Troubleshooting of room temperature processes for large recreational and industrial parts - C100

 

Troubleshooting of room temperature processes for large recreational and industrial parts
Case study
Document Type	Case study
Document Identifier	100
Themes	Thermal management
Tags	Cure and crystallization Process modelling Thermoset cure kinetics Thermoset polymers Thermal transformation
Objective functions	Cost Maintain Rate Maintain Quality Increase
MSTE workflow	Troubleshooting
Prerequisites	Cure of thermosetting polymers Degree of cure Polyester resin Thermal and cure/crystallization management (TM) Thermoset polymers

Summary

Over time, several composite companies in British Columbia (BC) have noticed a strong correlation between product quality (evaluated by characteristics such as dimensional control, surface finish, and durability) and the season in which they were manufactured. For example, parts manufactured during the winter were often more prone to deform after de-moulding, particularly during transportation.

In response, the Composites Research Network (CRN) followed a systematic approach to investigate the problem. Researchers were able to identify the main causes of deviations and revisit manufacturing workflows to limit their impacts. The industrial partners implemented practical tools for use during production to track product quality and to guard against or limit further occurrences. As a result, the companies have been able to improve product quality and production rates.

Challenge

Consistent production quality was a constant challenge for several composite companies in BC and the source of variability was unknown. Identifying the root cause of quality variations and developing practical mitigations strategies were the key motivations driving CRN’s involvement in this project.

Prerequisites

• Thermal management
• Thermoset polymers
• Polyester resin
• Curing of thermosetting polymers
• Degree of cure

Approach

CRN engineers started by visiting the companies located in the Okanagan Valley in the southern interior of BC to document their manufacturing process and look for commonalities between them. They noticed that their factories were all relying on room temperature cure for thermal transformation of polyesters resins and the companies were sourcing their polyester resins from the same manufacturers. In room temperature cure, the MSTE system is simply exposed to ambient air temperature, with no external heat added or removed by a heating or cooling system. This leaves the MSTE system and associated workflow vulnerable to daily and seasonal temperature fluctuations which might impact the quality of production.

This is the case in the southern interior of BC where the temperature can vary by more than fifteen degrees Celsius in a day and by more than thirty degrees Celsius throughout the year. If this source of variability is not taken into account and mitigated by environmental control of the factory, it can greatly impact thermal transformation. Such impacts will have an effect on the cure of thermosetting resins used to make the part. The evolution of the resins' degree of cure (DOC) during processing is a critical manufacturing parameter which can affect most of the manufacturing outcomes, such as porosity, durability, and dimensional control.

As a result, CRN engineers assumed the effect of daily and seasonal changes in temperature on degree of cure as the principle cause of the identified quality issues. To validate this assumption, CRN engineers characterized the cure kinetics of a wide range of polyester and vinyl-ester resins used by those companies and then conducted a parametric study using process simulation to assess the effects of the design and process parameters, such as ambient temperature, on the degree of cure during manufacturing. The parametric study not only allowed them to validate the above assumption but also to develop practical mitigation strategies to increase the robustness of the companies' manufacturing processes as explained below. Following their development, the strategies were integrated into the companies' manufacturing workflows and validated.

Seasonal changes in temperature and precipitation in the southern interior of BC.

In summary, the following troubleshooting steps were followed to reach a practical solution:

1) Document manufacturing processes (i.e., MSTE objects, process steps, factory cells and layout)
2) List commonalities between manufacturing processes
3) Identify common source of variability
4) Characterize material systems
5) Conduct parametric study using process simulation
6) Develop mitigation strategies
7) Transfer and implement mitigation strategies
8) Evaluate mitigation strategies

Outcomes

Level I

Level II

Families of manufacturing design tools and mitigation strategies were developed by CRN. These tools allowed CRN researchers to revisit the companies' manufacturing workflows, making appropriate changes to limit the impact of daily and seasonal changes in temperature. Additionally, an on-site non-destructive Fourier transform infrared (FTIR) technique to measure degree of cure during production has also been developed, and is now available to CRN’s industrial members (see how to measure curing time and degree of cure).

Extensive material characterization and process simulation were used to investigate the role of the MSTE system parameters and clearly identify the ambient temperature as the main source of variability. Critical steps of the workflow were identified and process maps created to monitor them (see Level II and Level III). The processing maps convert criteria on the degree of cure into practical production parameters. For instance, process specifications on the degree of cure of the gel coat before the deposition of the bulk layer are converted into a processing time window during which the bulk layer should be applied. A thermal management parametric study was also used to identify pre-production and in production mitigation strategies.

Armed with an improved understanding of the cure chemistry and kinetics of polyesters and vinyl-esters investigated in this project, the participating companies have gained confidence in their manufacturing processes. In practical terms, each has been able to improve product quality and production rates.

Simplified manufacturing workflow used by the BC companies.

Families of manufacturing design tools were developed by CRN. These tools allowed CRN researchers to revisit the companies' manufacturing workflows, making appropriate changes to limit the impact of daily and seasonal changes in temperature. Additionally, an on-site non-destructive Fourier transform infrared (FTIR) technique to measure degree of cure during production has also been developed, and is now available to CRN’s industrial members (see how to measure curing time and degree of cure).

The assumption retained from the initial diagnostic pointed to a thermal management deficiency. The analysis and outcomes described below all focus on understanding how the temperature within the factory cells could impact quality. The companies’ manufacturing workflows all shared the six following process steps: mold preparation, gel coat deposition, bulk layer deposition, part demoulding, storage and shipping. These process steps were occurring in a mix of indoor and outdoor factory cells. The temperature within an indoor factory cell is usually controlled by the factory heating, ventilation, and air conditioning (HVAC) system unlike the temperature of an outdoor factory cell which is subject to the daily and seasonal temperature fluctuations. An indoor factory cell might also experience limited temperature fluctuations depending on the efficacy of the HVAC system and its location and configuration. For example, a factory cell placed near an entrance may experience an air draft and have its temperature change as the doors are opened. Similarly, an outdoor factory cell when sheltered from the sun and the wind will be subjected to lower temperature fluctuations. Understanding how the factory layout and workflow was governing the thermal history and cure of the room-temperature cure polyester resins became the key focus of this project.

Cure kinetics of a polyester resin used by the BC companies.

The cure kinetics of room-temperature cure polyester and vinyl ester resins, like any other thermosetting resins, depends on time and temperature. A series of DSC tests were conducted on a wide range of the polyester and vinyl-ester resins used in production to quantify how time and temperature governed their cure and how sensitive it was to the daily and seasonal temperature fluctuations. The DSC results were also used to create curing process maps and develop cure kinetics models for thermal process simulations. The DSC tests revealed that their cure kinetics varied significantly over the range of temperatures encountered in production. For example, curing the room-temperature cure polyester resins at 10°C could not only take three times longer than at 25°C, but also never reach the same final degree of cure. In fact, the characterized resin systems all required curing above 50°C to fully cure.

The impact on quality was illustrated by using process maps and comparing two production scenarios following the same process flow but occurring at different times during the year; one taking place in the summer and a second one during the winter. In these scenarios, the mold preparation, gel coat deposition, and bulk layer deposition steps are all performed indoor in the morning. The part and tool assembly are then moved outside in the early afternoon and left there until demoulding in the late afternoon. In the summer, the temperature is assumed to be about 17°C inside the factory in the morning and about 30°C outside the factory in the afternoon. In the winter, the temperature is slightly cooler in the factory and about 30°C cooler outside the factory. In a room temperature cure cell, the tooling (T) and equipment (E) do not allow for temperature control of the material systems (M) which is therefore exposed to ambient temperature fluctuations. As a consequence, the material systems will follow different curing paths depending on the day and the season. For example, during the summer the degree of cure before demoulding might reach 80 percent compared to 60 percent in the winter. The identified temperature fluctuations have a significant impact on the evolution of cure during manufacturing and this source of variability needs to be mitigated to ensure a constant level of quality.

Evolution of cure during manufacturing: winter and summer scenarios.

To develop practical mitigation strategies, CRN researchers conducted a parametric study of the MSTE systems using process simulation. The goal was to understand the effect of the MSTE parameters on the evolution of cure. For example, how does the final degree of cure depend on the initiator concentration, part thickness, and surrounding air flow? Part thickness is a key factor in determining the final degree of cure due to the exothermic nature of the crosslinking chemical process. The heat energy released can significantly increase the part temperature, which can in turn, advance the cure. The thicker the part, the less the heat energy is transferred to the surrounding environment and the more it contributes to increase the part temperature. For instance, at 20°C, a 3 mm thick laminate may reach a final degree of cure of 76%, while a 8 mm thick section will cure 10% more under the same conditions.

Final degree of cure as a function of ambient temperature for different laminate thicknesses.

This practical knowledge was then used to analyze the critical manufacturing steps and guide the definition of pre-production and in-production mitigation strategies. For example, a critical step early in the manufacturing process is the deposition of the bulk layer following the application of the gel coat. An important material property to follow before the bulk layer application is the gel coat’s degree of cure. If the gel coat has not reached gelation by that time, the application of the bulk layer might perturb the gel coat thickness which may lead to non-uniform surface finish and inconsistent environmental resistance. On the other hand, if the gel coat has fully cured, no crosslinking will develop between the gel coat and the bulk layer resulting in the formation of a weak interface. This weak interface will compromise the durability of the part and will be prone to blistering. It is therefore key to monitor the degree of cure of the gel coat and to define acceptable limits to be reached before the deposition of the bulk layer.

The in-situ measurement of degree of cure is a complex task rarely done in production. Specifications on the gel coat degree of cure must be translated into more practical process parameters. In this case, by knowing the cure kinetics of the gel coat, the specifications on the gel coat degree of cure can be converted into minimum and maximum curing times to define a processing window for timing the process as a function of the ambient temperature. For example, for the winter scenario explained above and a minimum gel coat degree of cure of 20% and a maximum of 40%, the deposition of the bulk layer must start no sooner than 55 minutes and no later than 90 minutes following the application of the gel coat. This gives a processing window of 35 minutes. In the summer, the minimum time to bulk layer deposition following gel coat application is 18 minutes with a maximum of 32 minutes. This gives a much shorter processing window of 14 minutes, as compared to 35 minutes in the winter. Note that the processing window can be controlled by changing the gel coat formulation and, more specifically, the catalyst and initiator types and concentration.

This example illustrates how a room temperature cure process needs to be timed differently depending on the day or the season. Additional process maps to monitor degree of cure in production and outcomes from this project are given at the Level III.

Process map to follow a gel coat's degree of cure before subsequent deposition.

Gel coat processing window: winter and summer scenarios.

Related pages

Page type	Links
Introduction to Composites Articles
Foundational Knowledge Articles	Conduction - A118 Convection - A106 Cure of thermosetting polymers - A162 Degree of cure - A104 Glass transition temperature (Tg) - A210 Heat of reaction - A114 Thermoplastic polymers - A161 Thermoset polymers - A105 Viscosity (resin) - A203
Foundational Knowledge Method Documents	How to measure curing time and degree of cure - M100
Foundational Knowledge Worked Examples
Systems Knowledge Articles
Systems Knowledge Method Documents
Systems Knowledge Worked Examples
Systems Catalogue Articles
Systems Catalogue Objects – Material
Systems Catalogue Objects – Shape
Systems Catalogue Objects – Tooling and consumables
Systems Catalogue Objects – Equipment	Rheometer - A357
Practice Documents	Decreasing Cure Cycle Time - P121 Practice for developing a thermal transformation process step - P105 Troubleshooting scale-up issues from thin to thick parts - P140 Troubleshooting scaling-up issues from coupons to parts - P134
Case Studies	Troubleshooting of room temperature processes for large recreational and industrial parts - C100
Perspectives Articles	Case Study: Optimizing a Press Moulding Process - A324 Composite materials engineering webinar session 3 - Constituent materials - Resin - A122 Composite materials engineering webinar session 4 - Thermal management and resin cure - A123 Composite materials engineering webinar session 5 - Manufacturing processes - Introduction - A124 Composites Process Simulation: A Review of the State of the Art for Product Development - A283 Dr. Anoush Poursartip's cdmHUB global composites expert webinar - A136 Effect of cure on mechanical properties of a composite (Part 1 of 2) - A319 Effect of cure on mechanical properties of a composite (Part 2 of 2) - A320 Fabric Forming: how it affects design and processing, and how simulation can address this - A310 Heat Transfer in Composites Processing - A321 Interview with Prof. Kevin Potter - A134 Introduction to the processing of thermoplastic composites - A322 Resin Behaviour During Processing: What are the key resin properties to consider when developing a manufacturing process? - A257 Simulation models for rapid liquid composite molding - A333.0 Understanding Polyester Resin Processing: The Effect of Ambient Temperature on the Final Part - A286

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Engineered materials (designed to have specific properties) made from two or more constituent materials with different physical or chemical properties. The constituents remain separate and distinct on a macroscopic level within the finished structure.

A set of steps/procedures that are intended to provide guidance in manufacturing and/or decision making activities.

There are two types of workflows:

• Standard workflows are intended to formalize practices where the manufacturing science base exists. The focus is to provide guidance using manufacturing simulation as an enabling tool (e.g. design activities/decisions relating to thermal management).
• Complex workflows are intended to reduce the level of effort in practices where the existing manufacturing science base is not sufficiently mature to support production scale problems. The focus is to provide guidance using simulation based thinking and/or checklists (e.g. design activities/decisions relating to porosity management).

Degree of cure (DOC) is an indication of how far the chemical curing reaction (crosslinking process) has advanced in a thermoset resin.

DOC is defined with a number between 0 and 1 (or 0% and 100%) where 100% is a fully cured resin. It does not have to fully reach 100% for the resin to become solid or the part to be used. In some aerospace applications, resins are only cured to about 90%. Higher the degree of cure, higher the mechanical properties.

Outcomes represent the range of response/sensitivity to factory system attributes. Those that fail to satisfy manufacturing requirements are known as defects. Examples of manufacturing outcomes include process parameter outcomes, material structure outcomes, and material performance outcomes.

The use of multiphysics models to predict the outcome of real-world scenarios. May be analytical (closed form, "hand calculations") or computational (implementation on computers is required due to the large number of calculations involved. e.g. finite element method, finite difference method)

Most often in composite materials engineering, simulation refers to either:

• Process simulation (predicting manufacturing outcomes, or the inverse problem of predicting manufacturing parameters to achieve a desired outcome), or
• Performance simulation (predicting the stiffness/strength/suitability of a structure, or the inverse problem of the material properties and dimensional requirements to achieve a desired stiffness/strength/suitability of a structure)

(same as "Modelling")

A central processing theme in the manufacturing cycle. This theme is concerned with managing the thermal response of materials during storage and handling or parts/tools when they are subsequently heated.

An individual station within a factory where a given set of tasks are accomplished (also known as a "work cell"). Some cells may directly add value to the product (e.g. deposition), while others may serve support roles that are critical to maintaining part quality (e.g. receiving, storage, inspection & shipping).

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.

Differential Scanning Calorimetry (DSC).