Surface Finish and Material Selection for Plastic Injection Molded Part to be Painted 3

[goldenfab]

I'm working on the design of a plastic part that will be painted black in a matt finish. What should I specify for the surface finish? SPI C-1? The reason it being painted is because its a backlit display panel so it will need to be translucent to carry light to text and graphics. Clear part, painted white, painted black, laser engrave black to reveal text/graphics. Since mechanical properties like strength isn't an issue I was thinking clear ABS. Paintability and good adhesion and a good looking part with minimal shrinkage making unsightly sunken areas are priorities. Should I consider another material?

Thanks,
Adam

 

[btrueblood]

Clear ABS? Do you mean SAN, or MABS? MABS is like ABS, but has methyl acrylate and a matched optical property for the rubber (nitrile) phase.

Straight acrylic takes paint well, and can be had in optically clear or dyed translucent/transparent colors. SAN is pretty robust stuff too, though it may have a slight yellow tint.

MABS will probably be the tougher alloy from an impact standpoint...but do you need that?

 

[goldenfab]

I don't know much about SAN vs MABS. I'm not really a plastics guy, yet... I've made panels before from machined acrylic and now I have a requirement for a panel with a shape that doesn't lend itself to machining very well and I've found myself in the midst of finalizing the design on an injection molded part. From my quick internet searches it seems MABS is the ubiquitous form of clear ABS? Impact resistance isn't a big deal for my case. I don't have a requirement for optical clarity either. It just needs to carry enough light from LEDs positioned behind the panel to illuminate white painted text/graphics on the front of the panel.

My other concern is sunken-in areas resulting from uneven cross sectional areas in the part making the flat front of the panel unsightly. I'm guessing acrylic will be better to mitigate this?

 

[3DDave]

Sunken areas are from shrinkage which can be handled with careful mold design. The cause is the large coefficient of thermal expansion for plastics and that the molded part surface cools and shrinks first, but the core is incompressible, and afterwards the thicker portions shrink but don't have any reservoir of material to make up for the shrinkage and pull the cooler material down with them - I doubt that a minor change in polymer will eliminate that effect.

I think that mold designers can design a mold to keep pressure on those areas/volumes and force other places in the part to fill in the shrinkage while preventing that from pulling the cosmetic surface down. Gates, runners, cycle, and other features of the molding process, all affect how the part turns out - whether shrink or knit failures or any other of a multitude of potentially unacceptable outcomes, almost none of which you can control with the polymer selection or the part design.

What you can control is the requirements. After that it's up to the mold designer/maker/injection molding company, to meet those requirements or tell you which parts of the design will be unexpectedly expensive and suggest changes.

 

[btrueblood]

"Impact resistance isn't a big deal for my case. I don't have a requirement for optical clarity either. It just needs to carry enough light from LEDs positioned behind the panel to illuminate white painted text/graphics on the front of the panel."

I'm fairly confident that the plain acrylic, or even plain styrene, will give you reasonable clarity at lower cost than the MABS...but that is an easy question for your molder to answer.

"My other concern is sunken-in areas resulting from uneven cross sectional areas in the part making the flat front of the panel unsightly. I'm guessing acrylic will be better to mitigate this?"

3DDave gave good answers on this. Part of the sink "problem" is how long the mold must be held closed before the part finishes freezing, which affects mold cycle time and resulting part costs. Again, I'd take your molding shop's advice...but I think (and mind you am no expert) the relative differential of melt temperature to mold temperature will affect that timing, and that differential is higher for MABS than the other two, meanining it may take longer for the MABS to finish freezing.

 

[cowski]

The "sunken in areas" are often referred to as "sink marks" or just "sinks". You can help minimize sink marks with your part design. The general rule of thumb for highly polished surfaces (such as your panel) is that the rib thickness should be no more than 40% of the panel thickness. Bear in mind that if the rib gets too thin, it won't fill properly. The minimum rib thickness is largely controlled by the material used.

There is software available that can simulate injection molding a part. If you are working with a toolmaker/molder, ask if they can do a fill analysis on your part. It can help identify potential problems such as sink marks, warp, cold flow, etc.

 

[PentechMoulding]

Hello

Just joined this forum.

With past experience of manuafcturing Infra red white opaque lens, We have always used Clear Polycarobonate or Acrylic and colour to suit the appliaction but you can have parts moulded in clear with good polished surfaces for the paint to adhere too.

Ensure to use suitable paints which are comaptible with plastics or if you are going to have mass produced parts it is advisable to have the parts moulded in the colour to your sepcification or have them prodcued in clear and have them powder coated.

Trust this helps.