The size error of plastic products mainly comes from the shrinkage of plastics during the molding process and the instability of various influencing factors. Simply put, it is to inject heated and melted plastic into a mold, and after cooling and shaping, its volume will be smaller than the mold cavity. The amount of reduction and the fluctuation of the process together determine the accuracy of the final size.
Specifically, errors mainly come from the following aspects:
The root cause of plastic size errors lies first and foremost in the physical properties of its polymer materials. From high-temperature filling of molds to room temperature solidification, the volume of plastic melt will inevitably shrink, which is called molding shrinkage. It is not a fixed value, but a variable influenced by multiple factors.
Crystallization behavior and orientation effect of resin: For crystalline plastics such as PA66 and POM, the molecular chains will be arranged in a regular manner to form a lattice during cooling, and the specific volume will change dramatically. The shrinkage rate is usually between 1.5% and 3.0%, and the fluctuation amplitude is large; Non crystalline plastics (such as ABS and PC) have disordered molecular chain freezing, lower shrinkage rate (about 0.4%~0.8%), and easier size assurance. In addition, during the filling of the melt mold, the molecular chains are stretched along the flow direction, and this frozen orientation will cause stress relaxation after demolding, resulting in inconsistent shrinkage between the flow direction and the vertical direction of the product, that is, anisotropy. This is the deep internal cause of product warping deformation.
The influence of raw material status and recycled materials: The moisture content of raw materials can change the melt viscosity, especially for moisture absorbing materials such as PA, where moisture acts as a plasticizer and increases the molding shrinkage rate. At the same time, adding recycled materials will change the melt index due to molecular chain breakage. If mixed unevenly, the dimensional tolerances of the same batch of products will be significantly dispersed.
2, Mold manufacturing and structure: "physical ceiling" of precision
The mold cavity is the "master" of the product, but the manufacturing accuracy and mechanical state of the master itself determine the upper limit of the product size.
1
Insufficient processing tolerance and compensation
There are inevitably mechanical processing tolerances in mold manufacturing, and more importantly, if the shrinkage rate is not taken into account during design (such as not considering the orientation of glass fibers or post extrusion shrinkage), even if the mold is processed to the micrometer level, the final product will still be a waste with huge deviation.
2
Rigid deformation and wear
Injection pressure often reaches tens of megapascals. If the rigidity of the mold is insufficient, the mold cavity will undergo elastic expansion under high pressure, resulting in increased wall thickness of the product. At the same time, for plastics with added fiberglass (GF), the surface of the mold cavity will be eroded by the melt, resulting in abrasive wear. After long-term production, the size of the mold cavity will increase, and the wall thickness of the product will also increase accordingly. In addition, wear and tear of the guiding mechanism can cause misalignment of the mold, directly resulting in irreversible geometric dimensional deviations.
transient process variables
Process parameters are the most active and frequently changing variables in the injection molding process, which directly determine the flow behavior, cooling rate, and shrinkage state of the melt in the mold cavity. Among them, the synergy and game of temperature, pressure, and time constitute the dynamic sources of size errors.
Temperature control runs through the entire injection molding process, including barrel temperature, nozzle temperature, and mold temperature. The mold temperature is the temperature parameter that has the greatest impact on the dimensional accuracy of the product. The mold temperature directly controls the cooling and shaping process of the product. During the period from the freezing of the sprue to the demolding of the product, the temperature of the mold determines the crystallinity of the crystalline plastic. As the mold temperature increases, the crystallinity increases and the shrinkage intensifies. But if the mold temperature is too low or uneven, insufficient cooling of the product can also lead to dimensional instability. For precision injection molding, the mold temperature needs to be precisely controlled within a specific range (such as 35-45 ° C) in order to control the dimensional shrinkage within the qualified indicators.
The temperature of the material barrel is another key variable. If the temperature is too high (exceeding the recommended processing range of 5-10 ° C), the viscosity of the plastic melt will decrease and the flowability will be too strong. During the cooling stage, the free space for molecular chain movement will increase, the shrinkage rate will increase, and the size deviation can reach 0.1% -0.3%; If the temperature is too low, the melt flowability is poor, the filling is insufficient, and the product size is prone to be smaller.
Among numerous process parameters, the holding pressure has the greatest impact on the size of the product. During the holding pressure stage, continuous replenishment of material to the mold cavity is used to compensate for cooling shrinkage - insufficient holding pressure can cause excessive shrinkage and smaller dimensions; Excessive pressure holding may result in oversized products or even flash edges. Practical data shows that for every fluctuation of 10MPa in the holding pressure, the dimensional deviation of some precision plastic parts can increase by more than 0.02mm. The study also shows that the thinner the wall of the molded part, the greater the impact of the holding pressure on the size. The holding time cannot be ignored either. If the time is too short, it cannot effectively compensate for shrinkage, and if the time is too long, it may cause overfilling.
The injection pressure and injection speed mainly affect the filling stage. The injection pressure is too low, making it difficult for the melt to completely fill the mold cavity, especially for complex thin-walled structures that are prone to short shots; Higher injection speed and pressure, combined with appropriately extended filling and holding time, are beneficial for overcoming size instability faults.
The impact of injection speed is more subtle. High speed injection will cause strong shear flow of the melt in the channel and cavity, resulting in local temperature increase and viscosity decrease due to shear heating. However, at the same time, the shear flow field induces the orientation of polymer chains along the flow direction. This frozen orientation leads to stress relaxation after demolding, resulting in different shrinkage amounts of the product in the flow direction and perpendicular direction - i.e. anisotropic shrinkage. This is one of the underlying causes of product warping and deformation.
All process parameters must be strictly controlled according to the process requirements, especially the molding cycle of each type of plastic part must be consistent and cannot be changed arbitrarily. Any small fluctuations in temperature, pressure, and time will be reflected in the final dimensional tolerance through cumulative effects.
the 'hardware ceiling' of dimensional errors
The accuracy of the injection molding machine itself determines whether the process instructions can be faithfully executed and is the physical basis of dimensional accuracy. If the accuracy of the equipment itself is insufficient, even the most precise process parameters cannot be converted into qualified products.
Precision injection molding requires that the control system of the injection molding machine must ensure good repeatability of various process parameters. This requires multi-stage feedback control of process parameters such as injection volume, injection pressure, injection speed, holding pressure, back pressure, and screw speed, while PID controllers are used for precise temperature control of the barrel and nozzle.
Modern high-end servo injection molding machines can achieve a weight repeatability accuracy of less than 3 ‰ and a mold opening accuracy of within ± 0.2mm through closed-loop control. The closed-loop control of the injection system can even achieve a repeat positioning accuracy of within 0.01mm. As a comparison, the injection action switching response time of a regular injection molding machine is about 300ms, while a fully closed-loop ultra precision injection molding machine is much better than this level.
The hydraulic system is the power heart of the injection molding machine. Precision injection molding machines generally require closed-loop control of heating and cooling of hydraulic oil to stabilize the oil temperature at around 50-55 ° C, in order to prevent viscosity and flow rate fluctuations caused by changes in oil temperature, which can lead to instability of process parameters. The advanced servo hydraulic system can achieve pressure fluctuation control below ± 0.5bar, and the repeatability accuracy of the main motion axis reaches ± 0.05%. The accuracy of the displacement sensor is required to be at least 0.1mm to strictly control the measuring stroke, injection stroke, and residual material pad thickness, ensuring accurate injection volume each time.
The injection pressure required for precision injection molding is much higher than that of ordinary injection molding - ordinary injection molding requires 40-120MPa, while precision injection molding requires an increase to 180-250MPa, and some even up to 415MPa. Under such high molding pressure, if the rigidity of the injection molding machine's clamping system is insufficient, the template will undergo elastic deformation, causing the mold parting surface to open and the product to produce burrs, resulting in a decrease in accuracy. Therefore, high rigidity template design and uniform locking force distribution are prerequisites for ensuring accuracy.
The wear of screws and barrels is a common cause of equipment accuracy degradation. Especially the anti reverse ring (also known as the rubber ring), it wears out severely after long-term production of reinforced filling materials (such as fiberglass). Poor sealing of the anti reverse ring can cause the melt to reflux during injection, leading to unstable measurement and fluctuations in the injection endpoint. This is directly reflected in the dispersion of the weight of each molded product and the differences in size between batches
5, Environmental interference: hidden and persistent 'external variables'
Environmental factors are the most easily overlooked but continuously influential variables in the injection molding process.
1. Seasonal fluctuations in environmental temperature
The temperature changes in the workshop environment directly affect the cooling efficiency of the mold and the control of the melt temperature. The temperature difference between day and night can cause the mold to repeatedly expand and contract, and the temperature control of the mold temperature controller is affected by environmental interference, resulting in unstable shrinkage and fluctuations in the size of products in the same batch. Actual measurement data shows that for every 5 ° C increase in ambient temperature, the size deviation rate of PP material products increases by 12% -15%. Taking PC materials as an example, for every 5 ° C change in temperature, the dimensional deviation can reach 0.05% -0.1%. Injection molded parts produced in low-temperature environments generally exhibit phenomena such as reduced size and increased warpage. This requires manufacturers to adjust the process temperature of equipment and molds in a timely manner according to changes in the external environment.
2. Humidity and moisture absorption expansion of materials
The influence of humidity on plastic size is also significant. Polyamide materials, represented by nylon (PA), contain polar amide groups in their molecular chains and are easily bound to water molecules. The equilibrium moisture content of non reinforced PA6 or PA66 can reach 3% under standard atmospheric conditions (23 ° C, 50% RH). In high humidity environments (relative humidity>60%), the size expansion can reach 0.2% -0.5%. When the humidity changes by 10%, the size fluctuation of nylon materials can reach 0.1% -0.3%.
What's even more tricky is that the moisture absorption and expansion of certain materials exhibit anisotropic characteristics - the higher the environmental humidity and the longer the time, the larger the size of the glass fiber reinforced PA66 material becomes, and the size change in the vertical flow direction is greater than that in the flow direction. This means that even with the same material and product, the response of different dimensions to humidity varies in different directions, posing additional challenges to the design of precision fittings.
3. Thermal deformation of equipment
The thermal deformation caused by long-term operation of the equipment can also lead to dimensional deviations. The various components of the injection molding machine expand due to frictional heat during continuous operation, which can change key parameters such as mold clamping accuracy and injection stroke. This slow and continuous drift is often difficult for operators to detect immediately.
6, Logic of System Control
In summary, the injection molding process parameters, equipment accuracy, and environmental interference are not isolated effects, but rather a dynamic system that is coupled and transmitted layer by layer. The process parameters are "real-time regulators", equipment accuracy is "execution guarantee", and the environment is "background noise". To ensure dimensional accuracy, a systematic control mindset must be established: selecting high-precision closed-loop injection molding machines and regularly maintaining them; Adopt mold temperature machine and workshop temperature control system to isolate environmental interference; Implement strict drying and humidity control treatment for sensitive materials (such as PA); And continuously monitor the stability of process parameters through Statistical Process Control (SPC). Only by controlling these three dimensions as an organic whole can objective errors be continuously and stably compressed within the design tolerance zone.
