Views: 0 Author: Site Editor Publish Time: 2026-11-03 Origin: Site
Many customers ask whether a single injection mold can run more than one plastic material. The short answer: it is possible, but only under strict conditions. Mold steel is one of the critical limiting factors.
Before approving material changeover on one mold, we must check these key conditions:
Shrinkage rate match Cavity dimensions are machined with shrinkage compensation. Large differences in shrinkage will cause parts out of tolerance. For PP and TPU, their shrinkage ranges overlap. For non-critical decorative trim parts, switching is feasible. For precision fitted parts, it is not recommended.
Similar processing temperature window Melt temperature for both materials should be close. Big temperature gaps will trigger carbon buildup and damage hot runner components, especially valve pins.
Comparable flow performance Gates and runners are designed for a certain melt viscosity. If one material flows far easier, flash will occur; if it flows worse, short shots and excessive injection pressure follow.
Consistent demolding requirements Soft elastomers like TPU need smoother mold surfaces and larger draft angles. Rigid plastics have different ejection characteristics. Mismatched demolding conditions lead to sticking, ejection marks and deformation.
Important distinction: This is different from 2K overmolding. 2K molds bond two materials into one part in a single cycle. Material changeover means the mold makes one material at a time.
Besides PP and TPU, several widely used resin combinations can run on the same mold for loose-tolerance cosmetic parts:
ABS & PC/ABS Shrinkage range 0.4%–0.8%. They share similar processing temperature and flow behavior. This is the most common swap for consumer housings.
ABS & PC Close shrinkage. PC needs higher mold temperature and better venting to avoid burn marks. S136 steel is recommended.
ABS & HIPS Similar shrinkage and molding window. Suitable for low-cost decorative covers and toys.
PMMA & PC Both clear optical grades. They require mirror polish on S136 cavities.
PP & TPE-S Similar to PP & TPU. TPE-S is less sticky than TPU, so hot runner maintenance is easier.
HDPE & LDPE Polyethylene family, good for flexible packaging parts.
Important reminder: All these pairs work only for non-critical, loose-tolerance parts. For tight tolerance functional parts, interchangeable cavity inserts are still the safer solution.
Steel grade must match the harshest material inside each combination. Even if two resins are switchable, wrong steel will cause surface blemishes, corrosion or fast wear.
These are non-abrasive polyolefins and thermoplastic elastomers. The main risk is material sticking; TPU may release weak acidic by-products when decomposed.
718H / P20: Budget pre-hardened steel, good polishability. Suitable for occasional runs of TPU / TPE.
S136 (420 stainless steel): Preferred for continuous TPU / TPE production. Excellent corrosion resistance, mirror polish and anti-sticking performance.
NAK80: Balanced option, great surface finish; corrosion resistance sits between 718H and S136.
45# carbon steel: Not recommended. Prone to rust with elastomer residue and moisture.
Non-corrosive, non-abrasive rigid plastics. No strong acid decomposition risk.
718H / NAK80: Most widely used. Good polish for cosmetic housings.
S136: Optional upgrade if high-gloss surface or long mass production is required.
Clear or high-temperature grades. PC and PMMA decompose easily if poorly vented, which creates slight corrosive gas. Mirror polish is usually required.
S136 stainless steel: Recommended. Resists corrosion and supports mirror finish for transparent parts.
718H is not ideal for long-term PC / PMMA runs due to corrosion risk.
Glass-filled materials (PP+GF, PA+GF) Glass fiber is highly abrasive. Use tough wear-resistant steel such as H13 or SKD61, usually with nitriding. A mold built in 718H or S136 for the above soft/rigid non-abrasive pairs cannot run glass-filled grades. The cavity surface will scratch and wear rapidly.
Flame-retardant plastics Many flame retardants release corrosive gas during molding. Corrosion-resistant stainless steel like S136 is mandatory.
Surface finishing complements steel performance:
High polish: Reduces sticking of elastomers and improves clarity for transparent PC / PMMA.
Hard chrome plating: Improves corrosion and release performance, though the coating may chip easily.
PVD coating (TiN / TiCN): Widely applied on valve pins and gate inserts. It cuts resin adhesion and carbon accumulation, ideal for valve shut-off hot runner systems.
Nitriding: Boosts surface hardness and wear resistance. Not suitable for mirror cosmetic or transparent surfaces, as it limits polishing quality.
Steel selection rule for interchangeable material molds: choose steel based on the more demanding material in your pair. ✅ PP + TPU / PP + TPE-S / HDPE + LDPE: 718H for occasional elastomer runs; S136 for continuous elastomer production. ✅ ABS + PC/ABS / ABS + HIPS: 718H or NAK80 is enough. ✅ ABS + PC / PMMA + PC: S136 is strongly recommended. ❌ All these molds are only for soft, non-abrasive, low-corrosion resins. Do not run glass-filled or aggressive flame-retardant materials.
Material compatibility decides whether your parts meet drawing tolerances. Mold steel decides whether your mold can survive repeated changeover. A “multi-material mold” is never universal. Always select steel for the most demanding resin you plan to process.
