Case Study: Eliminating Paint Cracks in ABS CNC Machining Parts

Introduction

We delivered 100 pieces of ABS CNC machining plastic parts to Mike, an electronics-design engineer. Shortly after delivery, he reported hairline cracks in the paint layer. Actually, we had inspected each part before shipment and found no defects. Determined to solve the issue quickly, we performed a thorough root-cause analysis and then implemented targeted improvements.

Project Brief

Project Info	Details
Material	ABS
Processing	CNC Machining
Surface Finishing	Painting
Client	Electronics Industry
Problem	Painting Cracks

Why Choose Painting?

Painting is a common surface finishing technique for plastic parts. It has a wide range of benefits.

Enhanced Aesthetic Appeal

For Mike’s electronic device housing, the key advantage is the wide-ranging color options and high gloss. It not only gives the product bright colors but also adjusts the gloss level as required, from high-gloss to matte, catering to different design styles.

Moreover, it can effectively cover surface flaws like scratches, air holes, and weld lines that may occur during plastic part formation. It makes the product surface smoother and enhances its quality and appearance.

Improved Protective Properties

Painting also boosts the ABS’s protective properties. In the electronics industry, durability is crucial. The painted coating forms a protective film on the plastic surface, shielding it from air, moisture, and corrosive substances, thus preventing damage from oxidation and hydrolysis and prolonging the product’s service life.

And the hardness and toughness of the sprayed layer enhance the product’s wear and scratch resistance, ensuring it remains in good condition during frequent use.

Root-Cause Analysis

We recreated the customer’s conditions and reviewed each step of our painting finishing workflow:

Material–Coating Compatibility

The original lacquer exhibited high hardness but with low elongation. ABS plastic naturally expands and contracts with temperature and handling. The rigid paint could not follow these movements, generating microscopic stress that ultimately fractured the paint skin.

Spray Application Uniformity

Automated spray guns delivered too much material at edges. In these stacked areas, thicker wet film took longer to cure internally. The surface skin hardened first, trapping solvents below. As the solvents later evaporated, internal pressure caused the skin to split.

Curing Conditions

We relied on ambient air drying. Warm temperatures in Beska’s workshop (around 25°C and 60% humidity) led to inconsistent solvent evaporation. Some of the areas dried too fast under the lights, while shaded zones retained moisture. These uneven cure profiles exacerbated internal stresses.

How Did We Handle It?

After reviewing every step, we had a comprehensive improvement plan with 4 steps as below:

1. Upgrade Coating System

We switched to a UV-curable flex-finish lacquer designed for ABS material substrates.
Added plasticizer and anti-crack agents to boost its elongation and adhesion.

2. Refine Spray Parameters

Reduced spray pressure from 2.5 bar to 2.0 bar.
Lowered flow rate from 1.2 mL/s to 1.0 mL/s.

Adopted a multi-pass light-coat technique, limiting each pass to 15–20 μm, especially around edges and corners.

3. Control Low-Temperature Curing

Installed a circulating-air oven set to 50–70 °C.
Tailored dwell time (20–30 minutes) to paint thickness.

Through humidity monitoring, it ensured even airflow to prevent localized hot spots.

4. UV-Protective Topcoat & Enhanced QA

After applying a thin UV-resistant finish, it cured in under five minutes, further strengthening the chemical and scratch resistance.
Instituted three inspection stages: pre-spray, post-spray, and post-cure.
Introduced cross-hatch adhesion tests to verify coating integrity before approval.

After taking these 4 steps, the final ABS CNC machining parts look much better than before.

Result & Feadback

What is the final result? We found that there are zero defects: No cracks, orange peel, or adhesion failures appeared even under bright light. After painting, 200 pieces of ABS parts displayed consistent gloss and color accuracy.

What’s the response we received from Mike?

“The reworked ABS CNC machining parts even exceeded my expectations. The finish is flawless and durable. Thank you for your rapid response and expertise.”

After receiving the e-mail from Mike, we couldn’t wait to share it with our production team. And said a big thank you for their efforts.

Conclusion

This project reinforced three key principles for plastic-part spray finishing:

Match paint flexibility to substrate movement.
Control coating thickness with precise spray settings and multi-pass layering.
Ensure consistent curing through temperature and airflow management.

To prevent future issues like this, we’ve updated our process documentation: every painting job on ABS or other materials now uses only approved UV‑curable lacquers, follows the new spray‑parameter guidelines, and passes our three‑stage QA protocol. According to the reports, we had never encountered issues like this happening again.

Beska Mold

What do you think of this case study? Have you ever encountered similar problems? We’d like to hear from you. What the Beska team can do is beyond CNC machining and surface finishing. Check out the details to see what else we can do for you!