Hello and welcome to this post on injection molding tips with the focus on flash reduction of PPS and obtaining low flash PPS parts.
Let us get started. We will first discuss the possible causes of flash and then show possible solutions.
What is flash in injection moulding?
In general, flash occurs when molten plastic flows out of the mould during injection and solidifies, causing a visible effect (Figure 1).
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| Figure 1: visible flash on an injection moulded part. |
Why flash can be an issue with PPS?
Since PPS is a high flow polymer, it is an excellent choice for thin walled injection moulded parts. Also, the good flow properties allow for high filling loads. The good flowability allows the PPS to flow during filling out of the cavities without too much effort. Figure 2 compares the spiral flow length of different PPS types to Polyamide 6 with 50 wt% glass fiber [based on 1,2]. The highly filled PPS (GF/MD 65 wt%) reaches 40% more in flow length compared to the PA 6 (GF 50 wt%).
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| Figure 2: comparison of spiral flow length (1 mm thickness) of different PPS compounds (incl. unfilled) vs. PA 6- GF 50 wt% [based on 1,2]. |
Reducing flash with PPS: possible causes vs solutions
How to start:
The first step to enable flash-free injection molding with PPS is to control the mold, specifically by cleaning and inspecting the venting and parting line.
Next, the required clamping force should be verified. Short shots should then be produced to determine the switch-over point, followed by the application of the appropriate packing/holding pressure (height and time).
Optimize Molding Parameters
1) Too high injection pressure -> decrease cutoff position, injection packing pressure, and injection time forward
2) Too high injection rate -> decrease injection rate
3) Low packing phase from 0.0 to 0.5 s and then start with regular packing phase. The 0.5 seconds in delay allow the melt to freeze the parting line (seal it off) and flash is reduced.
4) Too high polymer melt temperature -> decrease barrel temperature and lower backing pressure
5) Too high mould temperature -> decrease mould temperature (min. 135-140 °C)
6) Too low mould clamping force -> increase clamping force; alternatively check the possibly to move mould to larger injection moulding machine (higher possible clamping forces)
7) Mould wearing during production and misalignment -> in the beginning of production , parts show no flash however during the production run flash occurs on parts. Check if mould steel is hard enough and cavity edges are not wearing. Also, checking correct alignment of the mould together with clean parting lines during moulding (no material sticking in parting area).
8) Drying Procedures: Because mineral-filled PPS compounds can be hygroscopic, they should be dried at 135–163 °C (275–325 °F) for 2 to 3 hours to prevent moisture-driven "drool," which can cause flash.
Bonus Tip: Select Low-Flash Grades:
Utilize specific PPS compounds designed to minimize flash, such as Ryton R-4-230BL (40% glass fiber reinforced), which provide improved processability.
Furthermore, there are PPS alloy compounds which have low flash properties and fast cycle times (f.e. Ryton PPS XK2340 or XK3340).
Now I am curious, let me know - what are your tips to reduce flash when moulding polymers such as PPS?
Thanks for reading & #findoutaboutplastics
Greetings,
Herwig
Literature:
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