So why does my plastic arrive oversize?
Why manufacturers use tolerances when producing engineering plastics.
What is a tolerance? Technically speaking, it is the amount of difference from the dimensional ideal. In other words, the dimensions of a product will fluctuate, which allows for tolerances to be determined at +/- measurements. Tolerances are essential in the manufacturing process because they allow room for variability in the part’s dimensions. Thickness, length, and width of plastic parts can all be affected.
As we know, tolerances are expressed as a “base tolerance” plus added tolerance based on linear inch of a given product feature. They represent typical variation from part to part once molded. For example, the thickness of a product molded from ABS will likely have a tolerance of +/- 0.13mm while the same feature on a part moulded from Polypropylene might have a tolerance of +/- 0.18mm
Producing Engineering Plastics
There are two main manufacturing processes to produce engineering plastics into sheets and in rods. The first is extruding. This is where the heated, molten polymer is extruded, or pushed, through dies or rollers. Because these dies or rollers can be set with a great degree of accuracy, the extruded material is fairly uniform in size and thickness, and the tolerances are much tighter. The application of some tolerances is necessary because of the shrinkage or contraction of the material after the extrusion process. This will happen in varying degrees depending on the polymer. In addition, rod material may not come out of the dies or rollers perfectly round. They will appear to be round to the naked eye, but there may be, in fact, some slight ovalization.
The other process is “casting” which normally used with Nylon 6. This process involves pouring heated liquid polymer into sheet or rod moulds and then allowing it to cure. This method is slightly more susceptible to surface imperfections in the rod or sheet since any marks on the mould will be transferred to the polymer. As a result, the size of the product increases. The post-manufacture shrinkage from casting is even less predictable than with extrusion so a greater tolerance is included.
However, there is a way around this. Sheets can be planed to a prescribed thickness on large planing machines. These are accurate to within +/- 0.25mm. For small diameter rod stock there is the alternative of ordering precision ground rod. This is achieved with a centreless grinding operation and the tolerances are incredibly accurate.
What else affects it?
Tolerance allowance also applies to the annealing process as well. As we have seen, extruded plastics are compressed through profile dies or rollers when extruded. The compression that occurs in the material is not relieved because the plastic “sets up” as soon as it comes out of the die and, consequently, remains in this compressed or highly stressed state. This high state stress can cause problems if not addressed:
- Materials will tend to warp and distort
- Physical properties will be different than published data (usually lower)
- Materials may crack
- Finished part dimensions may change randomly
In order to eliminate these problems extruded products are annealed. Annealing is a stress relief process, which occurs when the material is exposed to heat (air or fluid) above its glass transition point, which allows the material to “decompress” back to a relaxed state. As a result, the need for tolerances comes into play at this stage as well. If, during the machining process, significant material is removed, annealing is recommended to relieve machined-in-stress and minimize possibility of premature part failure. And finally there is the potential for slight blemishes to occur during transport, storage or cutting.
Putting It All Together
So what does this mean for the end-user of an engineering plastics product? Always allow for the fact that engineering plastics will always be oversize to a varying degree. Engineering Plastics are classed as “semi-finished” which means they are expected to be machine finished by the customer in the final process. If, however, you need materials to be more accurate before you work them, give us a shout about the grinding and planning options available.