You did not say what type of knife you are using? Nor the section thickness you are working with/desire?
You can try softening the plastic by putting it into a moist chamber for a short time, but you should NOT touch the block if it is wet unless you wear nitrile gloves. Too much water on a block face usually means a section that is mush as it comes off onto blade.
When trimming the block, do not try to trim it as you would a paraffin block, the thicker the micrometer setting, the more likely you will have crumbled sections. Approach the block face with care, and not taking thick "trim" - a sharp blade/knife it critical for good trimming too.
GMA was developed for getting THIN sections, 3 um down to 1 um. You can section at 4 or 5um successfull, but the thicker you go may mean crumbled sections. We always sectioned with glass triangular knives, but people do have success with disposable blade, they must be VERY sharp, and probably a brand new edge after trimming or you can use tungsten carbide knives. There are triangular tungsten carbide knives available from Delaware Diamond Knives (DDK).
Finally, you may need to adjust the plasticizer content, i.e. increase the amount a bit in order soften the plastic or make it a bit more flexible,. The spec sheets that come with the kit should tell you how to do this, OR you can contact Poly Sciences tech services and ask them how to do this.
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