How to Properly Cut Parts from the Runner Without Damaging Them

One of the most common mistakes new model kit builders make is cutting parts from the runner incorrectly. Whether you're assembling a Gunpla, a third-party kit, or a resin figure, how you remove parts from the sprue can make or break your final result. A careless cut can leave unsightly stress marks, create deep nubs, or even snap a delicate piece.

In this guide, we'll walk you through the best practices for cutting parts from runners, from choosing the right tools to mastering the two-cut technique. You'll learn how to avoid damaging parts and achieve a clean, professional finish that makes painting and assembly easier. These skills are essential for anyone working with kits like the Motor Nuclear MNP-XH01 Bai Qi (White Tiger) Metal Frame Model Kit or the SNAA 1/100 YR-04P Fire Lord.

Why Proper Cutting Matters

When you cut a part from the runner, you're not just separating plastic—you're creating a stress point. If you cut too close to the part, you risk gouging the surface or leaving a deep indentation that's difficult to fix. If you cut too far away, you leave a large nub that requires extra sanding, which can damage surrounding details.

Proper cutting techniques also prevent stress marks—those white or cloudy areas that appear when plastic is stressed or bent. These marks are especially common on soft plastics and are nearly impossible to remove once they appear. By using the right method, you can keep your parts pristine and ready for assembly.

• Stress marks weaken the plastic and can lead to cracks over time.
• Clean cuts make seam line removal easier and improve part fit.
• Good cutting habits save time on sanding and cleanup.

Choosing the Right Tools for Cutting Parts from Runner

The most important tool for removing parts from a runner is a pair of sharp side cutters (also called nippers). Avoid using scissors or craft knives, as they can crush or tear the plastic. High-quality side cutters have a single bevel edge that slices cleanly through the gate without applying excessive pressure.

If you're working on a kit with very fine or brittle parts, such as the Eddas Industries 1/100 Valkyrie WF2025 Expo Version Metal Frame Model Kit, consider investing in a pair of God Hand or similar precision nippers. These cutters are designed to make flush cuts with minimal stress, which is crucial for metal frame parts that may have thin sections.

• Always use sharp nippers; dull blades crush plastic and cause stress marks.
• For tight spaces, use a hobby knife to carefully cut the gate away.
• Have a sanding stick or file on hand for final nub removal.

The Two-Cut Technique: The Gold Standard

The best way to cut parts from a runner is to use the two-cut method. First, cut the part away from the runner leaving a small nub (about 2-3mm) attached to the part. This first cut should be made at a distance from the part to avoid any accidental damage. Then, make a second cut closer to the part, but not flush with the surface—leave a tiny bit of excess to sand down.

This technique minimizes the stress on the part because the second cut is made on a smaller, more stable piece of plastic. It also reduces the risk of cutting into the part itself. After the second cut, use a sanding stick or file to remove the remaining nub, working from coarse to fine grit for a smooth finish.

• Never cut directly flush with the part in one go—this can cause stress marks.
• Hold the runner steady with your other hand to prevent slipping.
• For curved surfaces, angle your nippers to follow the contour of the part.

Special Considerations for Different Types of Kits

Different kits have different runner designs and plastic types. For example, Bandai's Gunpla kits often use under-gated runners where the gate is hidden on the underside of the part. In these cases, you can cut closer to the part without worrying about visible nubs. However, for third-party kits like the Wolf Technologies 1/100 Wolf King Metal Frame Model Kit, gates may be thicker and require more careful cutting.

For metal frame kits, such as the Cang-Toys 1/100 GMS-004 Odin (Deluxe Ver.), the plastic is often harder and more brittle. Always use a fresh blade and cut slowly to avoid chipping. For kits with clear or translucent parts, be extra cautious—these plastics are prone to stress marks and cracks. Use a sharp cutter and consider making a single clean cut if the gate is small.

• Under-gated parts allow for flush cuts without sanding.
• For clear parts, use a hobby knife to score the gate before cutting.
• Metal frame parts may require a different cutting technique due to hardness.

Common Mistakes and How to Avoid Them

One of the most frequent mistakes is cutting parts from the runner while they are still attached to the sprue frame. This can cause the part to twist or flex, leading to stress marks. Always cut the part free from the runner first, then trim the gate.

Another mistake is using a dull blade. A dull blade will crush the plastic rather than slice it, leaving a rough, white edge. Replace your nippers' blades regularly or invest in a high-quality pair that can be sharpened. Finally, don't rush—take your time to position your cutters correctly and make deliberate cuts.

• Avoid twisting the part while cutting; this causes stress marks.
• Keep your work area well-lit to see the gate clearly.
• Practice on spare runners to perfect your technique.

Tools and Accessories to Enhance Your Cutting Experience

While a good pair of nippers is essential, other tools can make the process easier. A cutting mat protects your work surface and prevents your tools from dulling. A hobby knife with a sharp blade is useful for cleaning up small nubs in tight areas. Sanding sticks in various grits help you achieve a smooth finish after cutting.

If you're working on a project that requires precision, consider using a magnifying lamp or a hobby vise to hold small parts steady. For those who want to take their cutting to the next level, a runner holder or a parts separator can help you manage multiple parts efficiently. Remember, the goal is to remove parts from the runner cleanly and safely, so investing in the right tools pays off in the long run.

• A cutting mat protects your desk and your tools.
• Use a hobby knife for final nub removal in hard-to-reach spots.
• Sanding sticks are essential for a flush finish.

Mastering the art of cutting parts from runners is a fundamental skill that every model kit builder should develop. By using the right tools, employing the two-cut technique, and being mindful of the plastic type, you can avoid damaging parts and achieve a clean, professional result. Whether you're building a simple HG or a complex metal frame kit like the Motor Nuclear MNP-XH01 Bai Qi (White Tiger), these tips will serve you well. So grab your nippers, take your time, and enjoy the process of building your next masterpiece!