Bi-material design: DFM rules and best practices

Design a room two-tone (or bi-injection) cannot be improvised. Beyond the choice of polymers, the mould geometry, the parting line, the design of the bonding areas, and the injection strategy directly influence industrial feasibility and final cost. This DFM guide brings together the rules that our design office applies to two-component projects to ensure stable production runs.

This guide is supplementary to:

• Our page on the bi-injection process (Principle of operation, advantages, applications)
• Our guide to multi-shot moulding compatible materials (compatibility chart, chemical vs mechanical adhesion)

The specific challenges of dual-material DFM

Compared to a classic single-material part, bi-material design introduces several specific constraints:

• Two Withdrawals materials to be harmonised to avoid differential deformations
• Two thermal cycles overlaid (the first material undergoes the heat from the second injection)
• One Interface critique between the two materials, where all the mechanical strength is played out
• More complex joint plans (often 2 planes instead of 1)
• A take-off strategy adapted to the composite part

These constraints explain why the cost of two-component tooling is 1.5 to 2.5 times that of an equivalent single-material mould.

Rule 1 — Design the interface between the two materials

The interface is the point critique de toute pièce bi-matière. Depending on whether the materials adhere chemically or not, the design changes radically.

Case 1: Chemical adhesion available (PP/TPE-V, PC/TPU, etc.)

A flat surface is sufficient. Molecular adhesion occurs within the thickness of the interface. Recommendations:

• Contact surface minimum 1mm x 5mm per zone d’accroche
• Clean surface, free of mould release agent residue
• Temperature of the first material at the injection of the second: At least 60°C (to activate thermal adhesion)
• No geometric relief is necessary – but it is not prohibited either

Case 2: No chemical bonding (PP/PA, PC/POM...)

The mechanical catch compensates. Several solutions:

Dovetail joint sizing Typical: top opening 2 mm, base 3 mm, depth 1.5 mm, undercut angle 15-30°. Spacing between 2 grooves: 5 to 10 mm depending on stress level.

Rule 2 — Optimise Joint Plans

A two-material part typically 2 sealing planes : one for the first injection, one for the second. Their positioning is crucial for:

• L'final aesthetic Visible or invisible joint lines in appearance
• the Burring (to be avoided at material junctions)
• L'ejection accessibility
• the mould complexity and thus its cost

Best practices:

• Align joint lines on sharp edges or unseen areas if aesthetics are critical
• Avoid joint lines that pass through an area of aspect or function.
• Maintain an angle of carcass of 1° minimum (ideally 2°) on all vertical walls
• Anticipate the smears aux interfaces — les zones susceptibles d’en générer doivent être facilement accessibles pour ébavurage si nécessaire

Rule 3 — Size the thicknesses

The classic injection rules apply to each material, but with nuances specific to dual-material injection:

Thickness variation within the same material do not exceed ±30% over a short distance. Brutal variations create hot spots and sinkages.

Rule 4 — Manage differential withdrawals

Each thermoplastic its own shrinkage coefficient at Cooling. In a two-material component, the two shrinkages must be compatible, otherwise the part will be deformed.

Rule of thumb withdrawal gap between the 2 materials < 0,5% for a precision part. Beyond that, plan for compensating reliefs (free deformation zones) or accept a final deformation that will need to be absorbed by sizing.

Rule 5 – Injection Strategy

The choice of injection point (gate) for each subject is crucial:

• First subject non-critical zone injection point, ideally concealed in the final design
• Second subject : injection point far from areas where the first material would be weakened by the thermal input of the injection. A simulation Moldflow makes it possible to anticipate
• Flow balancing : essential to avoid weld lines visible or weak at the junctions of materials

Rule 6 — Ejection Design

Ejecting a two-component part requires more care:

• Ejectors positioned on the rigid material (the flexible material deforms on ejection)
• Sufficient ejection surface: 3–51% of the projected area typically
• Ejection speed suitable: too fast, deformation of soft matter; too slow, lengthening of the cycle
• On complex parts with deep soft-touch areas, provide blade or tube ejectors (see. tubular ejector)

Rule 7 — Validate by rheological simulation

Before realisation of a two-component mould (10 to 14 weeks lead time, €25 to €90k investment), a Rheological simulation is strongly recommended. Standard software (Moldflow, Moldex3D, Sigmasoft) allows for:

• Visualise the filling of the 2 cavities and anticipate defects
• Check the temperatures at the interface between material 1 and material 2
• Simulate differential withdrawals and final deformations
• Optimise the gate locations and cooling strategy
• Calculate the cycle time forecast

Cost of a two-material rheological simulation: Between €1,500 and €5,000 excl. VAT, for a potential gain of €20,000 to €50,000 on mould optimisation. This is one of the best investment/risk ratios in technical plastics engineering.

✅ 10-point two-material DFM checklist

1. Chemical compatibility or validated mechanical interlocking design
2. Joint plans of the 2 cavities positioned on edges or unforeshortened areas
3. Thicknesses respected by material (1-3 mm typical)
4. Draft angles >= 1° on all vertical walls
5. Difference in shrinkage between the two materials < 0.5%
6. Optimised injection points (Moldflow study recommended)
7. Adapted ejection strategy (ejectors on rigid material)
8. Mechanically sized gripping zones (grooves, dovetails) if required
9. Cooling plan validated for the 2 phases
10. Pre-production adhesion trial planned and budgeted

🔗 To go further

• Two-component injection process: principle and applications
• Compatible materials for two-component injection
• Complete Guide to Plastic Injection Moulding
• Glossary of Plastic Injection Moulding
• Hybster design bureau for your bi-material projects

Are you currently designing a two-material part? Our design office can intervene in DFM review, rheological simulation or complete design. Request our expertise