Temperature
Temperature measurement and control are crucial in injection molding. While making these measurements is relatively simple, most injection molding machines lack sufficient sampling points or wiring.
On most injection molding machines, temperature is sensed by thermocouples. A thermocouple essentially consists of two different wires connected at the ends. If one end is hotter than the other, a small electrical signal is generated: the hotter it is, the stronger signal.
Temperature Control
Thermocouples are also widely used as sensors in temperature control systems. Desired temperature is set on control instrument, and sensor display is compared to temperature generated at setpoint. In the simplest system, system shuts off when setpoint is reached and then turns back on when temperature drops. This system is called an on-off control system because it is either on or off.
Melt Temperature
Melt temperature is important; shot cylinder temperature used is only a guide. Melt temperature can be measured at nozzle or using an air jet. Temperature setting for shot cylinder depends on melt temperature, screw speed, back pressure, shot size, and injection cycle.
If you have no experience processing a particular grade of plastic, start with the lowest setting. For ease of control, shot cylinder is divided into zones, not all of which are set to same temperature. If operating for extended periods or at high temperatures, set the first zone temperature to a lower value. This will prevent premature melting and diversion of plastic. Before injection begins, ensure that hydraulic oil, hopper closer, mold, and shot cylinder are all at correct temperatures.
Injection Pressure
This is pressure that causes plastic to flow and can be measured by a sensor at nozzle or on hydraulic line. It has no fixed value; the harder mold fills, the higher injection pressure. There is a direct relationship between injection line pressure and injection pressure.
First-stage Pressure and Second-stage Pressure
During filling phase of injection cycle, high shot pressure may be required to maintain required injection speed. High pressure is no longer required after mold is filled. However, when molding some semi-crystalline thermoplastics (such as PA and POM), sudden pressure change can deteriorate structure, so using a secondary pressure stage is sometimes unnecessary.
Clamping Pressure
Clamping pressure is necessary to counteract injection pressure. Do not automatically select the highest available value; instead, consider projected area and calculate an appropriate value. Projected area of molded part is the largest area seen from direction of application of clamping force. For most injection molding situations, it is approximately 2 tons per square inch, or 31 meganewtons per square meter. However, this is a low value and should be used as a rough guideline, as sidewalls must be considered if molded part has any depth.
Back Pressure
This is pressure that must be generated and exceeded before screw can retract. While using a high back pressure improves uniform pigment distribution and plastic melting, it also prolongs screw's return time, reduces fiber length of filler plastic, and increases stress on injection molding machine. Therefore, the lower back pressure, the better; under no circumstances should it exceed 20% of machine's injection pressure (maximum rating).
Nozzle Pressure
Nozzle pressure is pressure inside the nozzle. It roughly corresponds to pressure that causes plastic to flow. It has no fixed value but increases with mold filling difficulty. There is a direct relationship between nozzle pressure, line pressure, and injection pressure. On screw-type injection molding machines, nozzle pressure is approximately 10% less than injection pressure. On piston-type injection molding machines, pressure loss can reach 50%.
Injection Speed
This refers to speed at which mold is filled when screw acts as a punch. When molding thin-walled products, a high injection speed is essential to ensure that molten plastic completely fills mold before solidification, resulting in a smoother surface. During filling phase, a series of programmed injection rates are used to avoid defects such as jetting or trapped air. Injection can be performed under either an open-loop or recirculating control system.
Regardless of injection rate used, injection rate value, along with injection time, must be recorded on a record sheet. Note that this time refers to time required for mold to reach desired initial injection pressure and is a fraction of screw advance time.
Mold Venting
Due to rapid filling of mold, mold must be able to vent air. In most cases, this air is simply air trapped in mold cavity. If air cannot escape, it will melt and compress, causing plastic to heat up and burn. Vents should be located near water line and final shot. Typical vents are grooves 6 to 13 mm wide and 0.01 to 0.03 mm deep, usually located at parting plane of one mold half.

 

Holding Pressure
During filling phase of injection molding cycle, high injection pressure may be required to maintain required injection speed. After mold is filled, hold phase begins, during which screw (acting as a ram) advances additional plastic to compensate for shrinkage. This can be accomplished at lower or equally high pressures. Typically, if high pressure is used in the first phase, lower pressure is used in second phase. However, when molding some semi-crystalline thermoplastics (such as PA and POM), pressure fusion can deteriorate crystal structure, so a second phase of pressure is sometimes unnecessary.
Use of Recycled Plastics
Many injection molding machines use a mixture of virgin plastic and recycled plastic (commonly known as sprue stock). Surprisingly, using recycled plastic can improve performance of injection molding machine, producing more consistent molded parts. However, it is important to note that recycled plastic should be cleaned of dust before use to prevent variations in plastic feed volume and color distribution in molded parts. Exact percentage of recycled plastic used must be determined experimentally and must be determined without affecting physical properties of molded part. A general rule of thumb is between 15% and 25%.
Quality Control
Final starting weight and size of a molded part are closely linked to production conditions such as bedding size, injection pressure, and flow rate. This means that in many cases, it's possible to verify part's acceptability before any actual measurements are taken. During each shot, selected parameters are measured and compared to set or stored values. As long as measured values are within a preselected range, control system determines part is acceptable.
If measurement exceeds fill limit, part is rejected, or, if limit is only slightly exceeded, part is stopped and a second inspection by a qualified person is performed. Modern injection molding machines are equipped with video recorders and computer systems so that each part is compared to a stored image of required quality during molding. Each part is also compared to a standard part for dimensions and visual defects.
Recording Injection Conditions
It's important to never forget that purpose of an injection molding machine is to produce parts that meet quality requirements within a specified timeframe and at a specified cost. Accurate record keeping is essential to achieving this. On many machines, this can be accomplished at the touch of a button. If there is no button, complete an appropriate log sheet and retain a sample of molded part for future reference.
Shutdown
It is crucial to implement a proper shutdown procedure; this can save significant time and money. If you need to shut down machine, for example to burn plastic, then there is no need to drain plastic, potentially saving you expense of completely shutting down and cleaning machine.
Temporary Stoppage
If machine is stopped, it will require frequent spraying to remove excess plastic or running other plastic through machine to purge shot cylinder of excess plastic. Frequency of spraying may increase if plastic is discolored. When performing minor repairs, shot cylinder heater should be set to its lowest setting to minimize possibility of thermal decomposition. On more modern machines, this process may be initiated automatically.
Overnight Stoppage
Before molding hot plastics (such as PS), if machine is shut down overnight, simply turn off bottom slide and shot cylinder heater and spray shot cylinder clean. After nozzle is completely cleaned, cool shot cylinder as much as possible. Once injection molding machine has cooled, shut down all equipment and prepare it for reheating.
Heat-Sensitive Materials
If plastic decomposes or burns inside injection molding machine, it will eventually discolor and render molded part scrap. In this case, injection molding machine must be completely shut down and sprayed clean. Preventative measures include spraying heat-sensitive plastic with a more thermally stable plastic to resist subsequent heating. To address plastic oxidation, operator can fill shot cylinder with a plastic, such as PE.
Injection Molding Cycle
Injection molding cycle is the time it takes for injection molding machine to complete a specific set of operations. Therefore, time required for each component of operation can affect the overall cycle time. To achieve cycle time reduction, consider each component of operation separately to identify areas where time reduction is possible. This often results in a small time saving for each component. Although this saving may be small, when added together, percentage reduction in overall time can be significant.