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How to cut PE products in shape and mold release

POLYMEER polymeergermany

How to cut PE products in shape and mold release

17/06/2019 at 15:12

Hello dear recycle heroes,


Here is Magdalena from POLYMEER, we are a small recycling project with a compression machine in Kiel, Germany (https://www.facebook.com/polymeer.official/). We now want to produce the first products, in our case flower pots. Unfortunately, all this is a little more difficult than expected, because we don’t get the product out of the mold easily. After we greased the mold with a mold release wax several times, heat and press the granules in the form and finally quench the hot shape with cold water, we always get big problems when we want to release the product out of the iron mold.

We work with LDPE, which unfortunately is much more difficult here than PS, for example. With knives we always try to release the mold from the product, but this always leaves traces on the product and also our iron mold carries scratches from it. Even if we use only a little bit of granules, the product “sticks” so tightly to the iron that it takes forever for us to get it resolved and remove the product from the mold. Does any of you know the problem with the compression process and how do you deal with it? Which mold release do you use?

Another thing that causes us problems and where we may be able to draw on your help and experience is smoothing the edges. When we have finally managed to release the flower pot from the mold, we usually see that the granules have spread slightly unequally, so that one wall is a few centimeters lower, for example, than another. That can also look quite nice, but we would rather find a suitable solution to shorten all sides to the same height. We have already tried a hot wire, but this warps the shape of the product, with a cutter knife one does not get far, because the wall thickness is 0.8 cm and the circular saw does a good job, but there is only one which we can use in a wood workshop. And we would rather avoid all the microplastics coming into the wooden container when vacuuming. Therefore, we are still urgently looking for a solution on how to cut the edges. What do you do in such a situation when the plastic is pressed too high up and you have to cut off parts? Do you have any experience or any tip that we should try?

We would like to benefit from the community knowledge and are very happy about any response from you!

Lots of greetings

Magdalena von POLYMEER

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18/06/2019 at 13:19

Hi Magdalena!


There are two commonly used ways to do this much more easily. Compressed air and extractor pins.


Compressed air:

Works better with injection moulds than compression. Drill a small hole through the core (part which forms the inside of the pot) from the bottom all the way to the top. If you can taper the hole or at least the start of it (taper reamer), wide end at the bottom, this will help. Blow compressed air into the top of the hole and the part should pop off easily. Ideally you want to prevent the plastic going up the hole as much as possible which is why it should be small and works better with injection as the plastic cools and solidifies before it goes to far.

Extractor pins:
Again you have holes drilled through the core and tightly fitting pins which are flush with the bottom surface. Push the pins out to push the part off. An easy low-tech way to do this is to use bolts with the ends turned down and thread the top part of the hole. Just screw in the bolt to push the part off. Works great with compression or injection, I use this method a lot. In this video you can see the heads of the extractor bolts (cap screws) at the top of the core, I found usually I didn’t need to use them as the mould is quite smooth and the part comes off easily, so I didn’t show this in the video.


To avoid the uneven top edge/flash on the parting line, what I do is weigh the amount of plastic going into the mould so I can get the volume exactly right every time.

26/06/2019 at 13:29

@andyn thank you very much for your detailled discription! We will try out the second method! 🙂

09/09/2019 at 18:50

Only thing that I have found that helps is silicone spray.  Silicone can handle the very high temps and it definately helps with release, but sometimes plastics come over top (under pressure) or snag on some imperfection in the mold and its very hard to remove.  In that case I sometimes have to use heat gun to heat up the mold (which can also be used to smooth out minor imprefections in your product too!

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