Rapid Technique: Preparing Plastic CNC Milling "Blanks" for Parts Used In Prototyping Scientific Equipment




Introduction: Rapid Technique: Preparing Plastic CNC Milling "Blanks" for Parts Used In Prototyping Scientific Equipment

Rapid Technique: Preparing Plastic CNC Milling "Blanks" for Parts Used in Prototyping Scientific Equipment

Richard H. Siderits MD

A. Maskey

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Step 1: What Is a Milling "Blank"?

When milling a part for a prototype piece of equipment, it is often useful to start with a piece of material that is not much bigger than the part that you are making.  This is called a Blank.  Using blanks, especially with Computer Numerical Control (CNC) systems, can reduce the amount of time needed to clear excess material during the milling process.

We describe a method for making plastic milling blanks for CNC milling operations and for making a "Jig" that helps place holes for conveniently mounting the blank onto the milling machine's table.

Step 2: Basic Approach: Make Mold Blanks

Make a model of the desired shape for the plastic blank.  Use MDF type wood.

Vacuum form a mold onto the MDF wood model.

Trim the vacuum formed mold.

Coat the inner surface of the mold with a release agent.

Mix 2 part plastic and  pour it into the vacuum formed plastic mold.

Extract the blank from the mold ("de-mold").

Attach blank to CNC Milling machine.

Additional technique:

Making a "Fixing-Jig"

Step 3: First: What Is MDF Wood?

Medium-density fibreboard (MDF).

Engineered wood.

Mixed with wax, resin at high temperature and pressure.

Stronger and denser than plywood.

 Contains: urea-formaldehyde so it may irritate lungs and eyes - Wear mask when cutting or sanding.

Step 4: Making the Model:

Cut MDF wood to size of required "blank".

Cut edges of wood at
a 15 degree angle to help the mold pop off.

Sand the edges smooth with sand paper.

Remember to use the measurements on the top (smaller) half of the model as the minimum area for your milling blank.

Step 5: Next Step: Create a Mold for the Blank

There are several ways to do this.  We chose to make a vacuum-formed plastic sheet mold.  We then poured two-part plastic resin into the vacuum formed mold in order to create the blank for milling.

Using this technique, it is possible to create complex blanks that may have very unique shapes.  This makes it possible to decrease the amount of material that needs to be removed by the CNC milling process.

The platform that we use is a Home hobby vacuum forming unit that was easy to build and very inexpensive.

Step 6: Building a Hobby-Vac Vacuum Former:

Hobby-Vac 12"x18"
Do It Yourself Vacuum Forming by D. E. Walsh

This comes with a large and small platen for vacuum forming small parts and larger trays.

Build time averages about 2 weeks.  Instructions are clear and friendly.  

Requires a vacuum pump.

Or take a look at: https://www.instructables.com/id/Make-a-good,-cheap,-upgradeable-sheet-plastic-vacu/

Step 7: What If I Don't Want to Make a Vacuum Forming Machine?

Try visiting the local dollar store, toy store, or even a grocery market and find something that is about the size that you need, comes packaged in a formed clear plastic shell and use that instead in order to pour your plastic blanks.

Step 8: Where Do I Get Plastic?



Can be ordered in many sizes, colors, and thicknesses.

Sheets vary in size;
up to 4 feet by 8 feet.

We used 0.04 thickness which is rigid enough to hold the shape and resistant to the amount of heat given off in the exothermic reaction of the plastic resin polymerizing. 

Step 9: What Is a "Release Agent"?

This keeps the two-part plastic mixture from sticking to the plastic mold.

It can be a spray-on product or simply Vaseline wiped on the inner surfaces with a paper towel. 

Make sure to get the release agent into the corners of the mold.

Step 10: Where Do I Get "Two-Part Plastic Resin"?


EasyFlo 60
1 part A to 1 part B
(equal amounts)

Cures white in ten minutes, will take pigments for color.

Hard as machinable wax

Water thin and fairly inexpensive

Step 11: Vacuum Forming

Top Left: Vacuum forming "Platen", Top middle: MDF wood models under vacuum formed plastic molds.  Top right: Clear plastic vacuum formed molds for blanks.  Bottom left and right: Cutting out molds to cast the blanks for CNC milling.

Step 12: Examples

MDF wood models, vacuum formed molds, and white plastic
blank with fixation holes for mounting on CNC milling table.

Step 13: Making the "Fixing Jig": Materials

Two  3/4" x 5 1/2" x 11"
Top and Bottom

Three 5 1/2" x 2" x 3/4"
Dividers and Side Supports

Two 3/4" x 3/4"
Alignment Ridges

Two 5 1/2" x 2.0" x 3/4"
Standoffs for Top

Carriage bolts 3" x 0.25"

All parts are pine or, if you prefer, MDF wood.  Assemble as shown in the CAD model.  Drill holes for bolts to slide through snugly.  Place the holes to align with the corners of your unique mold for the blanks.

Step 14: Example of Fixing-Jig

Top and Bottom Half of Fixing-Jig with a Mold and Blank

Step 15: Separation of Mold and Plastic Blank

Separate top and bottom half.  Note mold and plastic blank have separated.  Alignment bolts will unscrew to remove.

Step 16: Releasing the Blank

Turn bolt with a wrench to release blank, leaving behind holes to affix the blank onto the milling table.

Step 17: CNC Milling a Plastic Blank

MAXNC 10-CL CNC Milling a Plastic Blank

Step 18: Project Summary:

When prototyping parts for experimental scientific equipment, it is often useful to have custom plastic blanks that are suitable for use in CNC milling operations.  

We have outlined several methods that can be used to create molds for making plastic blanks and a jig for placing fixation points for CNC milling operations.

In summary, MDF wood models are created to desired measurements of the milling blank.  The wood model is then used to create a vacuum formed mold of the wood model.  The mold is used to create the blanks from two part plastic resins.  A vacuum degasser may be used to assure absence of bubbles or void spaces in the plastic milling blank.

Step 19: Disclaimer:

Follow all safety guidelines, including but not limited to:

1) Using two part plastic polymers and vacuum formers.

2) Powertools that you never read the instructions for.

3) All tools that you never learned to use properly.

3) Anything that heats up, pinches, cuts, squeezes or
    causes traumatic, caustic or thermal injury.  

We are sharing our experience, not telling you to do it.  

If you choose to try this then - it is at your own risk!

No really, we're not kidding about this.

Step 20: Project Details

Facility: RWJUHH Investigational Pathology Division and
             Center for Parabiotics Research

Section: Special Projects 

Application: CNC Milling - Preparing Plastic Blanks

Technique: Rapid Prototyping in laboratory science

Title: "Rapid Technique: Preparing Plastic CNC
Milling "Blanks" for Parts Used in
Prototyping Scientific Equipment

Project Team: R. Siderits and A. Maskey

Step 21: The End, Thank You

The End...


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    6 Discussions


    9 years ago on Step 19

    I found your chalkboar written instructable easier on the eyes to read. It is hence forwith declared all instyructables should be required to use said chalkboard fonts . I am serious we should adapt to change this is a definater improvement


    Wow.  That's a very attractive Instructable with a lot of good information.  Thanks.


    10 years ago on Step 17

    chips do waste in materials that are not reusable but anything polyethylene is all you need to reuse would be heat. it melts very easily  the only exception I have found for this is UHMW and to do that you need a controlled environment.

    though that is not impossible either. I have scavenged both my vacuum cleaner (I hooked a shop vac up to collect dust from my mill) and the train tracks (they haul poly in big tankers as tiny pellets of various sizes and there are a couple poly container companies here so a ton of this stuff comes in for them to make 55 gal and other size barrels as well as underground containers like sewage and water retention for reuse (like to water lawns and other irrigation needs.) and the trains after being dumped always have a bunch in the small bent tubes that let it come out. usually those tankers are left open and one car will yield a whole grocery sack full of pellets of virgin unused polyethylene that can be heated with a heat gun into a mold or just on a flat surface. though you have to use silicon spray release agent on the mold or surface to get it off of the surface.) doing this on say a 1/8th inch thick aluminum plate though makes a great product for ridged plate with the poly being a slick bearing surface for any sort of roller activity.


    10 years ago on Step 17

    wow I am sorry to have to say but I am not seeing anywhere that all of this work could not replaced with a good set of layout tools and a decent cheap band saw. and I see plenty of places it could not be used that the band saw could.

    like if I wanted to machine real world usable parts from delrin or UHMW polyethylene. both of which are far superior to your two part plastic and a lot cheaper if you just drop by a local plastics custom parts maker. *they have bins full of the smaller pieces that come off the 8 by 4 foot sheets they cut parts for customers out of. I buy delrin at about 3 dollars a square foot by quarter inch thick half foot by half inch and so on. same price for just about any polyethylene. and sheet acrylic for about 2 bucks or less per 2 square foot at 1/8th inch thick. both in clear and transparent colors(opaques are the same but I am using my lathe to make lenses for goggles out of it.)

    PS&F is who I go through 918-622-8423 and if they will not ship let me know I will grab anything you get a quote online from them for and ship it for you. just pay the shipping to me to ship it priority mail to you.

    you can find me at www.borgsteamfactory.com just use my contact page

    and no I have no other affiliation to PS&F other than the lady who I deal with there is super nice and everyone else there has been extremely helpful.


    Reply 10 years ago on Step 17

    Thanks very much for the info!

    You are absolutely right. 

    I just wanted to give people an introduction to the procedure with the most basic applications and example resources (especially for high impact material/resins). 

    The great thing about Instructables is that people (like you) with experience can share both practical real world criticism and valuable insights.

    When I use this procedure, its for blanks that have much more complex shapes.  I can just finish cut for detail  on the CNC platform.  It keeps me from wasting a ton of material that would be lost to chips.  

    Of course, only now do I remember to mention that "part".

    Thanks again for the info.


    Band saws rule.


    10 years ago on Introduction

    The slide format would work much better as a slideshow. To embellish the slides like this is to mix two different formats, it would look good as slides or an Instructable, but the hybrid is a bit odd. (This would apply to the rest also)