material suggestions on a submersible electronic housing

[rmetzger]

to be used in salt, fresh water, and discharge / sewer lines. The material of choice was Dupont Delrin 527uv as its accepted in the industry (UV, chem resistance, corrosion) but the molder is having difficulties resolving surface finish problems (splay, inconsistent texture, and rough finish on sealing surfaces) so we are looking elsewhere. The main issues seem to be the parts wall thickness creates long cycle times int eh tooling (7 minutes or so) that the molder is citing as a root cause of the problems. I am examining glass filled PP (10 and 20%) but have little experience with them and it would be something very new for this company and industry. Any other suggestions?

thanks

 

[patprimmer]

Filled PP will shrink a bit less than the Delrin if that is a problem.

It will not mould much faster, and may in fact mould slower.

I have seen Acetal mouldings an inch thick. They took nowhere near 7 minutes.

Splay, inconsistent texture and rough finish are all evidence of poor tool design and poor control of moulding conditions.

Drying the material will help.

A very large gate will help.

Ensuring the material entering the gate strikes a core so that it stops jetting and the mould fills from a flow front rather than a jet stream.

Ensure a cushion of melted material is maintained during hold up pressure.

Ensure that hold up pressure and time are sufficient.

Use a machine with a small barrel so that the residence time in the barrel is minimised.

A low compression ratio screw (about 2.0 to 2.2:1) with compression over about 1/3 the length. A PVC screw but with a ring check valve generally works OK.

Injection speed is a tricky compromise in that you need to fill quick enough to avoid chilling the surface before the mould is packed, but the velocity through the gate should be slow enough to avoid excessive sheer heat and to avoid splash and jetting. If slower injection still does not solve jetting problems, then increase speed to high so as to fill the mould quickly and pack it fully before the surface jells. This at least minimises the jetting marks as they are flattened by hold up pressure.

Increasing mould and/or melt temperatures will normally help produce a good finish, but with long residence times they will also increase the chance of degradation and out gassing which gives silver streaks, brown marks and inconsistent results from shot to shot.




Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.