a manufacturer use low volume injection molding?
Simple Prototyping
Creating a steel mold is cost-prohibitive, making it unideal for developing prototypes. A start-up company can use injection molding small series to develop a working prototype of its product at a low cost.
Lower Costs
Speaking of lower costs, low-volume injection molding costs are less than traditional injection molding. The cost benefits decrease as the manufacturer produces more of the same type of part as the cost of tooling seems to dilute into the quantity of the parts produced.
Material and Shape Diversity
The various rapid tooling techniques used in low-volume mold creation allow for a great deal of material and shape diversity. For example, it’s possible to create elastomeric parts with this technology.
The Mechanical Strength of the Molded Part
Using a low-volume mold is preferable to simply 3D printing the intended part. The part produced using the mold doesn’t have the layer-by-layer printed surface of a 3D-printed part, even if the mold itself does have that surface. This makes the molded part homogenous and continuous with greater mechanical stability and visual appearance.
Low-volume Injection Molding Applications
Injection molding small series comes into its own when helping manufacturers to overcome short time cycles for small-scale product production. Examples include prototype creation and meeting on-demand molding requirements. However, many small manufacturers use this process to speed up the production of limited-run products.
Using the Micromold rapid tooling process, a manufacturer can take a product from idea to manufacturing in 4 weeks or less. In traditional injection molding, it can take up to eight weeks to create a mold, which doesn’t account for the time spent designing the mold or creating products using it.
Injection Molding for Small Production Runs
Injection molding small series isn’t suitable for large production runs. The molds created don’t hold up to high-volume use (millions of cycles). However, the technique has many applications, including prototype production and small-scale product runs. Furthermore, developing technologies may improve the strength of small series molds in the future, though many of these technologies have drawbacks that need to be overcome first.