Multi-Component (2K) Molding Defects

How to Recognize It

• Weak adhesion: in a transparent/translucent component it looks like a CRACK because of light refraction.
• Incipient melting of the first component: material wash-out at the gate area, a change in wall thickness or a colored mark.
• Warpage from the bimetal effect in hard/hard combinations; deformation from compression in the soft component.

Physical Causes

• Adhesion forms by diffusion; the CHEMICAL AFFINITY of the material pair is decisive (the styrene family adheres well within itself, the ester/carbonate group within itself; with polyolefins only the SAME TYPE adheres — PE with PE, PP with PP).
• Partial lack of adhesion is often caused by oil/grease "creeping" into the cavity through mold temperature or mechanical stress.
• If the second melt meets the first at a 90° angle and the melting/glass transition temperature is exceeded, the first component melts.
• Because of the staged injection, the 2nd melt has a higher shrinkage potential than the 1st — the difference in shrinkage causes warpage.
• Soft TPEs are highly compressible; if the holding parameters are high, they deform.

Solution Steps at the Machine

1. For weak adhesion: check the contact surface for oil/grease contamination and clean the mold; reduce the cycle time; INCREASE the mold wall temperature of the 1st component; increase the melt temperature of the 2nd component.
2. Holding time: INCREASE it for thermoplastics, REDUCE it for TPEs (so the soft component is not compressed).
3. If the first component melts: reduce the injection speed of the 2nd component (or use a fast-slow profile), reduce the mold temperature of the 1st component, use a film/diaphragm gate.
4. For warpage — 1st component: REDUCE the holding pressure and time, increase the mold temperature. 2nd component: INCREASE the holding pressure, reduce the mold temperature.
5. For deformation of the soft component near the gate: use a decreasing holding profile and shorten the time.

Mold, Design and Material Solutions

• Choose the material pair according to the adhesion matrix; consult the raw material manufacturer in every case.
• Improve the design of the contact surfaces: in a hard/hard combination, increase the contact area without creating sharp edges; blind-polished surfaces are preferred.
• The gate position of the 2nd component MUST NOT BE THE SAME as that of the 1st; optimize the cooling of the gate area.
• Consider a material with high heat resistance (for the 1st component) or an additional gate to compensate for shrinkage.

Important Notes

• ⚠️ The FIRST STEP in troubleshooting is to look at each component ONE BY ONE and apply the solution belonging to that component's own defect.
• ⚠️ Common well-adhering pairs: ABS↔PC, ABS↔PMMA, PC↔PBT, SAN↔PMMA, TPE-S or TPE-V on PP, TPE-U on ABS/ASA/PA/PC/SAN. POM adheres only to itself and to TPE-U; PE-HD and PP DO NOT adhere to each other.