Last week I wrote about the cost of making a mold. This week I am addressing another common question from clients: why does it generally take about one month?
The length of time it takes to produce a mold tool depends on a number of factors. Primarily, the more complex the part, the longer it takes to manufacture the mold. Other factors to take into account are the size of the part to be molding, and the number of cavities within the tool.
The basic parts of a mold tool are:
- Bolster or support block, this allows the tool to be fitted to the injection molding machine.
- The core and cavity, this is where the shape of the part to be molded is machined — the time to generate this part of the tool depends on how complex the part is, as mentioned above.
- Injection system — includes the locating ring, sprue, sprue bush, and the runner system which is machined into the mold to allow the part to be filled correctly.
- Cooling system — the tool needs to be maintained at an optimum temperature for the injection process to function at its best, however once the molten plastic has been injected into the tool, it needs to be cooled down before being ejected.
- Ejector system — this comprises of the ejector housing, ejector plate, ejector pins, ejector return pins, and the spur puller pin. All ejector pins need to be positioned so that the part is pushed out of the tool cleanly.
From a manufacturing point of view, a number of different machines and processes are used. And an element of hand finishing goes into every tool. If we focus on just the core and cavity, the steps would include:
- Initial rough shape would be created from CNC machining centre or CNC mill
- Electrical Discharge Machining commonly known as EDM is then normally used to produce the final shape and dimensions of the part required. The basic EDM principle is where an electrical spark is created between an electrode that has been machined to a specific shape, and the work piece. The electrical spark is so hot (up to 12,000 Deg C) that is melts virtually everything with the idea being that the work piece takes on the shape of the electrode. The key thing is to the control the spark so that it only affects a very small amount of the work piece at a time. Accuracy of EDM can be controlled to 0.0025 mm.
- Once the shape has been generated to the correct dimensions, the next stage is the surface finish; this could include final grinding, texturing, sand blasting, shot peening, or hand polishing.
One of our engineers tells me of a time during a factory visit (inspecting a set of tools for a client) where he witnessed a worker sitting on a wooden stool inside a tool cavity, hand polishing the surface. When our engineer asked how long the worker had been polishing that tool, he was told this would require 1 week to complete — just hand polishing (bearing in mind the size of the tool for him to be sitting inside it!)
Beyond the core and cavity, there are many other items that go into making a mold tool. And machining, finishing, polishing and assembly of a tool just takes time.