Turning raw material into a finished product can be a complex process. It usually requires more steps than a layperson would think and more controls along these process steps. This is where a firm grasp of manufacturing process controls becomes useful.
Understanding processes is important
Understanding the process(es) at work is really manufacturing 101, and most foreign buyers skip this. They come to China/Vietnam, think the factory people know their processes pretty well, and don’t take the time to gather that knowledge.
There are a set of process inputs, different controlled variable inputs as well as many uncontrolled variable inputs, all of which contribute to the manufacturing process outputs. And believe me… a very small proportion of Chinese manufacturers has a good grasp on what gets them good vs. bad results in the processes they run in-house.
The basic concept of ‘process control’
To put this into the simplest terms, a manufacturing process is an activity, or group of activities, that takes multiple inputs, transforms and adds value to them, and provides an output in the form of a product.
The diagram below shows this simple concept.
This is easy to grasp on a simple product such as making a cup of coffee where the inputs are minimal, the process is to add the inputs to a cup and stir, which results in the finished product.
If you get all this right (in other words, if your process controls are good), you will get a good cup of coffee.
So, let’s now look at a more complex model more suitable for an engineering project.
Source: Quality Progress May 2016
Let’s look at each of the 4 elements in turn.
1. Process inputs
Process inputs can be considered as raw materials, as well water, electricity, fuel, energy, all of which contribute to turning the raw material into a finished product.
You have probably heard the saying “garbage in, garbage out”. That’s really easy to understand. And yet, I am sure more than 50% of Chinese manufacturers have no formal mechanism in place to control production inputs.
Let’s take an example. They buy paint. Do they know how to check if every batch they get respects the desired sheer vs. viscosity curve? I have never seen a Chinese company do this well — except, of course, in some car plants.
Note that different batches of raw material will always have a slight variation, whether that be a chemical composition, a color variation, or other. All these variations could be within a set tolerance but there will always be a variation. That’s a fact.
2. Controlled variables
Controlled variables are parameters that you can control — machine settings for example.
Take the process of injection molding as an example, since it is such a common process in China. These controlled variables would include the melt temperature which is directly controlled by another controlled variable, the heater bands. The next variable to be set would be the mold temperature, this would be determined by the polymer (raw material) being used as different polymers require different mold temperatures. All these variables would have a feedback system that monitors and controls the set point of each attribute.
The switch-over position is the ram position where the filling (injection) stage switches to the post-filling (packing or holding) stage. The cushion distance is the distance from the switch-over position to the farthest position that the end of the screw can reach, as shown in the figure below. Thus, the switch-over position determines the cushion distance. The cushion should contain adequate material for post-filling the part. An insufficient cushion could cause sink marks. The typical cushion distance is about 5 to 10 mm.
Without going into detail for every single variable, I have listed some of the other variables to control in the injection mold process here:
- Screw rotation speed
- Cooling time
- Back pressure
- Mold open time
- Injection pressure
- Mold open stroke
- Holding pressure
- Ejector stroke
- Holding time
- Hydraulic pressure
- Injection velocity
- Water pressure
All these attributes are variable and are individually controlled for the specific process which would include the size of the molding and the polymer being used. In most cases, there would be some sort of feedback system — a closed loop feedback system for example that keeps these variables at a set point.
Note that some equipment (injection presses in this example) might do a great job of controlling those key variables… while others do it wrong. I can’t remember how many times I saw a temperature gage positioned way too far from the center of the mold, where controlling temperature makes sense.
3. Uncontrolled variables
Uncontrolled variables do not have any control, or would have limited control over them that provides a fixed known setting or status.
An example of an uncontrolled variable would be the ambient temperature or humidity in the factory. Even if your factory has air conditioning and dehumidifiers, they might not keep temperature or humidity constant over time.
Now, if they kept enough data (temperature and humidity readings over a long period of time), they might be able to factor in these data for your production process as temperature and humidity may affect the outcome of the manufacturing process. But this would require an understanding of statistics, which is usually pretty weak in Chinese manufacturing organizations.
Other uncontrolled variable inputs could include some of the following:
- Shift changeover – where there are multiple shifts producing the same product, there will inevitably be some variation in the staff and how they carry out the same function.
- Operators – one person may be more heavy-handed than the other, one may be faster than another, all these uncontrollable factors influence the process input.
- Machines – there may be multiple machines being used to create the same component, and there will be variations between machines. Some of this variation comes from how well the machines are maintained, their age, etc.
4. Process outputs
It is not just the product or component that is the output of the manufacturing process. There are a number of different process outputs produced.
- Yield – in basic terms, the yield is the proportion of correct items (conforming to specifications) you get out of a process compared to the number of raw items (or equivalent) you put into it.
So an example calculation for yield would be:
- Waste – this can be measured as scrap items as well as waste process inputs such as energy and water, or heat generated from the manufacturing process itself.
- Capacity – the definition of capacity is: the volume of products that can be generated by a production plant or enterprise in a given period by using current resources.
- Downtime – this is the period during which a machine or equipment is not functional or cannot be used for the manufacturing process. This may be caused by mechanical breakdown, machine adjustments, shortage of labor or shortage of other process inputs.
- Production Rates – this is the number of goods that can be produced in a given amount of time, it can also be viewed as the amount of time it takes to produce one item.
Summary about manufacturing process control
When planning your production process, always take into account all the different variables from an input point of view and understand what the uncontrollable variable inputs are and what the variation is on those inputs. This will help you better understand and control your outputs.
Use the diagram above (in figure 1) to help you plan your manufacturing processes and map out each of your inputs, process settings, and outputs. That’s the right first step.
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