Most of the safety standards implemented in China's plastics machinery industry adopt European standards (EN) or international standards (ISO, IEC), and exported products generally require CE or SPI certification. With the improvement of domestic plastics machinery manufacturing capabilities and the increase in injection molding machine exports, the safety protection requirements for injection molding machines have become increasingly important. This article discusses how to design effective circuits according to relevant safety standards to ensure that the safety level meets certification requirements. This discussion only covers a portion of the safety standards for injection molding machines.
Keywords:
Injection molding machine, risk assessment, performance level, safety-related functions
Abstract
Most of the safety standards adopted by the domestic plastic industry adopt the European standard EN or the international standard ISO, IEC, and the export products are generally required to pass the CE or SPI standard certification. With the domestic plastics machinery manufacturing developing, injection molding machine exports increased, the injection molding machine safety protection requirements have become increasingly important, this article discussed how to design safety circuit according to the relevant safety standards. This article only discusses on part of the safety standards for injection molding machine.
Key Words:
Plastic injection machine, Risk assessment, Performance Level, SRP/CS
1. Overview
Plastic molded products are highly interconnected and comprehensive, playing an increasingly vital role in the national economy. The development of the plastics machinery industry, to a certain extent, reflects the level of economic and technological development of a country or region. Currently, the production requirements for injection molding machines are constantly increasing, and safety certification is receiving more and more attention from injection molding machine manufacturers. The main international safety standards for injection molding machines include the European CE standard and the American SPI standard. With the publication of EN ISO 13849-1, mechanical safety can now be comprehensively assessed from components to the entire system, effectively solving the problem that the original EN 954-1 standard could not quantitatively assess system safety. In this regard, China is a major global producer, consumer, and exporter of plastics machinery, and the Chinese plastics machinery industry is on the path of international advancement.
2. Standards related to mechanical safety control systems
EN ISO 13849-1 assesses safety control systems or safety control functions not only through the categories of control systems known in EN 954-1, but also through performance levels (PL: a, b, c, d, e) estimated after probabilistic considerations. This standard applies to control systems using electrical/electronic components and other media (including hydraulic, pneumatic, etc.) that, when correctly applied, aim to reduce hazards to an acceptable level, or, under foreseeable conditions, to perform full safety functions and achieve risk reduction as specified.
Table 1 Relationship between PL, SIL, and PFHD
Figure 1 illustrates the relationship and "conversion" between Performance Level (PL) and Safety Integrity Level (SIL). This relationship is based on the hourly average probability of hazardous failure, also known as the PFHD value.
3. Main safety functions
The working principle of an injection molding machine is to use the thrust of a screw (or plunger) to inject pre-plasticized molten plastic (i.e., viscous flow state) into a closed mold cavity, and then obtain the finished product after solidification. A typical injection molding machine includes an injection unit, a mold clamping unit, a hydraulic system, and an electrical control system.
1-Mold area; 2-Mold closing mechanism area; 4-Nozzle area; 5-Plasticizing area and injection molding mechanism area; 5.1-Barrel feeding area; 5.2-Heating coil area; 6-Product falling area
Figure 1. Schematic diagram of an injection molding machine with horizontal mold clamping and injection unit (without protective device).
Safety components used in horizontal injection molding machines include safety switches for the front and rear doors of the mold clamping area, an emergency stop button, safety switches for the nozzle area (where safety may be present), a top cover safety switch, a mode selection switch, and safety mats. For ordinary horizontal injection molding machines with fewer safety features, these are often limited to the front and rear safety doors and the emergency stop button. Top cover safety doors, safety edge switches, safety mats, and mode switches are optional and generally used in special models or large injection molding machines.
Figure 2. Main safety functions of injection molding machines
4. Structure and application of EN ISO 13849-14.1 Required performance level
Before designing a safety loop, a risk assessment must be performed for each safety function to determine the required performance level (PLr). This can be determined using the following risk diagram, which can also be understood as the "rated" performance level for each safety function.
Figure 3 Risk diagram (based on EN ISO 13849-1)
Risk assessment is achieved through a simple combination of three parameters:
(1) S: How serious is the injury caused by the hazard?
S1 Minor injury (usually reversible), such as abrasions or lacerations.
S2 Serious injury (usually irreversible), such as media, death (2) F: Duration or frequency of personnel being in a hazardous area?
F1 less or shorter exposure time
F2 Frequent or long exposure time (3) P: Can the danger be avoided?
P1 may, under specific conditions, present a practical possibility of preventing accidents or avoiding dangers.
P2 is almost impossible
The assessment results show that the risk increases from low to high. For each safety function, a PL (Probability of Risk) is determined that is appropriate for the current risk reduction. Finally, the PL of the calculated loop must be greater than or equal to PLr.
4.2 Determine basic parameters (1) Category Cat
Safety-related functions (SRP/CS) consist of a sensor system, logic processing, and actuation elements.
Figure 4 SRP/CS
The category is a basic parameter describing the operation of safety-related functions (SRP/CS) when a failure occurs. It is a more precise description based on the categories in EN 954-1, and is divided into five categories: B, 1, 2, 3, and 4.
Category B is characterized by a single-channel structure, which may result in the loss of safety functions when a single fault occurs, and can only achieve the performance level of PL=b.
Category 1 also features a single-channel architecture, based on Category B. Failure may also result in the loss of safety functions, but requires the use of proven components and safety principles, as detailed in Annexes A to D of EN ISO 13849-2. Category 1 can achieve a performance level of PL=c at its highest.
Category 2 includes a single-channel architecture that implements the safety functions. Furthermore, a verification function must be implemented to check the effectiveness of the safety functions at appropriate time intervals. The quality of the verification function is crucial; it must be able to identify faults. A fault occurring between two verifications may result in the loss of the safety functions. Category 2 can achieve a performance level of PL=d at its highest.
Category 3 includes a dual-channel structure to implement the required safety functions. The design of these safety functions must ensure that a single failure of one of these components will not result in the loss of the safety functions. In other words, all safety components in Category 3 must be redundantly designed to ensure that the safety functions are maintained even if one channel fails; however, cumulative failures may lead to the loss of the safety functions. Category 3 can achieve a performance level of PL=e at its highest.
Category 4 also includes a dual-channel structure to achieve the required safety functions, but unlike Category 3, it can detect faults in a timely manner, and the accumulation of undetected faults will not lead to the loss of safety functions. Category 4 can achieve the highest performance level of Ple.
(2) MTTFd
The mean time to critical failure of components is divided into three intervals:
Low: 3 years ≤ MTTFd ≤ 10 years; Medium: 10 years ≤ MTTFd ≤ 30 years; High: 30 years ≤ MTTFd ≤ 100 years
For pneumatic and mechanical components, the Mean Time to Critical Failure (MTTFd) largely depends on the number of switching operations, as this is directly related to the mechanical wear of the component. B10d is a reference value, representing the total number of switching operations required for 10% of components to experience a critical failure during the lifecycle inspection. The MTTFd value can be calculated from B10d.
(3) Diagnostic coverage DC
Diagnostic coverage (DC) is a measure of the efficiency of safety function diagnostics (fault detection) by checking for failures or defects in relevant components of safety functions. DC can be calculated as the ratio of the failure rate of detected hazardous failures to the total failure rate of hazardous failures. DC is divided into four levels.
Table 2 DC Diagnostic Coverage
(4) Common Cause Failure (CCF)A single failure will not cause both channels of a redundant system to fail simultaneously. For categories 2, 3, and 4, it is necessary to evaluate the measures taken or to be taken using Appendix Table F.1 of standard EN ISO 134901, and compliance is achieved with a target value of 65% or higher.
4.3 Computational Performance Level
Based on the values of parameters such as Cat, MTTFd, DC, and CCF obtained above, combined with the chart, the corresponding performance level PL can be obtained.
Figure 5. Performance level PL calculated using parameters.
5. Perform verificationThe mold closing area is the most dangerous area. Taking the mold closing mechanism as an example, the movement of the mold plate drive device in this area can cause crushing and shearing hazards to operators and maintenance personnel. If the protective devices in the mold area are opened, a person can enter the area behind the mold plate when the mold opens, potentially causing crushing hazards. The movement of the core pulling and ejection mechanisms can also cause the same dangers. These hazards are considered irreversible injuries and are difficult to avoid.
Figure 6 Risk Map of the Mold Combination Area
Therefore, according to the risk diagram, the required performance level PLr for this area must reach PLr=d or PLr=e (depending on the frequency of door opening). Therefore, a reliable safety design is necessary to monitor the safety protection devices (front and rear doors) in the mold-closing area.
For safety-related aspects of the mold-closing area, the front and rear safety door solution utilizes Pilz PSEN mag series non-contact magnetic switches. These switches feature a dual-channel structure, 2N/C, 1N/O contact types, a sensing distance of 10mm, and a maximum performance rating of PLe. They also boast an IP67 protection rating and can be equipped with LED status indicators. Available in small or compact designs, they offer flexible installation. The safety doors can meet Cat 3 or Cat 4 standards.
Figure 7 Non-contact magnetic safety switch
For medium and large injection molding machines, the logic processing section uses the Pilz modular programmable controller PNOZmulti series to complete the corresponding safety functions. All safety functions are directly connected to PNOZmulti, which has a total of 20 safety inputs. Through simple programming, the logic can be freely configured to execute corresponding safety actions. The output of 4 safety transistors can reach the performance level of PLe (Cat 4). The safety output signal is given to the extended safety relay to control the cut-off of hydraulic valves or motor main contactors.
Figure 8 PNOZ multi 2 programmable controller
After obtaining the basic safety parameters of the required components, the corresponding SRP/CS performance level can be easily and quickly calculated using Pilz's safety calculation software PAScal, based on the designed electrical circuit.
Figure 9 Security Level Calculation Software
6. OutlookAs the industry develops, safety standards will continue to be updated, and the safety components used must also keep pace with the times. For example, coded non-contact safety switches can be used for front doors. Pilz's PSEN cs6.11 switch with a 50N holding force or the PSEN sl1.0 safety door system with a 1000N locking force both meet the PLe performance level. Their three coding methods can adapt to different application requirements in factories and have good prospects for use.
Authors: Sun Changang (1979-), male, electrical engineer, currently working at Haitian Group; Wang Zhuting (1988-), male, electrical engineer, currently working at Haitian Group; Xu Ruxia (1985-), male, electrical engineer, currently working at Haitian Group; Xu Qiqi (1988-), male, application engineer, currently working at Pilz China. References:
[1] GB 22530-2008. Safety requirements for rubber and plastic injection molding machines [H]. Beijing: China Standards Press, 2008:
［2］Safety of machinery – Safety-related parts of control systems – Part 1: General principles for design (ISO 13849-1:2006)
