SCR Voltage Regulators in Plastic Molding Machines: Temperature Control

Temperature is king in the world of plastic molding. It's crucial to maintain a precise temperature for the material; too hot and it begins to burn and lose its structural integrity, while too cool will lead to incompletely filled parts. The heaters in injection molding machines, extruders and blow molding machines require stable, precise power to ensure even barrel and die temperature. SCR (thyristor) voltage regulators can deliver micro-second response times, provide proportional control of power and are exceedingly reliable. In this article, we'll take a look at why the use of SCR technology is so vital for plastic molding applications.

In plastic molding, even tight temperature tolerance of 1°C relative to the target may be insufficient and Conventional electromechanical relays (contactors) or relays provide only on/off control; full power is delivered to the heating elements until the target temperature is achieved, then the power is cut off until the temperature drops below a hysteresis band. "bang bang" control at its finest which causes sawtooth patterns of temperature variation. The low thermal stability in the barrel will cause variations in product quality and increase scrap. Heaters on molding machines can be damaged by on/off control due to extreme heating and cooling. The constantly fluctuating power levels caused by on/off control and large current in-rush can cause premature element failure by damaging the heaters themselves (Cracking heater band connections and element failure) SCRs regulate voltage and provide a gradual and proportional heat to avoid damage to heaters and provide accurate stable temperature.

SCRs, (thyristors), regulate the power supplied to AC loads. For a resistive heater load like those used in molding machines, this is accomplished by firing the SCR at some point in each AC half cycle. Firing the SCR earlier delivers more power to the heater, firing later reduces the power delivered. SCR voltage regulators achieve 0 to 100% power without the mechanical switching necessary with a relay or contactor.

For plastic molding applications, phase-angle control mode offers precise and smooth temperature control. Output to the heaters is a smooth proportional output determined by a temperature control signal, usually 4-20mA or 0-10V. Unlike an electromechanical relay that switches every few seconds, an SCR can switch thousands of times per second to accurately control temperature within a tenth of a degree. Many SCRs also provide burst-fire (variable time base) mode control that can significantly reduce audible noise from certain loads.

Implementing an SCR voltage regulator onto an injection molding machine or extruder will provide tangible improvements in product quality and operating costs.

Temperature uniformity is the biggest advantage. Since the SCR always provides just the right amount of power to keep each heater band at the target temperature the temperature ripples found with contactors are eliminated. It keeps each of the barrel zones from becoming too hot or too cold thereby avoiding burning or incomplete mold fills.

Reduced heater failure significantly lowers the overall cost of operating. SCRs can apply a soft start to new heaters. They avoid excessive heating and cooling, which cause thermal stress on heater bands causing them to fail. Many customers report that heater life extends from months to years after installing our regulators.

Energy savings range between 5 and 15%. There is virtually no power consumed in transitions and the steady temperature output avoids overshoot, and consequent cooling. On a large 24-hour/day molding machine savings can run into thousands of dollars per year.

Choosing the correct SCR voltage regulator to implement in your plastic molding machine involves looking at some simple points.

Load rating must be known. We must ensure we rate the device a minimum of 20% over full load current to protect the unit against voltage surges. Single-phase resistive loads such as the heater bands in most molding machines work well with standard SCR panels. Three phase heating will require three phase SCRs and firing mode.

Control signal compatibility must be compatible with your machine's temperature controller. Most temperature controllers use 4-20mA or 0-10V output. Some also have a pot input. We utilize auto detecting circuitry so the regulator accepts any of these outputs.

Cooling requirements will need to be factored in. SCRs generate heat directly proportional to their output current and for any current in excess of 25-30A per phase a heat sink with a fan will need to be supplied. Power must also be derated by 20-30% if control panel temperature exceeds 40°C.

With more than 20 years of industry experience, all the raw materials from the metal casing to the finished product will be fabricated within our own facility in our more than 50,000 facility. We are official drafters for the national industry standard, and have been awarded 'Industry Champion' status by the Chinese Government. Our thyristor SCR regulators are produced with an emphasis on quality control, precise execution and maximum efficiency to perform where other devices can fail. All our software, as well as the controlling parameters within the software is internally produced to work optimally with our advanced logic systems to maximize ease of use when controlling any temperature control system and minimize any failures. Our highly skilled team of designers has developed a control system that is incredibly robust and stands up to any challenge it might face.

SCR voltage regulators replace rudimentary on/off control in plastic molding applications and transform the concept into smooth and precise heating for every application. Product quality, reduced maintenance costs and an improved energy usage are just a few of the benefits gained by switching to SCR control technology and for any 24-hour molding machine they are a necessity. We invite you to contact us today to see what our superior SCR voltage regulators can do to upgrade your plastic molding machine.