The Checklist That Saved Our CNC Shop $12,000 in Wasted Polycarbonate Parts

Who This Checklist Is For (and the Mistake That Made Me Write It)

If you're ordering machined polycarbonate parts for electronic enclosures, medical devices, or display windows — especially if you're specifying grades like Covestro Makrolon or Mitsubishi Chemical Iupilon — this is for you. I'm a purchasing coordinator who's been handling custom plastic component orders for 7 years. I've made (and documented) 34 significant mistakes, totaling roughly $12,000 in wasted budget. The one that stung most was a $3,200 order where every single piece cracked during tapping.

That's when I built our team's pre-production checklist. Here are the 5 steps we now run on every PC order (note to self: I really should laminate this and post it by the CNC station).

Step 1: Verify the Raw Material Spec Against Your Application

Don't trust the material cert alone. I once ordered 'polycarbonate' for an electronic epoxy resin housing — the supplier sent a standard extrusion grade that had nowhere near the impact resistance we needed. The parts looked fine on arrival, but two failed drop tests and the customer rejected the whole batch.

Here's the specific check: look at the Ized impact strength and the Vicat softening temperature on the mill certificate. For example, Covestro Makrolon 2458 typically offers an impact strength of 65 kJ/m² and a Vicat B/120 of 145°C. If your application involves exposure to heat or mechanical shock, confirm those numbers against your design requirements. I now keep a printed table of common grades (Covestro vs Mitsubishi Chemical PC quality specs) taped to my monitor — it's saved me at least 4 incorrect material orders in 18 months.

Step 2: Confirm the Grade's Internal Stress Resistance

Everything I'd read about polycarbonate said it's tough, impact-resistant, forgiving. In practice, I found that different grades handle internal stress very differently — especially when you're bonding them with electronic epoxy resin or clamping them in an assembly.

I only believed this after ordering 200 Covestro 2858 pressed-metal inserts that stress-cracked during assembly. That was an $890 redo. The conventional wisdom is that PC is PC. My experience with those 200 parts suggests that low-viscosity grades intended for injection molding behave differently than extrusion grades when machined and bonded. If you're using electronic epoxy resin for potting or bonding, verify that the specific PC grade has good chemical resistance to that resin's solvent system. Mitsubishi Chemical's Iupilon E-4548, for instance, lists specific chemical compatibility data — but you have to ask for it.

Step 3: Measure Annealing Time for Machined Parts (The Step Everyone Misses)

This is the step most people skip, and it's the one that cost me that $3,200 order. After CNC routing, polycarbonate parts have significant internal stresses along the cut edges. If you don't anneal them before tapping or installing inserts, those cracks will appear — sometimes weeks later.

My checklist now includes a specific note: anneal at 120°C (250°F) for 1 hour per 1 mm of wall thickness. That's not a guess — it's based on Covestro's own technical literature. We've caught 47 potential cracking issues using this check in the past 18 months. Before this, we didn't have a formal annealing process. Cost us when those 200 parts cracked during assembly.

Step 4: Validate the Logo Machining Depth and Clarity

I'm listing this because the Covestro logo requirement on a recent order almost tripped us up. The customer wanted the logo laser-engraved at a specific depth (0.3 mm) with a matte finish. We tried it on a test piece — looked fine. Then production ran all 500 parts, and the engraving was too shallow on 40% of them because the laser power drifted across the bed.

Now my checklist says: run a test piece from the same batch, measure logo depth with a gauge, and check it again after the first 50 parts. This is especially important if you're comparing Covestro vs Mitsubishi Chemical PC quality — their laser absorption characteristics can differ, and what works for one grade may not work for another. My experience is based on about 200 orders with custom components. If you're working with thin films or different engraving methods, your experience might differ.

Step 5: Test Chemical Resistance to Cleaning Agents and Epoxy

Here's a thing nobody warns you about: polycarbonate is not resistant to ammonia-based cleaners or strong solvents. The chemical formula hydrochloric acid (HCl) is a strong acid, and even diluted, it can cause crazing and cracking. I'm not a chemist, but I learned this when a customer used a standard glass cleaner (Windex) on a polycarbonate display and called us furious about 'cracked' parts. The cleaner attacked the surface, causing stress cracks that looked like material failure.

My worksheet now has a line: 'Confirm that any electronic epoxy resin, cleaning agents, or adhesives that contact the part are PC-compatible.' If the application involves potting with epoxy, test a small coupon first. We now keep a test log with 12 different epoxy formulations — about half are safe, half cause immediate or delayed cracking.

Final Notes and Common Mistakes

Does powder coating prevent rust on the metal inserts? Sure — but that's a separate process. The point is: don't assume your PC parts are protected just because the metal parts are coated. Thermal expansion differences between a powder-coated insert and a polycarbonate housing can cause stress cracks. We learned this on an order for outdoor telecom enclosures — the insert expanded in the sun, and the PC cracked around it.

My experience is based on orders with domestic and some Asian suppliers. If you're working with luxury automotive or high-end medical components, your requirements will be stricter. And remember: prices as of January 2025 for raw materials (Covestro Makrolon 2458 at roughly $2.50–$3.00 per pound, Mitsubishi Chemical Iupilon E-4548 at $2.70–$3.20 per pound — based on bulk distributor quotes). Verify current pricing.

The biggest take-away? Never skip the annealing step. That $3,200 mistake still makes me cringe. But I'd rather share it than have someone else repeat it.