The Need for Temperature Control in Plastic Injection Molding
Temperature control prevents quality issues such as shrinkage, warping and stresses from developing in the injection-molding processes.
From manufacturing personal items like toys and toothbrushes to industrial parts like plastics utilized in automobiles, injection molding is one of the most relevant manufacturing techniques in use today. This article will define what injection molding is, outline the steps involved, and explain the need for temperature control in the process.
Injection molding is a specialized manufacturing technique used for fabricating plastic parts and items. It allows for the mass production of several thousand — or even millions — of identical parts of a specific dimension and quality.
Injection molding offers the following advantages over conventional plastic manufacturing techniques:
- The cost per unit of producing an injection-molded part is low, allowing manufacturers to shave off massive costs, unlike in small-scale production.
- It is a precise and highly repeatable process that can be almost entirely automated. This helps to increase the speed of manufacturing, lower labor costs and shorten the time to market of new products.
- It has relatively lower scrap rates than conventional machining techniques such as CNC machining, which removes a considerable portion of the material.
These features help manufacturers minimize the waste of available resources.
Temperature control can be achieved by integrating a cooling system with an industrial chiller for injection molding.
Basic Injection-Molding Process
Plastic injection molding involves injecting molten plastic into a mold (or cavity), which defines its molded part’s shape after it solidifies. The basic requirements for the process are the injection-molding machine, raw plastic material and the mold.
The injection-molding machine consists of a hopper, through which pelletized plastics are fed into the machine; a heating barrel with a reciprocating screw; and an injection nozzle. The most common thermoplastics utilized in injection molding are nylon (PA), polycarbonate (PC), polypropylene (PP) and acrylonitrile-butadiene-styrene (ABS). Molds used can be single- or double-cavity type, depending on the application.
While actual configurations may differ between manufacturers, injection molding is essentially a three-step process involving the following steps:
Step 1: Injection. Plastic granules — thermoplastic resin pellets — are fed into the injection-molding machine via the hopper and then enter the heating barrel. The material is melted with band heaters and frictional heating from the reciprocating-screw barrel. Then, it is injected into the mold via the nozzle. As the melt is applied to the mold, hot air vents along the parting line and through the injection pins.
Step 2: Cooling. After injection, the molten plastic is cooled at a specific rate while the material hardens. In most cases, water or coolant will be cycled through the mold to lower its temperature. Temperature control can be achieved by integrating a cooling system with an industrial chiller for injection molding.
Cooling times depend upon the resin type and thickness of the material. A suitable cooling system attached to the mold transfers heat away from the melt by conduction, radiation or convection, keeping the cooling rate within specified limits.
The temperature of the melt can be anywhere between 392 to 572°F (200 to 300°C) and cools down to about 140°F (60°C) after being removed from the cavity (which absorbs most of the heat). The cooling time can take up to 60 percent of the entire cycle time. It ends when the molded part is hard enough to be ejected from the mold while leaving little or no residual plastic.
Step 3: Ejection. The mold is attached to a moveable platen. After the part has solidified, the mold opens and the ejector pins retract. The newly molded part falls out of the mold and into a bin underneath.
A suitable cooling system attached to the mold transfers heat away from the melt by conduction, radiation or convection, keeping the cooling rate within specified limits.
Limitations of Injection Molding
Like any other manufacturing process, injection molding is not without its downsides:
- The initial capital investment is high due to design, tooling and refinement requirements.
- Manufacturing companies must carry out extensive testing and prototyping.
- Its usefulness is limited for some molded part design and sizes.
Tooling Investment Costs. When a molded part is being manufactured for the first time, the design is first prototyped with a test material to ensure accuracy using techniques such as 3D printing or CNC machining. The mold tool is made of steel or aluminum material and must be designed to precise dimensions.
Prototyping and Testing. With injection molding, manufacturing companies must carry out extensive testing and prototyping of the entire system using replicas. Any subsequent modifications to the final design will require either modifying the tools or scrapping them completely — both of which could add significant costs to the production budget.
Size and Thickness Limitations. Because injection-molding machines and molds typically have limited sizes, injection molding may not be suitable for designing large plastic parts. Also, injection molding tends to create mostly molded parts of a uniform thickness. This characteristic may be undesirable to some manufacturers that require variation in this aspect. This is because injection-molded parts must be created with a sufficient wall thickness (at least 1 mm) to prevent problems with filling the mold.
The close tolerance temperature control units come in varying flow variations and configurations, and standard models have a pump and heat exchanger with a jacketed tank or a heat exchanger.
The Importance of Temperature Control in Injection Molding
Like many other industrial processes, temperature control is a critical consideration in injection molding. Effective temperature control prevents quality issues such as shrinkage, warping and stresses from developing in the material. A critical technical objective is to find a balance among:
- The temperature of the cooling fluid.
- The rate of mold cooling.
- The quality of the final product, keeping in mind that the speed of production is proportional to profitability.
Many manufacturers use cooling tower water to cool the small channels within the mold with a temperature regulator attached to the injection-molding machine, regulating its temperature. While this technique is effective and incurs a lower operating cost, the mold will be prone to contamination. Cooling towers are open-loop systems. By contrast, an industrial chiller can be used to regulate the temperature via closed-loop cooling, ensuring a higher degree of product purity.
When plastics for the injection-molding process are being heated and mixed inside a machine, a specific temperature limit must be maintained as the temperature of the process will directly impact the quality of the final mixture.
If the temperature is too low, the components may not mix properly. Alternately, when the temperature is too high, the mixture might become burned. Thus, there is typically an ideal point, or a prescribed temperature at which the process must be maintained.
Mold Temperature. The temperature of the mold is another essential consideration that affects the quality of the injection-molded part. Getting the best quality mold is a balance between heating the mixture sufficiently to create a homogeneous mixture and cooling it down at an ideal rate. Anything else would be undercooling or overcooling.
Undercooling and Overcooling Issues. Improper polymer flow is a direct consequence of under- or overcooling issues in injection molding. When the mixture is not cooled sufficiently, it may not solidify completely before being ejected. This may leave residual plastic in the mold.
Conversely, excessive cooling causes a lack of uniformity in the plastic material. This may cause further problems down the line such as shearing, cracking and cavities that do not fill up completely. Maintaining the mold temperature at a set temperature will yield the most optimum results.
Temperature Control with an Industrial Chiller System
Close tolerance temperature control units can be used with industrial chiller systems to provide close temperature tolerances for process water. When combined with a compatible and suitably sized industrial chiller, the temperature control unit can maintain the process temperatures at ±0.5°F of the set 33°F value, ensuring a stable injection-molding process without catastrophic freezing failures.
The close tolerance temperature control units come in varying flow variations and configurations and standard models have a pump and heat exchanger with a jacketed tank or a heat exchanger. PC