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.
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What do you think? Have you got experiences to share about the injection molding process?
Photo credit: Wikipedia
All of the comments are absolutely on target. Let me add one other item.
Sometimes you need to start with the real fundamentals. (This story involves compression molding, but the principles are the same.)
Factory was having occasional defects with the molded product – cracking and porosity. During a process audit I noticed that the pressure gage on one of the machines didn’t return to zero when the press opened. Further study showed that this gage didn’t really work at all. And, follow-up showed that the gages were not included in any calibration or maintenance program. So, who knows how long this problem had been occurring?
Injection molding machines will have a variety of gages. Time, temperature and pressure are all critical for successful molding.There will be at least a primary pressure gage (possibly several) and usually multiple temperature gages, measuring various zones in the injection screw. Make sure that all are calibrated on a regular basis. This is particularly critical for the temperature gages; temperature sensors tend to wear out relatively quickly and will need replacement on a regular basis.
It has been my experience in many Chinese factories that they may be very diligent in routine calibration of handheld gages – micrometers, calipers, etc, used for inspection; but totally overlook the gages which actually control the process. Further, my assumption that ISO 9000 auditors would have checked this situation has frequently been incorrect.
One other fundamental to check for. As mentioned in the original post, usually there will be one process “wizard” who establishes the initial machine parameters. Make sure that the factory has a practice of recording the machine parameters on a job setup sheet, and then check to see if the production workers follow the instructions. If and when the standard parameters don’t produce an acceptable part, then the process wizard should be called in (rather than letting production people do their own adjustments to the parameters.)
Best to all!
Brad
Brad,
Many many thanks for expanding this discussion!
You are right, calibration sounds like “extra useless work” to most Chinese manufacturers, so they don’t have a program in place for it.
And most ISO 9001 auditors here are too busy looking for little mistakes in the documentation, to “waste time” on gages 😉
Hi Brad, I’d like to get in touch with you — how can I do so?
Email me at bpritts@pritts.com, or call (I am in USA at the moment, Eastern time: 734 834 3675.
With the ever-improving progress in robotics, industries in China have already started opting for the complete automation of their manufacturing processes. This results in reduction of head count and saving money. Today, Robots start becoming the integral part of many manufacturing companies. Industrial robots are capable of doing variety of tasks. For example, final assembly of the parts like placing component parts on circuit board, screwing the devices, cleaning and dusting of delicate fixings and many more such tasks. The main reason behind the advent of industrial robots is the labor shortage and wage inflation. Automation trend gear up due to worker protests that lead to slowing down of production. Robots can do work 24 hours a day with almost negligible downtime.
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Unfortunately, it is not that simple, as I wrote before: https://www.qualityinspection.org/semi-automation/
Apart from all the technical issues which are likely to arise with setting up own injection moulding units in China there are trust issues which arise. Examples (from personal experience) are injection factory going belly up (I lost all my tools overnight), the contractor injecting from my tool and supplying to others (happened more than once). Another incident was a factory kept claiming that my tool needed repairs, and charging me for repairs. I got fed up and moved the moulds to find that the tools were perfectly fine. After a long learning curve I finally got it right – I now store all my tools in my own wharehouse when they are not being used, and finally have one “reliable” and (hopefully) financially sound contractor to do the injections as and when needed. Plus I have my own engineer full time overlooking the jobs in this contractors unit. Its been 6 years now following this system and so far so good !
Thanks for sharing your experience. Yes, that sounds like a good solution.
Do you also have legally-enforceable contracts in place with the contractors?
Legal channels do not hold much weight in my SME trade. I would end up throwing more money down the drain. Instead I leverage in other ways. First is of course maintaining good Guangxi. Second is negotiating credit terms so that I pay suppliers after goods are received and checked. With new suppliers I never pay deposit, I open Letter of Credit. Other practical ways of dealing with situations is being in the network of the suppliers close friends / family of the supplier in case there is a dispute its easier to settle by involving them (the whole “saving face” factor). When it comes to dispute resolution in China it mostly boils down to who is holding the money. If the factory has your deposit or payment, the chances of getting a settlement come down to about 1% : )
Oh yes, if you pay them that late I can see why you don’t need to worry too much. Good approach.
Several parameters need to be take into account. Here is how our chief engineer explained this to a client recently.