Interview measurement technology Mould design: Cavity pressure as an indicator of product quality
Cavity pressure measurement is regarded as an efficient method of optimising the injection moulding process. Christian Streili, Field Service Development Manager at Kistler, explains how the data obtained can be correctly interpreted and how experts can optimise injection moulding processes.
Mr Streili, which do you think are the biggest challenges in the injection moulding process?
Streili: I think the biggest challenge is the complexity of the process. Many factors influence the quality of the finished product: the size of the moulded part, the material, the temperature of the working environment and the temperature of the plastic. In addition, moulding processes with multi-components or inserts work differently than those without because, as in the first case, several materials are used. Many injection moulders underestimate the influence of such factors or are unaware of how strongly seemingly tiny changes in the process affect the quality of the individual parts. Switching to a new material supplier, for example, can have a decisive impact on product quality — even if the composition of the plastic is exactly the same on paper.
How can measuring cavity pressure overcome these challenges?
Streili: Cavity pressure plays a crucial role during the injection moulding process. It is a reflection of the entire process in the moulded part, gives us information about the manufacturing conditions and is thus an indicator of product quality. Specific quality-relevant properties of the individual part, such as its dimensional accuracy, surface quality, its weight or the warpage even allow conclusions to be drawn about the conditions during the individual phases of the process. Cavity pressure thus matches a part-specific fingerprint of the product quality and helps to make precise statements about optimal process parameters. In addition, by measuring cavity pressure, manufacturers can identify defective parts even before they remove them from the mould.
How does this quality control work?
Streili: The measurement data obtained by the sensors are transmitted to the process monitoring system, which evaluates them and displays the result in the form of a graph. To sort out NOK parts, it is necessary to define a so-called evaluation box. If the curve representing the cavity pressure is inside this box, the produced part is perfect. If it is outside, it indicates a defective part. It is therefore particularly important to determine the limits of this evaluation box correctly. If they are too small, this results in pseudo-scrap. If they are too large, it has a negative impact on product quality.
How do you support your customers in exploiting the potential of cavity pressure measurement?
Streili: We offer our customers several services. Firstly, we have the possibility to provide them with a simulation of the process even before the injection mould is built. Based on these simulation results, we can determine how many sensors manufacturers need, where they should be optimally positioned and which type of sensor is best suited for the respective process. It also allows us to identify potential faults and weaknesses in the injection moulding process at a very early stage.
The challenge of injection moulding
Optimisation requires a lot of expert knowledge
Producing perfect injection moulded parts with as little scrap as possible is no easy task. Different shapes and materials require different approaches. At the same time, many external factors influence the process. This is where tiny piezoelectric sensors come into their own: They help injection moulders find and maintain the optimal processes for each mould — regardless of the materials used. The sensors measure and analyse the cavity pressure during the injection moulding process. These measured values allow production to be monitored or the hot runner system to be kept stable during the ongoing process and bad parts to be automatically sorted out.
This is where things get complex, as the installation of the sensors as well as the subsequent interpretation of the measurement results are demanding. In order to make optimum use of both the hardware and the data, a great deal of expert knowledge is required in the fields of injection moulding and metrology. Kistler therefore supports manufacturers with individual service offerings to exploit the full potential of piezoelectric equipment.
Can you give an example of the optimisation process carried out for a customer?
Streili: As part of our Mold Validation I service package, we make sure that all sensors are installed correctly and function as desired. For example, we check that the holes in which the sensors are located have been made correctly and that the sensor fits in well. We also check that the measuring cables have been laid correctly and that they are adequately protected in critical areas of the injection mould.
In the Mold Validation II phase, we take a closer look at running processes. The aim here is to optimise the process by defining the ideal parameters. Mold Validation II is particularly useful for customers who find it difficult to identify the problem in the existing process. The service is also useful when a crucial part of the process changes - for example, because the customer wants to use a new mould. We also help manufacturers to define the parameters of the evaluation boxes, which reduces waste or increases process efficiency. In this way, we help our customers to optimise their injection moulding process in the best possible way and ensure that they make the best use of our measurement technology and the measured values generated by our sensors. At the end of each optimisation process, the customer receives detailed documentation of the services and recommendations for possible next steps.
Why do manufacturers need the help of experts at all? Wouldn't a simple checklist of the most important parameters suffice to improve the process?
Streili: Of course, there are generally applicable criteria whose adjustments can improve the product quality of injection moulding technology — for example, the (process) temperature of the material can influence dimensional accuracy. However, our experts take a more complex approach. After all, every mould and every process is different: with all the different factors involved, it is often difficult to see the connection between faulty process sequences and external influences. This is where our expertise comes in - because even experienced injection moulding technicians who install sensors themselves can benefit from services like Mold Validation II.
Thank you Mr Streili