High density polyurethane ?

Hi guys . If I am building my plugs with high density polyurethane, do I need any coat base before primer ? Or can I primer straight away on the high density polyurethane foam?

Do you think high density polyurethane will cut labour cost when it comes to fairing and filling compare to plywood or any timber plugs ? And will it be more precise and accurate compared to wood or timber ?


Thanks

 

What are you using to cut the plugs? There are always toolmarks. And tool time needs to be considered in overall cost and time of a plug. Have you considered spring back?

 

Will be cut with a cnc.
Springback ?

 

What is the tool head? Have you made any test cuts? CNC is NOT a smooth surface. SPRINGBACK is what happens on the surface from the cutting head. Have you ever made an example cut?

 

The answer really depends on the item being produced.You can certainly prime straight onto the tooling block if you intend to flatten the surface before the final coat.If the machine is fairly rigid and the finishing cut has a fine stepover you may find that a light rub with 240 grit paper will be enough to prepare for a final coat.With a coating like ACG CS713 you don't need a primer as it is fairly thick and will build well enough to work with.For pre-preg epoxy parts that were destined for a paint finish I have directly machined a mould and used 320 paper and then a wipe-on sealer with the necessary number of applications of semi-permanent release agent,such as one of the Frekote range.This gets the part off the mould in less time than taking the extra time to make a plug,then a mould with supporting structure and then a part and as paint is to be applied the primer for that paint gives enough coverage to deal with tiny blemishes.

Springback used to be a huge problem with polyurethane tooling block.Back in the 1980's and 90's one of the early versions was supplied in 1.5 metre lengths and if you needed to build up a blank with several sections,once you had taken a slice off a block it would have sprung about 20mm from a straight line.Which meant that you might have to add 35mm to the width that you cut so that one edge could be flattened and the other then cut parallel to it.The root of the problem appeared to be that the polymerisation of the block was exothermic and the resulting stresses were locked in-in much the same way that a cast metal piece has stresses locked in.Unlike metals those stresses weren't treatable and movement occurred whenever a chunk was removed from any portion.This didn't just apply in one direction and it was a shock to see how much a slab that had had a good deal removed would curl up when released from the machine.The options were to rough machine and then flatten a datum surface or to glue up a blank that was close to the finished size.Some of the glues used would leave a glue line that showed through a couple of coats of paint as they tended to froth as they cured, later developments improved this a bit.

if you have plans to work with tooling block regularly,it is well worth trying as many brands as you can find as there are big differences in stability and cost.For much better stability and for those willing to pay for it,epoxy blocks are much better.One thing you will find with all tooling block is that the dust produced by sanding will dry out the skin of your hands and a dry barrier cream will help,as will gloves.

Specific advice really depends on the nature of the product and it's size.

 

It also depends on what you call high density. I make demonstration pieces out of polyurethane from 4 pound to 70 pound.