Processing of Plastics

Plastic is a polymeric material that fundamentally originates from organic compounds such as oil or natural gas. Its production is based on the polymerization process, where monomers link together into long chains of molecules called polymers. This adaptable material enables the production of various products ranging from packaging and electronic housings to automotive parts. Plastic is crucial to modern industry due to its durability, versatility, and relatively low production costs.

• The warehouse must be dry and not overly illuminated.

• Silos can be used for storing larger quantities.

• It is advisable to bring the material to the production facility at least 24 hours before processing begins.

• Inadequate, rejected, or obsolete material should be labeled and stored separately.

• Material packaging must have a label with the name and batch number.

During preparation, it is crucial that materials do not mix and that everything is thoroughly cleaned when changing materials or colors. Mixing two different materials can lead to problems during injection molding or an unsatisfactory final product.

Drying material requires strict adherence to time and temperature settings on drying equipment. There is no room for deviation or compromise in these two parameters. Material must be dried according to the instructions in the material technical datasheet. It is also necessary to regularly inspect and monitor the operation of the drying equipment to ensure desired drying results are achieved. This can also be verified by measuring the moisture content in the material after the drying process is completed.

The barrel on the injection or extrusion machine is primarily designed to transfer heat from the heaters to the screw and material. It is important to regularly clean the cylinder (see the cleaning process with Chem-Trend purging compound) and monitor the injection parameters. Although not frequent, the use of materials with a high content of glass fibers and other mineral fillers can cause cylinder wear, leading to poor and qualitatively inappropriate final products.

The screw in the plastic injection molding machine plays a crucial role in transferring material from the hopper or reservoir to the end nozzle. It functions by homogenizing and compressing the material and, in combination with a non return valve, enables the injection process. There are several types of screws, including the most common three-zone screw, which consists of the feeding, compression, and metering zones. In the first phase, known as plasticizing, solid plastic pellets transition into the compression zone, where partial melting of the granules begins. Subsequently, in the compression phase, the material is compressed, cut, and melted, eliminating air pockets. Finally, in the metering phase, mixing and homogenization of the melt occur. There are also screw designs with mixing elements; however, caution must be exercised when using these elements with materials sensitive to shearing, as they can cause undesirable effects.

The task of the non-return valve is extremely simple. Its role is to prevent the material or melt from returning back into the cylinder during the injection process. Throughout the process, the non-return valve can take two positions: dosing (front) and injection (rear - must seal). The non-return valve is composed of three parts:

• Tip or spigot,

• Ring,

• Support ring.

If the parameters of the cushion consistently show a value of 0 during injection, there is a high probability that the non-return valve is not functioning properly. It is necessary to disassemble the injection unit (barrel) and check the condition of the non-return valve. Most often, the problem is valve wear or foreign object at the point where the ring should seal on the support ring.

Fitting the nozzle of the injection unit or cylinder is extremely important, as improper fitting can cause leakage of material between the tool and the nozzle, as shown in the image below. To prevent this, it is important to check whether there are radius made for fitting on the tool and the machine's nozzle. Also, check for any damage to the machine's nozzle or the inlet nozzle on the tool. Additionally, it is crucial that the machine's nozzle is properly centered.

You can see recommended profile temperatures for processing a specific material in the image.

Source: Janez Navodnik, Errors in Injection Molding.

The part of the important process of the entire injection molding cycle is also the holding pressure, which basically must compensate for the volumetric shrinkage of the melt during cooling. Therefore, with this process, additional material is pushed into the mold cavity.

• The holding pressure is always somewhere between 40 and 60 % of the injection pressure.

• If the holding pressure is low and the holding pressure time is too short, the pieces will be incomplete or sink marks may appear on them.

• If the holding pressure is too high, it can lead to tool breakage, internal stresses in the product, and more difficult ejection of the product from the mold...

The key difference between amorphous and crystalline materials lies in their structure.

Amorphous materials:

• Randomly entangled chains

• Chemical sensitivity

• Moderate heat resistance

• Transparent materials

• Small shrinkage

• Wide range of softening

• Examples include PC (LEXAN™), PEI (ULTEM™), PPE (NORYL™), ABS (CYCOLAC™), ASA (GELOY™)

Crystalline materials:

• Some chain structures are ordered

• Chemical resistance

• High heat resistance

• Susceptibility to stress cracking

• Non-transparent materials

• Significant shrinkage

• Durable materials

• Abrasion resistance

• Various melting points

• Examples include PBT (VALOX™), PA, POM, PEEK, PE, PP...

This is the material that remains in the cylinder after the injection phase is completed (after the holding pressure). The injection process must allow the cushion to provide sufficient pressure in the mold. If the cushion is too small, the pressure cannot be transferred to the melt, resulting in incomplete filled parts and potential damage to the screw. If the cushion is too large, the material stays in the cylinder for too long, which can lead to material degradation.

A CONSTANT CUSHION VALUE INDICATES STABILITY AND REPEATABILITY OF THE PROCESS

On the image, you can see the basic recommendations for retaining a certain material in the cylinder. As you can observe, both too short and too long retention times in the cylinder are undesirable. It is important to pay attention to this parameter, as disregarding the recommendations can lead to deviations in the quality of the final product.

Source: Sumitomo DEMAG

In the table below are the basic parameters for plastic injection molding from the most common materials.

Source: Janez Navodnik, Errors in Injection Molding.

For the proper functioning of the injection molding tool, it is important to consider several basic factors:

• The tool must be manufactured in accordance with the customer's requirements, including consideration of material shrinkage, tool durability based on anticipated quantities, protection and coatings, hardness, high gloss or eroded surface, use of a hot runner system or cold runner.

• The tool must be properly maintained, including cleaning, protection, and lubrication.

• The tool must have flow channels for the medium, which must not be clogged. After each series, the channels need to be "blown out", and before each series, it is necessary to check whether the medium is flowing through the channels and how much it flows (in liters per minute) on the water group or temperature control device.

• The tool must have appropriate additional equipment, such as regulators of hot runner systems, suitable temperature control devices, controllers for hydraulic and pneumatic valves.

• It is recommended that each tool has a counter that cannot be dismantled or reset, as it allows monitoring the quantity of cycles produced with the tool.

Venting allows air or gas to escape as the melt enters the mold. If air becomes trapped in the tool, compression can cause it to heat up significantly, potentially igniting the melt and even causing a dent or erosion on the tool insert. Effective venting prevents the formation of incompletely filled products and potential material degradation during the filling of the mold cavity.

We are familiar with many different methods of tempering or regulating temperature in injection molding tools, but their common feature is high precision in operation. However, a well-functioning tempering device alone is unfortunately not sufficient; it is important to have a functioning entire system that is adapted according to the material used.

Key factors for the cooling system:

• Clean and flowing channels in the tool and inserts,

• Cooling channels should be placed as close as possible to the surface of the mold,

• Cooling channels must be adapted to the medium and temperature to be used,

• Temperating devices or cooling systems must be powerful enough and regularly maintained to be able to dissipate heat according to the processing procedure.

When processing materials such as PA, the tool temperature needs to be above 50°C. When processing materials such as PP, the tool temperature needs to be below 25°C.

Image source: YouTube Kruse Training

System water is crucial in the plastic injection molding process as it serves as a cooling medium for the oil in hydraulic injection molding machines and injection molds or tempering devices. It is important that regularly monitor the quality of system water through hardness and additive content measurements and takes appropriate action based on the measurement results. If the water in system is poor quality and contaminated, issues such as corrosion and clogged water channels or even deposits in injection molds may occur, that leads to negatively affecting the entire plastic injection molding process. This typically leads to overheating and longer cycles compared to when the tool was initially started.

Hot runner system in plastic injection molds is a method of distributing molten plastic that allows for precise and efficient injection molding. This system typically includes a heated manifold or distributor and multiple heated nozzles. When the injection molding machine pushes molten plastic into the manifold or distributor, it evenly distributes the plastic into several heated nozzles. These nozzles then accurately meter the plastic into specific injection points in the mold cavities. This system enables better product quality and accuracy, increases productivity, and reduces waste.