Light RTM Vehicle Door Design and Quality Issues - C128

 

Light RTM Vehicle Door Design and Quality Issues
Case study
Document Type	Case study
Document Identifier	128
Themes	Materials deposition and consolidation management Quality/inspection management
Tags	Deposition Development to production Process modelling Quality
Objective functions	Cost Maintain Rate Maintain Quality Increase
MSTE workflow	Development

Summary

Light RTM processing was selected to manufacture the mass transit vehicle external door shown in Figure 1. After prototyping it was discovered that the stiffness of the door did not meet the design requirements. Additional reinforcement was added to the design, but the new design thickness exceeded the part cavity thickness leading to issues closing the mould and degrading the exterior surface finish. A new B-side mould needed to be manufactured because the part cavity thickness wasn’t properly considered during the design process.

Figure 1 - External Door

Challenge

The performance of the exterior door needed to meet specific requirements during service. These requirements included:

• Aesthetics – Class A surface finish free from material print-through.
• Structural – Meet minimum safety factor and maximum deflection limits when undergoing door opening, slamming, and collision load cases.

The layup of the door was designed based on a preliminary stiffness analysis. The materials used in the layup were comprised of a stitched infusion sandwich mat consisting of a non-woven synthetic core sandwiched between two layers of chopped strand mat, stitched unidirectional mat, and low-density polyurethane foam core. The A-side and B-side light RTM moulds were manufactured with the part cavity thickness specified to match the predicted layup design thickness.

Physical testing was performed once a prototype part was manufactured. The prototype door was installed on the vehicle and opened and slammed closed. The deflections across the door were measured and it was discovered that the door did not meet the stiffness requirements. The addition of another unidirectional mat to the layup was trialed in an attempt to stiffen the design. It was observed that adding a single ply of unidirectional mat was not sufficient, so multiple plies were added to meet the stiffness requirement. This increased the thickness of the laminate by 17%. However, when the new design was being manufactured it was found that the B-side mould would not close and properly seal.

Approach

It was discovered that physical clamps were required to close the B-side mould when manufacturing the updated design. The infusion was completed successfully but the resin flow rate was slowed considerably compared to the original layup design. Both of these factors, the time needed to clamp the mould and the additional infusion time, increased the cycle time of the process by approximately 20%. This drove up the cost to manufacture the part. As well, the extra clamping force resulted in additional wear to the moulds, decreasing their lifespan and increasing the tooling cost amortization rate across each production part.

Another problem that arose was print through on the exterior surface. The additional reinforcement mats that were forced into the part cavity thickness lowered the resin content of the part and decreased the ability of the chopped strand mat plies to block the print through from the directional reinforcement mats and along the edges of the core.

When assessing potential mitigation strategies another contributing factor was discovered. During the initial design process a thickness of 1.5 mm was assumed for the stitched infusion sandwich mat. Upon review, it was determined that the assumed thickness was too small and was outside of the manufacturer’s recommendations. A value of 3 mm was determined to be more suitable. Since the part cavity was designed based on the 1.5 mm thickness, it meant that during production the stitched infusion mat was overly compressed reducing resin flow. It also meant that when additional reinforcement was added to the design the stitched infusion mat was unable to compress any further since it was already beyond it’s designed thickness range. This further increased the challenge to close the B-side mould.

During the redesign process it was also determined that adding more unidirectional reinforcement was not the most efficient method of improving the stiffness of the door. Increasing the thickness of the core was a much more efficient method to improve stiffness . By increasing the core thickness by 12.5% the stiffness of the design increased by 36%. Adding the extra reinforcement increased the stiffness by only 10%. This comparison is shown in Figure 2. This helped explain why adding a single ply of unidirectional reinforcement was not sufficient to meet the design requirements and instead, multiple plies of reinforcement were required. Subsequently, the need to add multiple reinforcement plies led to the challenges of closing the tool and a reduction in the resin flow rate.

Figure 2 - Door Design Stiffness Comparison

Outcome

The challenges that arose during the manufacturing of the door after adding extra unidirectional reinforcements to the layup demonstrate the importance of analysing and testing a light RTM design sufficiently to properly define the part cavity thickness before the B-side mould is built. The need to include extra plies of reinforcement within a pre-set part cavity thickness led to the difficulties in closing the mould, increased the cycle time, and increased the prevalence of print through on the exterior surface.

Although the part could be manufactured with the revised layup and using the existing mould, a decision was made to manufacture a new B-side mould with an increased part cavity thickness. This allowed for a more efficient revised design that included a thicker core and less directional reinforcement, eliminated the need to use physical clamps to close the mould, reduced print through, and decreased the part fill time resulting in a reduction of the cycle time for each part.

Related pages

• Practice for troubleshooting a Light RTM step - P107
• Light Resin Transfer Moulding (LRTM) - A424
• Part Design for Light RTM - P177

Related pages

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Practice Documents	Part Design for Light RTM - P177 Ply Draping Modelling for Structural Analysis and Manufacturing - P174 Practice for Developing a Deposition Step - P157
Case Studies	Cost Comparison Study of a GFRP Leisure Boat Hull Manufacturing Method; Spray Up vs Resin Infusion - C115 Improving Quality of a Truck Fender Using Ply Draping Modelling - C122 Light RTM Vehicle Door Design and Quality Issues - C128 Optimizing Cost vs Weight for Low Production Volume Parts - C112 SAMPE Canada - CKN - CANCOM Student Competition: Case Study - C121
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About	Help

Resin transfer moulding (RTM) involves loading a preform into a two (or more) piece, matched tool, closing it, and injecting resin under pressure (~15-100 psi, or ~1-7 bar).

Well suited to small to medium sized parts, limited to large sizes due to injection pressure loads and tool cost.

For polymer matrix composites (PMCs), resin refers to the matrix; the continuous material phase that binds the reinforcement together, maintains shape, and transfers load. Resins are divided into two main groups: thermosets and thermoplastics.