Tech Talk Blog

If you have been machining metal for a while the change to plastics can be a little daunting. There are some tricks to the trade and some basic things you need to know about thermal expansion, speeds and feeds and the use of coolant.  The biggest thing to remember is that most plastics, especially those that are thermoplastic, will melt when they get hot enough. Thermosets won’t melt but can be brittle to machine so they are a totally different problem. Since heat is the culprit you must machine each plastic with the knowledge that it will grow, sometimes very rapidly, and then shrink again after machining. Some materials have to be machined once, normalize at room temperature, and then go back for final cuts. Sharp tooling, properly designed tools, speeds and feeds are all critical so there will be a learning curve. TriStar offers a “Machining Plastics” seminar and design manual through their website. For even more tips, Ask The Experts – that’s what they are there for! It’s not rocket science but there are tricks to the trade!

Question: How do I decide whether I should injection mold my plastic parts or machine them?

Answer: The fast answer is usually based on volumes. If you can justify the cost of an injection molding tool, which can run from a few thousand dollars to tens of thousands of dollars, then the decision comes down to some other key points.

1. Are the tolerances moldable? Depending on the material, types of fillers and geometry of the part  it may not be a part that is conducive to molding. Holding tight tolerances is difficult at best and may require machining.

2. The geometry of a part may require variations in wall thickness, i.e. heavy and thin sections across the part. Wall variations that are not properly blended could lead to internal stresses, distortion and eventual cracking.

3. Draft or taper is generally required in molded parts due to part ejection needs. If your part requires close straightness or parallelism it may not be attainable by molding but is through machining.

4. Internal stress is much less prevalent in machined parts since the stock shapes are stress relieved prior to machining. Additional stress relief can be done in mid or post machining if so needed. Stresses from molding are more prone to warpage, especially high temperature applications with high end materials.

5. Surface requirements of a part are better suited to a machined part over a molded part. Sprue/gate marks could leave a blemish or a flat spot that can’t be repaired. Molded parts could also have sink areas and weld lines that infringe on the part finish.

6. Design flexibility is much greater with machined parts versus molded. Once the tooling is made, changes to the design and subsequent tool modifications can get very expensive. Machining parts can also give you the freedom to change materials based on performance requirements. Since tools are designed for specific shrink rates it could also lead to major costs should a material change be needed on the part.

Machined versus molded? Definitely issues that need to be reviewed and TriStar is available to help.