1 of 7
Figure 1
Examples of four-cavity and eight-cavity runner systems. While the exact cavity spacing layout and subsequent runner size is very dependent on part geometry, the eight-cavity runner in this example is 2.75 times the size of the four-cavity runner.
2 of 7
An example of runner optimisation. On the left is a typical runner design, and on the right is an optimised runner. The larger runner is more than 10 times the size of the optimised runner. The result is significant cost savings in material waste, which is especially important for expensive bioabsorbable materials.
3 of 7
Example of bioabsorbable parts, a bioabsorbable fixation suture
4 of 7
Examples of bioabsorbable part, a bioabsorbable fastener.
5 of 7
Inherent viscosity (IV) testing equipment at MTD Micro Molding.
6 of 7
A micromoulding automation and vision system.
7 of 7
Precision micro tooling.
Gary Hulecki, executive vice president, MTD Micro Molding
When higher cavitation does not result in lower cost
When OEMs prepare to increase production volume of a micromoulded component, many focus on multi-cavitation tooling as a strategy to reduce piece part price. While increasing cavitation may be a cost-effective approach for higher volumes of simple thermoplastic parts, it is typically not the best approach for micromoulded parts, especially those that are bioabsorbable or made from other high-cost materials. This may initially seem counter-intuitive.
Material waste
Figure 1 shows an example of optimised micro runner system designs that can be utilised for any material. You can see that the inherent material waste is in simple surface area. With expensive materials such as bioabsorbable resins, which can cost US$5–10 (£4–7) per gram, it is easy to see how this amount of waste results in a non-cost savings situation.
In micro moulding, it is estimated that a runner-to-part ratio for an optimised one-cavity design is approximately 80:1. For an eight-cavity, it is an estimated 800:1. An 80:1 ratio may seem as though the runner is big and wasteful, but the reality is that the majority of the material will always live in the sprue and runner because the micro parts they are feeding are extremely small in comparison.
Increased cycle time
When increasing from a four-cavity to an eight-cavity runner system design, one may assume that the yield is doubled. However, in reality, it takes longer for the moulding process to create an eight-cavity shot—approximately 10–15% longer cycle than a four-cavity version.
Mould and automation issues
Complex, tightly toleranced micro designs lend themselves best to smaller cavitation tooling. With hundreds of variables to control in micro moulding, introducing higher cavitation can further expose the moulding process to risk and risk is expensive.
Hidden costs of high cavitation micro moulds are a result of significantly more time and resources being spent on hard-to-avoid issues such as more frequent repairs, maintenance and downtime.
The automation complexity increases with more cavities as well. Having a robot successfully and accurately remove a tiny, fragile part from a one-cavity mould, present the part to multiple camera systems and dispense the part into a custom packaging solution is a feat in itself in micro moulding. This challenge is compounded significantly with more cavities.
Four key areas for savings
With high cavitation tooling not being an ideal cost-savings option for micromoulding, and especially for bioabsorbable products, how can piece part pricing effectively be reduced with production volume ramp up?
MTD Micro Molding (MTD) recommends focusing on the following four areas to bring the piece part down for bioabsorbable products:
1. Runner sizing
Use runner optimisation to pinpoint the minimal amount of material required for the runner system to be successful. MTD’s MicroRunner tool has a ratcheting runner system that varies in diameter and aids in determining the minimum runner size required to fill the volume of the part with the goal of sizing a runner system to adequately mould a product without sacrificing material.
2. Production optimisation
Having a successful moulding optimisation period during early production helps with planning and efficiency, which can result in cost savings down the line.
3. Accurate forecasting, steady ordering
Accurate forecasting, blanket purchase orders and steady ordering can allow the moulder and material provider to be more efficient with their processes, resulting in reduced manufacturing costs and piece part cost savings passed down to the customer. For example, MTD was able to reduce a bioabsorbable piece part price with steady ordering and accurate growth forecasts by 40% over a five-year span.
4. Risk mitigation strategy
Designing the validation protocol to validate the widest range of bioabsorbable material IV lots available is a risk mitigation approach that could result in cost savings for the product.
By validating the widest IV range possible and passing the validation, the moulder will be able to successfully accept and process any IV lot in that range. This reduces the risk of interrupted production runs that would be inevitable if only a single IV lot was validated and those lots became unavailable during production. That would require additional costs to get the production line back up and running with new IV lots, as well as further activities to validate.
Steps to program scaling
The path MTD encourages its customers to take at the start of a challenging bioabsorbable R&D project is to always start at proof-of-concept and scale the program using the following four steps:
Step one —Prove the design is viable.
Step two —Create a prototype mould that can be used for early production.
Step three—Maximise cavitation based on what is learnt from that prototype tool.
Step four—Build production cavities at optimum cavitation to reduce cost, and then consider multiple moulds to achieve volume requirement.
MTD Micro Molding