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Case Study: How to reduce the costs of injection moulding

Injection moulding is the preferred method for mass-producing precision plastic parts. However, its upfront costs can be hefty. According to a report, It’s worthwhile to tweak the production process to decrease both the complexity and the overall expense of a project.

Here are some of the  primary cost factors in injection moulding:

For smaller batches, tooling has the greatest impact, contributing to roughly 50-70% of the total cost. Therefore, it's essential to plan in advance to accommodate your project's needs.

Most moulds are CNC machined from metals like aluminium or steel for durability. These moulds can cost anywhere from $10,000 to $100,000. If you're producing thousands of parts, that investment is justified. However, for smaller batches, 3D printed moulds have become quite popular owing to improvements in additive manufacturing.

As explained here, opting for 3D printed moulds can drastically reduce mould expenses, bringing them down from thousands to just a few hundred dollars. But, this choice is most suitable for short runs where wear resistance isn't a primary concern. For high-volume production runs (10,000 to 100,000+ units), material and production costs eclipse the impact of tooling costs on the overall expense. In such scenarios, your focus should be to reduce both the volume of the part and the duration of the moulding cycle.

Here are some tips to help you optimize the cost of your injection moulding project:

1: Stick to the straight-pull mould

Using side-action cores and other in-mould mechanisms can bump up tooling costs by 15% to 30%. This means you might face an extra tooling expense ranging from $1,000 to $1,500. To keep your production on budget, avoid using side-action cores and other mechanisms unless absolutely necessary.

2. Redesign the injection moulded part to avoid undercuts

Undercuts add to costs and complicate things, so it's always worth the effort to get rid of them where possible.

3. Make the injection moulded part smaller

Smaller parts can be moulded faster resulting in a higher production output, making the cost per part lower. Smaller parts also result in lower material costs and reduce the price of the mould.

4. Fit multiple parts in one mould

Using a single mould for multiple parts is standard practice. Typically, you can fit 6 to 8 small identical parts in one mould, cutting down production time by roughly 80%. You can also place parts with varying shapes into the same mould, much like model airplane kits. This strategy significantly trims assembly costs. Sometimes, two assembly parts share the same main body. With a touch of inventive design, you can craft interlock points or hinges at symmetrical spots, essentially creating a mirrored part. This lets you use one mould for both parts, halving your tooling expenses.

5. Avoid small details

Manufacturing a mould with fine details takes longer due to extended machining and finishing times. Text serves as a prime example, often demanding specialized techniques like electrical discharge machining (EDM), which can drive up costs.

6. Use lower-grade finishes

Hand-applying finishes to moulds can be pricey, especially for high-grade finishes. If your part isn't meant for cosmetic purposes, it's best to skip the expensive high-grade finish.

7. Minimize the part volume by reducing wall thickness

Slimming down the wall thickness of your part is a prime strategy to reduce its volume. This not only uses less material but also significantly speeds up the injection moulding cycle. For instance, cutting the wall thickness from 3 mm to 2 mm can slash the cycle time by 50% to 75%. Why? Thinner walls allow the mould to fill faster. Even more crucial, thinner parts cool and set more rapidly. Consider that nearly half of the injection moulding cycle is dedicated to letting the part solidify, with the machine on standby. However, it's essential to strike a balance. You don't want to compromise the part's rigidity, which could affect its mechanical performance. To maintain stiffness, consider adding ribs in strategic areas.

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