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

Richard H. Siderits MD

A. Maskey

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.

•     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"

•      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.

•     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.

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.

•      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/

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.

•       www.warmplastic.com

•       http://www.castcraft.com/index.htm

•      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. 

•     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.

•     www.polytek.com

 

•     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

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.

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

•      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.

 

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

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

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

MAXNC 10-CL CNC Milling a Plastic Blank

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.

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.

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

The End...

Thanks!