Machining IJ2 Molds

Joanna and I machined the core and cavity molds for the IJ2 piece during lab today. Everything turned out nicely.

First we milled the cavity:

 Then we turned the core (leaving the center section for milling):

 And finally, we milled the core with ejector pin holes:

Our finalized molds fit together well.

 We had a lot of fun machining (and wearing goofy, colorful safely glasses):

We expect to test the injection molding on Monday 3/24.

Thank you to both Daves for their help!

Determining Thermoform Plastic Stretch

We are using 0.03” thick white thermoform plastic to make our piece. There are 6 pinwheel sections that will be raised during thermoforming, and each of these sections has a projected area of 0.964in². The total surface area of each raised segment will be 1.2218in² which means that the thickness of the thermoform plastic will decrease on average 21.1%. This means that the average thickness of the stretched plastic is expected to be 0.0237in.

The plastic does not stretch evenly and is expected to be somewhat thick on the top of the extrusion than on the sides. With this in mind, the mold has been constructed with a 0.0225in margin between the mold and expected edge of the extruded part. This is an upper estimate of the ultimate thickness of the plastic along the edges to allow re-machining if necessary once the prototype has been produced and measured.

The extrusions are 0.205in tall as the final height of the extrusions is expected to be 0.23in above the top surface of the mold. The thickness of the plastic on the top surface is expected to come out to about 0.027in and an estimated eventual 1% shrinkage from the initial molded height, the part should be meet the required dimensions at 0.23in.

Goal Maximum Height: 0.23”

Height of mold: 0.205”

Original Thickness of Plastic: 0.03”

Stretched Thickness of Plastic (top surface): 0.027”

Maximum Height (prior to cooling and shrinking): 0.232”

Maximum Height (after cooling and shrinkage): ~0.2296”

-> Expected part dimensions are within the 0.005” tolerance of the designed part dimensions

Overall Design Modifications to Improve Manufacturability

After determining that the smallest diameter of ejector pin locations would not fit under the pinwheel blades in our original design (as shown in our Mastercam mockup below), we decided to modify our overall design to make the triangles larger.

As our team progressed with making the mold, we realized that the previous design was too challenging to machine, so we made design modifications. First, we removed the center circle because the dimensions would be challenging to machine: the notch was big enough but too deep for the 1/32” tool. We then remodeled the top design such that it would look good with fillets for a 3/32” tool. The 3/32” tool was selected because it can reach a depth of 0.2812”, beyond the depth we need for our top features.

Overall, the design changes improve the manufacturability of our yoyo. Our team also finds the new model more aesthetically pleasing. The design changes are illustrated in the drawing below. 

The ejector pins also fit beneath our center triangles in this new configuration as shown in the Mastercam mockup below:

These design changes affected the IJ2 and IJ3 molds.

Group Meeting Notes 03/14/14

-We made sure to all use the same geometry since there are multiple revisions going around.

-Need to alter IJ2 design in order to accomodate injection pins.

-IJ3 has a weird anomaly where the mold plane comes out curved in Solidworks, need to fix.

-Due Tuesday: Mold, GCODE, justification, process plan.

-Machining announcement: Dropbox folder--> three folders 1) reports-->team folders 2) Mastercam Models and GCODE --> To be machined: Put GCODE and model in that folder (only the version ready to be machined) 3) Solidworks Models

-Tolerances: Reasonable tolerances can be found in link on first report, reference for tolerances. Make sure to include tolerances in drawings.

-Version 14 to be exported and used from now on due to update for IJ2 ejector pins.

IJ1 Shrinkage Measurements

Injection molding body piece (orange, multi-layered, drafted) measured to determine shrinkage:

Average mold diameter (over 3 diameter measurements): 2.507"

Min body diameter: 2.410"
Max body diameter: 2.440"
Average body diameter: 2.426"

Min Shrinkage: 2.8%
Max Shrinkage: 3.8%
Average Shrinkage: 3.2%

Given Dave's approximate recommendation of about 2-3%, we are incorporated a 3.3% shrinkage into our injection mold design.

Week 3/4-3/10

Created Dropbox folder and Google Drive folders

The Dropbox folder: (for official things)
Report 1 (Due 3/11)
Solidworks part files

Google Drive folder: (for "living" documents)
Gantt Chart (Sheet 1 organized by lab/report number and Sheet 2 organized by part)
Revision Log Document
Division of Labor spreadsheet (also contained in Report 1)
Contact Info
Meeting Minutes folder (with minutes from Meeting 3/4)

To access Gantt chart: 
https://docs.google.com/spreadsheet/ccc?key=0Am767zQWwCXtdHozV2QtcVQ0TlRZZjF6Q2E3RmJkckE&usp=sharing

Screenshot of in-progress Solidworks model
(Yoyo_v4.jpg)