[P K Pillai (OP)]

How do I eliminate/reduce small air bubbles in a small hand injection moulded product, of  Nylon 6/6, during moulding ? Air bubbles weaken the part.

[CliffordK]

I haven't done plastic molding.  However, is it possible to mount your mold on a centrifuge and spin it during the molding process?  Allowing air bubbles to escape.

If your mold could be made airtight, you might also be able to put the mold under a vacuum before introducing the plastic.

Adding additional risers to your mold might also help a bit.

[chris]

For concrete you vibrate it to remove bubbles; can you vibrate your plastic while it's molten?

[teragram]

Many years ago I worked for a company that built injection and compression moulding machines. A variation on the compression machine was one intended for making synthetic rubber items (seals, o-rings etc.) This type of machine has its mould in a vacuum chamber, purely to eliminate bubbles in the material.
This information is probably of no use to you, and is presented for interest’s sake.

[chris]

Does working in a vacuum make the injection process more tricky? Does the rubber behave differently under those conditions?

[dlorde]

For concrete you vibrate it to remove bubbles; can you vibrate your plastic while it's molten?

I suspect that relies on the large relative difference in density between concrete & air, and possibly the suspended particulate nature of concrete assists it; I'm not sure it would apply with viscous plastics on a relatively small scale... some experiment required!

[teragram]

Does working in a vacuum make the injection process more tricky? Does the rubber behave differently under those conditions?

The vacuum machines we built were compression machines (material fed into the space between heated moulds, which were then pressed together at up to 400tons as I remember). As far as I know vacuum INJECTION machines, (mould closed and material heated to become plastic, then injected into the mould, usually by screw) were not common.
The difference between a vacuum press and an ordinary press was that the mould parts in the vacuum press were mounted in a two part vacuum chamber (box) which closed at the start of the process, with the air pumped out quickly.
So, yes, the injection process would be more tricky.
My input was in the electrical design of the machines, I had little knowledge of the behaviour of the materials.

[joshuaseigel]

The design process can be simplified by following a three stage approach where material, design and
fabrication decisions are made in parallel. SABIC Innovative Plastics engineers should be consulted during
the early stages of the design process. Additional information on each of the above steps can be found
in the LNP Speciality Compound Design Guide.

[stevenchengcn]

Hi,
I am a tooling and injection modling engineer,the solution depend on your part design:
1.is that part too thick which is main cause for bubble.
2.do you dry the PA before you use?it is very important to dry the material before using,you could refer: newbielink:http://www.myplasticmold.com/plastic-material-shrinkage-and-heating.html [nonactive]
3.the melt plastic stay in barrel for long time?
you could come to newbielink:http://www.myplasticmold.com/mold-design.html [nonactive] for more information of mold deisgn.