Intricate mold cavity details polishing/finishing

Hello. I’m planning to print a mold core & cavity ((with Formlabs new High temp resin) for a part to be injection molded in ABS/PC.

My problem resides in polishing/finishing the mold details. While flat or large surfaces are quite easy to polish (water grit from 800 to 5000); many parts of the cavity and core are not easily accessible to be polished this way.

Has anyone there got some ideas about the process I could follow to get a smooth mold finish on all its surfaces (polishing compounds ? rotary tool ? Specific putty ?).

All the best from Paris

What do you want to polish off exactly ? Is it support marks or layer lines ? Or is it just that you have a nice surface but you want it even smoother ? What’s the indented surface finish for the plastic parts ?

Hi John. Thanks for your feedback.

I plan to use Formlabs High Temperature resin (RS-F2-HTAM-01) which withstands 290°C. Our intended injection resin is Bayblend (an excellent ABS/PC blend that injects at 260°C).

The positive (the final injected object) looks like a badge, with internal ribs and bosses, but the STL’s I plan to 3D print are the negative of that object (core + cavity), which should be polished and handed to my plastic injector.

There are no supports, because the core/cavity are printed direct off the bed. No polishing required here.

For the cosmetic side (the CAVITY), we’re currently wet sanding it in increasing order of grit and this is not a problem.

The aim of the polishing is to get an as smooth surface state as possible on the CORE side of the object (the non cosmetic side) in order to have it ejected easily without sticking out of the mold. Ideally, we would need a glossy finish, but someting like satin would do it perfect. Polishing here is not a cosmetic requirement, just a technical requirement linked to standard injection processes.

Your ideas are welcome !

I’ve done a few mold details for a hobby machine and have had some good luck.

Most standard polishing methods work well. Note that any polishing compound that you use should be for plastic. I use a very small air brush sized bead blaster to even out any layer lines and get an even surface finish to start. From there you have to hand work everything with small stones or files. It’s not fun.

Also note that printing on the build plate can/(will) cause a couple of issues. The top and bottom are likely not to be parallel due to build plate tilt. Compression during the first 3mm or so also causes some dimensional issues. I would recommend fly cutting the back side to get the detail to height and make it parallel. Make very thin passes and there will be some chipping at the edges.

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I see. Being in industrial product design I am familiar with injection molding too. So you mainly want to smooth some surfaces to facilitate the part extraction, but will it really solve this issue ? Extraction issues are mostly dealt with with drafts, and 2-3° will go a long way IMHO since the prints with the Form 2 are pretty smooth out of the box.

If visual aspect isn’t important, would also have gone they way @FredB describtes : plastic polishing compounds with a handheld motor tool (dremel) and a cloth bit small enough to get into the cranies. The downside with this method is that it may leave a marbled aspect to the molded part.

If you want to ensure that the parts eject from the mold try a light spray of mold release (the stuff used for casting polyurethane). It greatly reduces the force required to eject the parts. A very light coat makes a huge difference.

I haven’t had issues with part ejection with the finish right off the machine. A longer print at 50 microns will leave a reasonably good finish for your non-cosmetic areas

An air eraser (air sander) would probably work OK, you could also use a Dremel with an abrasive wheel

1- printing flat on the build plate will most likely make for a very bad injection mold with layer lines far more prominent than other orientations.

Think about a COIN… with its low relief… laying it flat on the build plate will put MOST of its flat, nearly horizontal surface area very close to parallel with the X-Y plane… this will cause the layer lines to be most severe on those surfaces you wanted close to smooth, requiring the most effort to polish down.

Keep in mind that the Form 2 prints a smoother line in X-Y than the layers are in Z… so any orientation that places more of the critical surface in near parallel to the X_Y plane will make the layer lines in Z look more like the topgraphic lines on a terrain map.

Secondly, your difficulty will be in polishing the nooks and crannies in the cavity- the flat exterior planes of the mold will be a snap.

The Air Eraser is a good Idea… but for the small interior corners you will need something like a dremel with the soft
cotton point heads that you can load with polishing compound.

Will you be Using Ejector Pins?
Is your separation line going to be perfectly flat? or does it have some contour the two halves have to mate along?

hi scultingman and many thanks for your skilled suggestions. reading the various (enlightening) replies to my initial post, i decided to go for a microblasting unit (Renfert Basic quattro + glass beads) as well as plastic polishing compounds from Menzerna applied with dremel softcloth tools (GW16 > 495P > G52 grades).

Applied on Formlabs printed tabs (Dental Model, Black, Tough, and White resin), the process was straightforward and results where stunning. II got mirror finish where I wanted it (masking tape), with some labour though to be honnest.

Now, for the mold… I have partnered with a plastic injection company here in France , and we are busy manufacturing (CNC machining) a specific aluminium frame + ejectors to host the 3D printed formlabs hightemp resin core and cavity. We’ll mount this frame in a Babyplast injection machine (a highly qualitative screw injection machine) for our first plastic shots (Bayblend T65XF, ABS/PC blend).

The mold joint is not flat, rather contoured to follow the round shaped design of our product. We plan to use an injector battery of 5 ejectors/vents for each part. No cooling circuit for now. The two molded parts are 80x60 mm (3.1 x 2.4"). Approximately 8g (0.25 oz) each.

I admit, this is pure experimentation and results are quite “uncertain”, but the great benefit (if success), is that we’ll have some real plastic parts in hand, accounting for iplastic design constraints (ribs, drafts, bosses, grooves… and sink marks !), and with these in hand, we’ll be ready to carry-out ultrasonic welding assembly tests (20 khz) with embarked electronics for failure testings.

Then, and only then, we’ll go for big business (ultimate design refinements, steel mold, big machines and the like)

I.m currently documenting (photos)) all the steps involved and will post a full paper in a few weeks to share with the community.

kind regards


You might need to reconsider the cooling circuit.
Injection machines have high thruput… that’s like the barrel of a machine gun- with a rapid high temperature impulse at relatively high frequency.

Metal tooling takes advantage of how rapidly a metal block can conduct heat away from the mold surface… but resin mold surfaces are effective insulators… they do not conduct heat well and act as heat sinks.

I think you will find the heat in the plastic mold surface builds up rapidly to the point of degrading the resin part- but of greater concern is Pyrolysis. If the part gets above a certain temperature the resin might outgas poisonous fumes- urethanes, for example, give off cyanide gas above 350 degrees F.

Given the abilities of the printer- I think it wise to design in place a cooling circuit if possible… you will want to be able to dissipate the heat of each injection shot fast enough to keep the resin print within its tolerance for greater mold life- but ALSO for dimensional stability. As the resin soaks in heat- it may very well swell or warp.

Is there a reason you want to try and inject plastic parts- rather than just print the prototypes on the Form2?
About the only reason I can think of for going the route you are is that you are trying to iterate injection mold designs rather than parts designs.

I just printed a small insert with the new rigid material and it looks pretty promising.

The post above does bring up a good point. You will need to develop both the mold set-up and cycle parameters based on the printed mold.

You’ll probably need to shim up the cavity a bit because it isn’t flat. A bit of an interference will ensure that the core and cavity press completely closed.

You’ll also want to experiment with the injection pressures. In the end you want just enough material to pack out the mold. Too much extra and you risk breaking the print.

The printed cavities are not a very good conductor of heat so most of the time there are not cooling lines in the inserts (makes them weaker, can cause splitting). The fix is to increase the time between cycles. I use fans to cool down my little hobby molds.

I’m assuming that his reasons are the same as mine. Printed parts do not have the impact strength of molded parts. If you need to do validation testing it’s always best to have the correct material and process similar to production.

You simply can’t get the impact strength of polypropylene with any Formlabs material currently available.