Plastics Technology

SEP 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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cations, it is important to challenge the process during validation and mimic viscosity changes. For example, if your specification allows for regrind to be added in production, then you need to include a run with regrind during your process validation. Once you have a baseline and have identified normal variation for your process, when the peak injection- pressure variation exceeds that baseline, it is important to understand why. I am not saying you should immediately reject this product, I am saying that this product needs to be reviewed and more impor- tantly, the molder must identify where the added variation is coming from. • Cavity Pressure: Given the millions of dollars molders spend every year adding cavity-pressure transducers and equipment to monitor and con- trol their process, cavity pressure must be a critical output. Having the ability to pack out each cavity based on the cavity pressure, or even controlling the V-to-P transfer by cavity pressure, provides dimensional stability from part to part and shot to shot. Although the investment can be substantial, the benefits are proven. That said, my position is not that every product needs this technology. If you're an automotive molder and you're molding class "A" surface products, even parts that are run within your control limits for cavity pressure may not be acceptable due to visual defects. Cavity-pressure transducers aren't going to let you know whether the parts have a scratch or a spot of rust on your texture. If set up properly, they can monitor for shorts and flash; but cavity-pressure transducers might not catch visual defects that aren't related to material flow. If the cavity-pressure transducers are installed in the correct locations, peak cavity pressure can provide a high level of control and monitoring. However, I would say the biggest benefit of cavity-pressure transducers isn't even the cavity-pressure output. The cooling-rate output from a cavity-pressure transducer provides the rate at which the plastic shrinks away from the transducer. This can reassure molders that the water is cooling at the same rate on each run. So maybe the argument for cavity pressure is actually an argument for cavity-pressure trans- ducers. That would be a difficult stance to take, however, since there are so many successful molders out there that do not invest in this technology but are still data-driven scientific molders. Ensuring that you have a skilled, motivated, and properly trained work- force is ultimately what will determine the quality of the product being sent to your customer. 38 SEPTEMBER 2018 Plastics Technology K now How Spritzgiessautomaten BOY Machines Inc. • Exton, PA • • Specialist for LSR since 40 years Worldwide customer base due to speci c advantages for LSR processing offered by BOY injection molding machines: Very rigid clamping system – important for processing LSR Precise, sensitive hydraulic control – important for sensitive LSR parts Very low energy consumption due to servo drive technology Industry leading knowledge and experience in LSR processing Two platen clamp design offers ultimate flexibility for automatic parts removal Ideal for use in clean room applications • • • • • •

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