Quality assurance in the production of mountain bikes
High-quality mountain bikes with carbon frames are very complex to produce. Depending on the model, the frame of the bike alone consists of up to 1,046 individual carbon blanks, whose shape is precisely designed for the loads of the corresponding frame sections. Here is a corresponding insight into the ROTWILD carbon manufacturing.
More than 60 working hours pass until the final frame is manufactured and painted. In production, the individual work steps are carried out by about 35 different specialists. Against this background, it becomes clear that quality assurance is of particular importance throughout the entire manufacturing process.
The carbon fiber-reinforced plastic (CFRP) used in frame construction must meet the highest standards in terms of strength, rigidity, appearance and weight. For this to succeed, seamless quality control is essential. Only through the complete documentation of all production steps can production know-how be passed on and the repetition of errors be avoided.
Control of raw materials
Quality control begins with the inspection of the raw materials used and their manual or mechanical processing. This is the only way to produce frame components that meet all the specifications from the design and withstand the calculated loads. In addition, conscientious quality control in series production ensures that the properties of all frame components remain the same.
The properties of frame components made of CFRP material depend heavily on careful manufacturing. After a component has been completed, possible production defects such as defective material or unclean alignment of the carbon layers can only be detected with extremely high effort. In the course of quality control, one therefore tries to eliminate material defects and defective parts from the manufacturing process as early as possible.
Quality assurance at the frame builder
The series production of all ROTWILD carbon frames takes place in one of the most modern factories in Asia. For quality assurance during the construction of the frame, a Standard Operating Procedure (SOP) is defined for each individual process step. This covers the inspection of all workpieces, auxiliary materials, operating materials and work instructions. The individual quality checks can be divided into four steps:
- Incoming goods inspection of raw materials, prefabricated raw materials, and individual and add-on parts.
- Quality checks between the individual departments: Processing of raw parts, CNC machining, surface finishing, painting.
- Fluoroscopy by X-ray machine.
- Mechanical inspection - load tests.
- Final inspection per test report.
1. Incoming goods inspection
Flawless materials are the prerequisite for high quality of the finished product. This step therefore involves checking whether the various starting materials are free of defects and can therefore be used in production without hesitation. The following are checked: carbon prepregs, glass fiber mats, resins, PU moldings, polystyrene cores, metal inserts (threaded inserts, blanks for subsequent CNC machining), bearings (rear triangle, headset, etc.), screw connections (standard and special parts), inlays (dropouts, covers, other inserts), frame parts (linkage), frame protectors, decorative and protective foils.
2. Quality controls and in-process quality assurance between the individual departments
High-quality components require flawless starting material. For this reason, the individual carbon blanks are first visually inspected for defects, fiber distortion or fabric damage. Then, for documentation purposes, all blanks are assigned to the correspondingly numbered material batch. Finally, the machine-produced blanks are compared with the specifications and the material batch is assigned to the specific component.
For error-free production, the individual lamination steps are described in a lay-up plan, both pictorially and in writing. For documentation purposes, each frame is given a unique QR code.
For this work step, quality assurance defines clear curing parameters. These apply to the contact pressure on the mold halves and molds, the pressurization of air bubbles inside, the temperature profile and the curing time.
Quality control after demolding (blank part)
This step ensures that there is no confusion in the molds used for the frame parts and the tools. A visual inspection of the frame blank is carried out with a particular focus on: Resin accumulation (lack of fibers), wrinkles in the scrim, delamination, contamination (particles), dents, holes, fractures, damage due to demolding, condition of the inserts (no deformation), shape of the cavities (defective internal printing - air bladder), condition of the tools (mold halves, sliders). In order to detect the smallest production defects, printing is performed in a water bath. Here, the air-filled frame blank is immersed. Rising air bubbles reveal poorly processed areas.
Various frame parts are machined in production with CNC milling machines. An exact description of the machining steps per part, the listing of the CAM programs to be used and the definition of the milling cutters and coolants guarantee high product quality. After each machining step, the machined surfaces are randomly inspected using measuring tools and test gauges, thus validating the process parameters.
The components to be used for bonding the frame parts are precisely defined. The same applies to the cleaning agents, adhesives, etc. used. Precise documentation of the individual work steps makes it possible to check whether all processing specifications have been adhered to. The gauges used for positioning the frame parts during bonding are also recorded in the documentation for this work step.
Quality control after mechanical processing
In this step, all dimensions and interfaces of the frame blank are checked. First, all frame dimensions are checked on the basis of the dimensioned drawing. The position of the bearings and the mounts for the motor and battery are also compared with the specifications on the dimensional drawing.
The frame interfaces are then checked. This involves checking the clearance of the tires, the rear triangle, the crank and the chainrings. The cable bushings, the alignment or the distortion of the frame (axles, straightness) and all connection dimensions on the bottle cage, the seat tube diameter, the axle width and the brake mount are also checked.
Fill & Grind
After all dimensions of the frame blank have been matched, the surface is ground. During this process, the surface finish is continuously visually inspected. The frame does not meet the requirements until all burrs have been removed, all surfaces smoothed and all uncleanly processed laminate layers filled.
Quality control - handover to the paint shop
Before the frame blank is painted, a visual inspection is carried out with a special focus on color changes and fabric defects in the visible carbon, cleanly smoothed surfaces, possible holes due to missing material, scratches, dents, droplets and bubbles. Also checked is whether all body edges are visible as intended by the design and have not been smoothed by excessive sanding. In addition, the blank is inspected for possible contamination from particles in the laminate and the hardness of the surface is checked.
For quality assurance, the cleaning agents, primer, colors and masking films used in the painting process are precisely defined. The same applies to the colors, the artwork (masking) and the finish (matte, glossy). After each intermediate step, a visual check of the masking, the color tones and the surface quality is carried out.
After painting, the decor is applied. For all the stickers to be used, there are precise specifications in terms of their size, position, colors and material. During the application, the applied number, type and position are visually checked.
Quality control after painting and decor
Once all the decors have been applied, a final visual inspection of the frame is carried out. The aim here is to rule out paint damage, masking defects, discoloration, undesirable matte and gloss effects, orange peel, droplets and bubbles, scratches, dents and incorrect positioning and incompleteness of artwork and decor. The properties of the surfaces in terms of their layer thicknesses, impurities (particle density), hardness and resistance (drop-ball test) are also checked.
All components and means used in the assembly are specified in the course of the in-process quality assurance. This applies to all frame parts, bearings, screw connections and all special parts. This also applies to all tools used during assembly, the assembly paste, all lubricants and the adhesive for screw locking. All work steps are also defined, as well as all locations where the work equipment is used and the tightening torques of the screw connections.
3. Fluoroscopy by X-ray machine
Fluoroscopy of the frame by X-ray machine allows a view into the interior of the frame. This allows the composite structure to be inspected and possible defective areas to be detected. The wall thicknesses can also be checked on the X-ray image. X-ray inspection validates all upstream processes (layup, curing).
4. Mechanical testing - load tests
For the load tests, the geometry, load directions, material and operating materials of the test rigs used are specified. Test parameters such as the height of the loads and the number of load cycles in fatigue tests are also uniformly defined. The test results of the tests are documented and all measured values from the stiffness test are converted into the value for specifying the stiffness.
5. Final quality control - inspection by test report
During the final quality control, individual frames are inspected on a random basis. The inspection is carried out according to the specifications of the ROTWILD inspection protocol using the dual control principle. Particularly important frame characteristics are documented in the protocol.
Measuring tools and test gauges are used to check the following parameters during the final quality control:
- Connection of the fully integrated seat post
- Head tube length, inner diameter
- Coaxiality of the rear triangle pivot points
- bearing spacing
- frame weight
- Checking the tightening torques of all bolted connections
- Position of the brake mount
- Other: engine mount, tire clearance, axle mount, etc.
Finally, the qualitative inspection takes place with a check of the mobility of all bearings and the free movement of the rear end, as well as an inspection of the damper mounts. All surfaces, threads, the artwork and the decor as well as the feel of component connections and the inspection of cable bushings are also part of the final quality control.