Many importers complain about the lack of technical expertise in Chinese factories. In most cases, there is one key engineer who “works his magic”. And, when he is gone, really bad things start to come out.
Setting up injection molding machines is one of these things that require deep experience, and that can ruin a whole batch if handled improperly.
And, to make things more difficult, the engineer in charge of doing and fine-tuning the setup seldom (never?) speaks English. Communication though a translator who doesn’t understand all the subtleties is a real challenge…
The two stages of setting up an injection molding machine
Several parameters need to be take into account. Here is how our chief engineer explained this to a client recently.
In order to determine the parameters of an injection machine, it needs to be done in several stages.
1. The geometry of the part being injected determines the basic parameters:
A few examples:
- The volume determine the shot size
- The projected surface determine the clamping pressure
- The material used determine the temperatures, cooling, heating, etc.
2.a Optimization of the above-mentioned parameters (small batches)
For small batches, it makes sense for an engineer to spend little time fine-tuning the parameters.
In addition, an engineer with enough experience should know the common problems encountered with different materials. Here are a few examples:
- Nylon (PA-66) – very common in industrial parts — typically has a high shrinkage ratio. In order to stabilize the shrinkage, it is necessary to dip it into warm-to-hot water.
- Nylon with glass/fiber filler will have cosmetics flaws. In order to minimize the flaws, it would be necessary to use different fibers for different surfaces (float or sink fiber).
- PE, ABS, PC, and other materials will have their own characteristics. If the injected part is an overmold, then it will require a different process as well.
2.b Optimization of the above-mentioned parameters (large batches)
For batches of at least 2 million shots (this number depends on the cavities per mold), it is advised to spend more energy and get as close as possible to the optimum set of parameters.
To achieve this, it is necessary to use a statistical tool called Design of Experiment (DOE). It is not easy and is time consuming. But most six sigma black belts and quality engineers can do it.
What do you think? Have you got experiences to share about the injection molding process?
Photo credit: Wikipedia