Hello and welcome to this post on injection molding tips with the focus on flash reduction of PPS and obtaining low flash PPS parts.
Background
Having worked in the field of high-performance polymers and specifically with PPS for the past ten years, I have often received the same question: How can flash formation be minimized and reduced during PPS molding? In this text, I will share more than six practical tips to address this issue, based on my extensive experience in the industry.
Let us get started. We will first discuss the possible causes of flash and then show possible solutions.
What is flash formation 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 formation 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: Check the Mold, Clamping Force, Packing Pressure
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) 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.
2) Too high injection pressure -> decrease cutoff position, injection packing pressure, and injection time forward
3) Too high injection rate -> decrease injection rate
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)
During molding operations
Mold Wear and Misalignment During Production: At the start of production, parts typically exhibit no flash; however, flash may develop as the production run progresses. It is important to verify that the mold steel has adequate hardness and that cavity edges are not experiencing excessive wear. Additionally, ensure correct mold alignment and keep parting lines clean and free from material buildup in the parting area.
Check PPS Drying Procedures
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).
Check out my YouTube training video on this topic too:
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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