The versatility of PVDF as an engineering plastic is well known. Indeed, we have covered the advantages, properties, and applications of PVDF in an earlier article. Properties such as extreme chemical resistance, UV resistance, thermal stability, and piezoelectricity all make PVDF an intriguing polymer that finds application across a host of different industries.
Our own experience with PVDF has thus far stayed within the realm of machining. PVDF rods and sheets are available and can be easily machined to make a final component. The polymer itself throws up no surprises with regards to how it behaves dimensionally, post-machining.
Recently, however, we have encountered a challenging prospect. The part we were asked to develop was far too large to be machined from a rod or a sheet. With an outer diameter of about 200mm and an inner diameter of 130mm, the part would have caused far too much waste were it to be cut from a sheet or a rod. The option, therefore, would be to compression mould it.
Given our expertise in PTFE and PEEK moulding, we assumed that PVDF – whose melting temperature is far less than either PTFE or PEEK – would be simple enough to mould. We knew from discussions with our suppliers that the equipment we use for PEEK moulding could be easily used for PVDF, provided the processing parameters were adjusted accordingly.
Our first experience with the moulding convinced us that this was a fairly simple affair. We moulded a small rod of 100mm diameter and 50mm thickness and found the part to be uniformly coloured (PVDF should be milky white), with no signs of any blowholes. Skived sections taken from the rod confirmed that the tensile properties were in line with what was expected, while the specific gravity was also in the range of 1.8, as it should have been.
Moving from the test sample to the part we needed to mould showed us that we may have underestimated the material. A few challenges were immediately apparent:
-
The material was extremely sensitive to temperature. While the 100mm sample appeared to have formed easily under a temperature of 210°C, the larger part was getting discoloured and turning brown.
-
The melt flow of the material was challenging to control. If the temperature was held for too long, the viscosity of the material would reduce and cause it to leak from the mould. If the temperature was not held long enough, the part would come out with blowholes, having not been sufficiently melted throughout.
-
Similarly, too much pressure would cause material leakage, while too little pressure would not allow all the air to be expelled, resulting in blowholes.
In effect, moulding PVDF turned into a very precise give and take between temperature, dwell time, and pressure. Furthermore, although the material could be re-melted, doing so would discolour the polymer, rendering it useless. This meant that all parameters needed to be precise and that a cycle could be run only once else the material would be lost. (Incidentally, we are not new to this conundrum. PTFE behaves in much the same way, only we have decades of experience with PTFE and know how to get it right every time!).
Once we had moulded the part, the time came to machining it. Again, although our experience with machining PVDF had always been smooth, here too we observed that compression moulded PVDF behaves slightly differently post machining. Stresses in the material tend to relax overnight, causing slight deviations in dimension. Hence, adjustments needed to be made to the machining process to allow for the same.
There is a reason that engineering polymers are a niche space and that so few have the expertise to consistently manufacture certain high-performance plastics. We pride ourselves in being able to understand our polymers and to investing the time it takes to develop them.
Read More
1. Case Study - PEEK in Coffee Machines
2. Polymers in Low Friction Applications
3. PTFE Extrusion - Ram vs Paste Extruded - A comparison of features