In-mould decoration case study

The engineering challenge.
Rosti China installed a new IMD (In-mould decoration) manufacturing cell for one of its long standing customers. IMD has been very well received as it achieves an increased level of cosmetic design not possible with conventional painting/plating processing. IMD designs are predominately used in flat geometries, however these parts are very complex 3D components, with a stretch ratio in excess of 45%. 

The Rosti way was to assess risks and potential failure modes and effects, the outcome yielded significant challenges which lead to Rosti treating this initiative as an exploratory technical development project completely separate from the usual main stream new product introduction programs. The main challenges revolved around: stretch ratio, resin and part flow length.

Table1: critical to quality (CTQ) factors
  1. Stretch ratio: percentage increase of part profile with reference to the flat foil
  2. Resin: component substrate, optimal condition is to enter as slow and as cool as possible to reduce ink defects at the gate location.
  3. Flow length: this is a ratio of the component wall section against the length from gate to end of fill. The optimal is to be as short as possible so the temperature and pressure gradient is as uniform as possible over the decoration length.

Overcoming the challenges

These challenges were overcome by Rosti pulling together all partners involved, resin suppliers, equipment suppliers and decoration foil suppliers. The critical to quality items above were overcome through digital simulation and analysis. This reduced physical resource and costs to optimise the development cycle to reach market as quickly and cost effectively as possible.

Stretch ratio: Rosti part had complex three dimensional curves both in line of flow and cross flow. Geometric splices were made to assess the high and low stretch areas;

  • High stretch areas are important to understand how Rosti can reduce the impact on the foil
  • Low stretch also important to understand how to prevent wrinkles where the geometry transitions from high to low stretch sections

Resin: Rosti part was a Polybutylene terephthalate (PBT) + glass fibre content, this is a high temperature engineering polymer.

  • The resin was considered as a constant, the critical to success factor here was Rosti’s advanced injection mould flow simulation ability, optimizing the temperature and pressure at the gate area. Iterations and optimizations on cooling/gate design/etc. were verified digitally at a fraction of the cost/time of traditional physical trials.

Flow length: Rosti part was 1.2mm wall section over a length of 210mm.

  • The thin wall geometry coupled with the resin resulted in high injection temperature and pressures. Rosti’s digital analysis experience was again a major advantage in this feasibility analysis. Key actions were to profile injection conditions to ensure uniform filling conditions to simulate the decoration performance with foil.

Rosti’s conclusion of the digital development and the hundreds of scenarios that were run delivered a high success confidence to be achieved which enabled physical tooling and development to be started. From point of launching steel tooling to having proof of concept and production part process approved was less than 3 months. This was only possible through the deep understanding of the process difficulties and building robust simulation models to work through all potential scenarios to optimize tool, process and automation design. This process/project shows the commitment to innovative technology development and the boldness to proceed into a project like this when the experienced market leaders were openly stating that this is a high risk low success application. With attention to detail and the drive of the development team the result was a perfect example of Rosti’s concept to reality philosophy.

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