Skating Wheels manufacturing
They will have no more secrets for you...
Widening spokes for robustness and rigidity >
We took advantage of a trip in the French region of Doubs stop by Chézery (Ain), near Oyonnax.
This small village of 400 inhabitants is home to the only French manufacturer of inline skate wheels, RollX. The family business established here some decades ago by Alain Myallonnier has made a name for itself on the French market among giants like Matter or MPC. Olivier (son of Alain) leads us on a tour of the premises...
Major brands design their wheels on computers using CAD (Computer Aided Design). Wheel development often takes several years.
It all starts from a block of steel that will be engineered into a mould. This mould is the "negative" part. Mould making starts with conventional machining techniques (lathe).
As for the imprint of the rim, it is done by removing material with Electrical Discharge Machining (EDM), a process approximately reverse to that of welding. This is required to finely hollow material out of the very hard mould imprints, which are made of hardened steel, the material of choice for moulds that will need to withstand millions of repeated uses.
In fact, EDM starts with machining a brass electrode shaped like a positive image of the wheel core. This electrode will erode the steel block (the “negative”) to the desired shape. It is slowly lowered into the raw material block. An electrical arc appears between the two parts and erodes into the steel. It takes approximately forty hours of this process to achieve a 12mm deep imprint.
A mould takes many hundreds hours of work and its worth can be anywhere between 5,000 and 40,000 Euros, depending the complexity of the design.
EDM electrode displays the shape of the future part, here a wheel core mould.
Once the moulds are ready, they are sent to the injection shop to be set into the machine’s mould mounts, ready for chain injections. It only takes swapping the mould in and out of the machine’s mounts to start injecting different wheel diameters. Core raw materials pellets
First of all, wheel core making starts with raw materials. Depending on the wheel target usage, different materials are used: polymers, fiberglass...
Raw materials come in pellet form, colored (white, yellow, red) for different core colors. To get red cores, for example, you’ll mix 2% red pellets with 98% white pellets. Two types of materials are mixed: one rigid and one more flexible, for both rolling qualities and good resistance to breakage.
Before being sent into the machine, the pellets are passed through a dryer. Moisture removal will prevent the core from breaking during usage.
Inside the machine, materials are heated up to 230/240°C. An endless screw mechanism distributes materials into the temperature-regulated mould. One production cycle lasts over 2 minutes. Materials are injected through a tiny hole under 2200 bar pressure. To ensure sealing, the machine applies a closure pressure of 50 tons.
Then, the part is ejected and separated from its "carrot" (residual from the injection hole), once it has cooled off enough.
When it exits the machine, the part dimensions are not to standards yet, because the heat expanded the materials. It will take about one hour of waiting for the core to be of perfect shape.
Designing a core requires skilled dosage of the various materials according to the qualities one wants to favor: rebound, flex, robustness...
The first cores will therefore be sent to the test bench in specialized plastics labs in Oyonnax, to assess their properties. This is a mandatory step, and also quite an expensive one.
Before molding the over-cores onto the rims, cores will be heated up to between 80°C and 100°C. Temperature must equal that of the molding to obtain the best cohesion of the gum on the core.
There are 2 processes for over-core making: injection and molding over. At RollX, the latter is privileged and this is the one we'll look into in a more detailed manner...
First of all, one must tune the machine that will mix the polyurethane compounds. You get varying degrees of hardness by changing the proportions of the 3 basic materials. This is also when you may add coloring dye. Materials show up in pellet form, just like for the core. Components are mixed together and melted.
Melted material before casting
Over-core moulds are like little tanks, the top is open and can be closed from above. After thorough cleansing, the core is set there to recieve the gum. Cleanliness of the mould is mandatory, any leftover dirt will generate an imperfection on the gum surface. Therefore moulds are cleaned up between each series of molding.
The core is set at the mould center before casting the gum.
Moulds are first heated up to 80 to 100°C, depending on the material used. The rim is then set in the middle before closure. Then follows the casting phase. Materials heat-melted to liquid form are poured into the moulds.
A few minutes later, materials will set and the wheel can be uncast.
At this stage of manufacturing, the wheel is far from having reached its target hardness. A few minutes out of its cast, a wheel may be 60A shore when its final hardness after drying will be 85A.
The cooled-off wheel is taken out of its cast Casting has left excess material around the wheel, which must be taken off. No worry, a quick pass in a machine will trim this glut off.
Cutting off excess material leftover from casting
Wheels are then left to dry for 24 hours, before returning to the oven for 12 to 72 hours, depending on the material.
Wheels fresh out of the casts still display low hardness
It will take 1 to 2 months in the warehouse before the wheels can be used, time for full polymerization to complete and chemical structure to stabilize.
Wheels are returned to the oven
Properly dried up wheels are cleaned up before labeling and packaging.
Printing of the decals on the wheels is done by pad printing. Ink is deposited onto a finely engraved cliché plate. The pads descends on it and lands on the wheel for printing.
The cliché plate must be changed for each color. A 3-color design requires 3 plate exchanges for printing on one single wheel.
Cliché plate used for printing on the wheels
Pad printing: the pad descends to print on the wheel
Eventually comes the final phase, packaging! Wheels are lined up on an axis to insert them into a tubular plastic film that its heat-shrunk to secure the whole lot. Of course, never forget to insert the brand sticker as well as the label that displays all the technical information about the set of wheels.
Many thanks to RollX for this very enlightening tour!
Liens utilesText : Alfathor
Translated by Hub and Toox
Photos : Alfathor