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Plastic Injection Molding Pilot Runs: What You Need To Know

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Plastic injection molding pilot runs are used to optimize the tooling, processes, and designs to go into mass production with a minimum of risks of defects and other unwelcome issues when parts start getting made on a large scale. But what is the process of such a pre-production pilot run? What gets optimized? Once the part is agreed upon by the customer and manufacturer, can they go into production? These, and more, questions will be answered here so you can get an idea of just how important this part of the injection molding process really is.

 

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Why are plastic injection molding pilot runs important?

In any process being done for the first time, our general rule is that a pilot run is necessary as it validates that it’s ready. This goes for custom-designed products, packaging, loading, etc. Plastic injection molding is no different. A pilot run can help a great deal, but some businesses are so keen to get products made that they skip this step. If that happens, though, you have these potential risks:

  • Missing issues that occur due to non-optimised mold tooling or mold press machines, such as flash (which indicates a problem to be addressed).
  • Losing the opportunity to improve process efficiency (because you go straight into mass production and so cannot check and make changes to the way the work is carried out).
  • Not having a benchmark for quality. The pilot run allows us to assess what the optimal benchmarks are for, say, cycle times, color, etc, that will yield the desired quality.
  • Not being able to optimize the mold tooling and/or machine settings to get best results based on what we see in the pilot run. (01:49)

 

What are the steps involved?

Plastic injection pilot runs would commonly include these steps:

  • The plastic part is designed, CAD drawings are made, and it gets reviewed and confirmed.
  • The tooling needs to be designed (with manufacturability in mind), including the cavity, sliders, lifters, gates, etc.
  • DFM review to ensure that the tooling will be suitable to produce parts in large quantities with few risks.
  • Mold tooling is fabricated from a block of steel and then the tooling can have early trials done (which won’t include textures, for example) over around 30 days to start initially validating that the mold functions as intended. You will commonly look out for the mold’s functions working properly and that there no negative issues, such as short shots where not enough molten plastic is injected into the tooling, or that not all cavities are filled correctly.
  • Then after the trials are done, production can start.

In pretty much all industries, the first trials are called T0, T1, T2 and so on. The initial parts from injection molding during these trials are likely to be smaller than specification, and you will almost certainly require at least a couple of trials. (05:05)

 

After the product is approved, the supplier still needs to optimize the tooling

After a product has been through the trials and is approved, can we go into mass production? Not yet. After a plastic part has been through T0, T1, T2, etc, and we have validated that the dimensions, aesthetics, textures, polymer, etc, are all OK, it is approved by the customer as being acceptable as per their specifications. However, from a plastic injection molding supplier’s point-of-view there are still process and tooling optimizations to do before mass production can start even if the tooling and parts themselves are now validated. The supplier still needs to optimize processes, namely cycle times and plastic injection machines and ensure that the tooling itself is running to have the maximum life expectancy.

From a tooling point-of-view, suppliers optimize these processes and elements to ensure that they can deliver a reliable product to the customer:

  • The gate. This is where the molten polymer enters the tooling from the sprue. This ensures an optimal fill and minimal witness marks left on the part. The correct gate size and shape should be optimized to minimize such marks.
  • Heating and cooling. Polymers react to heat and need to be in a fluid form to inject into the tooling properly, so the correct temperature must be maintained throughout the process and then cooled at the right time and speed to solidify without shrinking, warping, suffering from burn marks, etc. The cooling lines can be moved to optimal areas. This is a delicate process that will be handled by a skilled injection molding expert.
  • Reducing flash by controlling the pressure the polymer is injected.
  • The aesthetic effects of the molding process on parts. For example, they may optimize the tooling to have pins that eject the part by pushing it out of the tooling from a side that customers will not see (such as the inside of a plastic enclosure) in case there are some small marks left.
  • The ejection process. The supplier needs to make sure that everything is smooth and parts can be ejected easily and that textured surfaces don’t lead to parts sticking. (12:39)

 

Optimizing the injection mold press (machine).

As well as tooling and some processes, the supplier will also need to optimize the mold press as well after a part is approved, but before mass production can start. The injection mold press keeps the polymer hot and clamps the tooling together during the injecting process, injects polymer under very high pressure, and then releases the clamp allowing the mold tooling to eject the part after a while.

The key optimizations the supplier will make are:

  • The injection pressure – this is about optimizing the fill of the cavity. Too much will lead to excess flash, and too little with short shots where the cavity is not fully filled.
  • Temperature should be controlled throughout the cycle. The polymer cannot be allowed to solidify until it has filled the cavity. Too cool will result in short shots and too hot, burn marks. The operating window should be optimized to have the best flowing polymer injected into the old when needed. The supplier will need to gauge the optimal melting temperature per polymer type. If parts are not solidified at the right time, it can lead to part defects in the field, such as them being too brittle because they were solidified when too hot.
  • Cycle times will be optimized especially during the pilot run to ensure that the supplier is getting through the injection molding process as quickly as possible meaning that more parts are made efficiently. Some suppliers may try to pump out plastic parts as fast as possible to make, for example, more profits. So they may optimize their machines for short cycle times and accept that there may be the risk of defects that they just don’t mention to customers. That’s a danger. However, if the supplier can find the shortest cycle time within the operating window, that’s a positive as it results in lower costs for everyone.
  • Holding time. The tooling needs to be held together under pressure for just long enough during the initial cooling time, otherwise, there may be a backflow of molten polymer which can lead to physical defects of the resultant parts which look fine but don’t perform properly. (25:01)

 

Keeping records of parameter settings helps to control the process.

Once all optimizations are done and the injection molding process is running smoothly, keeping a record of the parameter settings and which machine was used is helpful as it will allow the process to be replicated consistently, reducing problems like defects. Let’s say a new operator starts using the injection mold machine. They may have their own ideas about what the settings should be based on their experience and could be tempted to tinker with them. If they have a document to refer to that outlines exactly how the machine needs to be set up to obtain the desired results in the plastic parts, there can be no question about how it should be done. As seasons change, so too may settings to cope with the differences in heat and humidity (which vary quite a lot in South China, for example), so keeping records of the necessary parameter adjustments throughout the year is also incredibly useful (although plastic shops producing the highest-quality products implement temperature and humidity controls to reduce variations). (34:55)

 

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