• 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 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.

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

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.

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.

I. Properties

Basic types of PE: PE is obtained by polymerization of ethylene through various processes, resulting in polyethylenes with different properties (e.g., high and low-pressure processes), such as LDPE, MDPE, HDPE, and LLDPE.

a) Density: - LDPE branched: 0.914 to 0.94 g/cm³

• LLDPE: 0.918 to 0.943 g/cm³

• HDPE: 0.94 to 0.96 g/cm³

b) Structure: non-polar, partially crystalline thermoplastic with varying degrees of branching, affecting crystallinity (from 40 to 55% for LDPE and from 60 to 80% for HDPE). PE hardly absorbs water.

c) Mechanical properties: mechanical and chemical properties depend on crystallinity (density) and polymerization degree (melt flow index, MFI). With increasing density (linearity), PE exhibits higher tensile and flexural strength, stiffness, hardness, temperature resistance, and chemical and solvent resistance, while transparency, stress-corrosion resistance, and product flexibility decrease. Depending on crystallinity, it can be either rigid or soft. LDPE is particularly prone to creep.

d) Color: uncolored PE is milky white, almost transparent in very thin films. It is capable of covering in all colors.

e) Electrical properties: it has excellent electrical insulation properties. Dielectric properties are almost independent of density, melt flow index, temperature, and frequency. High-frequency heating is not possible. It often carries a strong electrostatic charge, leading to dust attraction. Therefore, antistatic agents are added. Conductivity is increased with the addition of 25 to 30% salt.

f) Temperature resistance: the upper temperature limit for LDPE is 60°C, for HDPE 95°C, temporarily even higher. Brittleness occurs at approximately -50°C, even lower with higher molecular weight. The crystalline melting range is from 105 to 115°C for LDPE, and from 125 to 140°C for HDPE. LDPE is more oxidation-resistant than HDPE. PE burns with a bluish flame and drips while burning.

g) Resistance: resistant to diluted acids, bases, salt solutions, water, alcohols, esters, oils, HDPE is also resistant to gasoline. Below 60°C, PE is practically insoluble in almost all organic solvents. It is not resistant to strong oxidizing agents, especially at elevated temperatures. LDPE swells in hydrocarbons. Oxygen and some other gases have greater permeability compared to most plastic materials. It exhibits very low water vapor permeability. Resistance to direct sunlight exposure is improved with the addition of 2 to 2.5% salt.

h) Physiological properties: PE is odorless, tasteless, and physiologically safe. It can generally be used in contact with food.

i) Susceptibility to stress cracking: stress cracking mainly occurs when surface-active substances (emulsifiers, cleaning agents) are used. This phenomenon is less common in PE with lower density and lower melt flow index (longer molecular chain lengths). Crack-resistant types of polyethylene contain polyisobutylene additives.

II. Processing

a) Injection molding: types of PE with good flowability (higher MFI) are used for injection molding. The ratio of amorphous to crystalline composition in the final product is greatly influenced by the cooling of the melt (mold temperature). This ratio affects shrinkage during processing and subsequent shrinkage. Mass temperatures (depending on type and composition) range from 160 to 300°C; tool temperatures range from 20 to 80°C, which is the upper limit for a higher proportion of crystallized particles and better surface gloss. Shrinkage during processing is 1.5 to 3.5% for LDPE and up to 5% for HDPE. Injection pressure is 600 bars for LDPE and up to 1200 bars for HDPE.

b) Extrusion: mainly high molecular weight types with lower melt flow index (0.2-4 MFI) are used for extrusion. The mass temperature depends on the type and ranges from 190 to 250°C, for the production of monofilaments and cables up to 300°C.

c) Extrusion blowing: high molecular weight types are highly suitable. The mass temperature depends on the type and ranges from 140 to 220°C, tool temperature from 5-40°C. High mass temperatures and rapid cooling enable the production of highly transparent profiles with LDPE.

d) Thermoforming: carried out at temperatures ranging from 130 to 180°C, mainly using vacuum forming processes in a mold. Tool temperature ranges from 40-90°C. LDPE sheets are highly dependent on temperature due to their low temperature resistance, so the use of a protective gas is recommended.

e) Bonding: as PE is non-polar, it has low adhesion properties. Sometimes, surface pretreatment is necessary, such as flaming, soaking in chromic-sulfuric acid, or surface electrification. Adhesive, contact (PUR, synthetic rubber), and two-component adhesives (EP, PUR) are used for bonding.

f) Welding: the best joints are made by hot air welding, using thermal elements and friction. Welding with thermal impulses is suitable for films. Ultrasonic welding is used only in special cases. High-frequency welding is not possible due to dielectric losses.

g) Machining: machining of PE is rare; ultrahigh molecular weight PE semi-finished products can be machined. Special tools for plastic processing are required.

h) Surface treatment: surface treatment by flaming or surface electrification in a vacuum chamber is necessary; followed by appropriate immediate further treatment. Printing: as screen printing or indirect lithographic printing. Coating: using standard procedures with two-component paints. Hot forging: at temperatures from 110 to 130°C. Metallization in a hot vacuum after surface electrification and priming.

i) Powder sintering: involves hot-melt processes at 220°C in a powder sintering oven with LDPE powder. Used for coating steel pipes, refrigerator grids, chairs, and more. Only limited coating thicknesses are possible.

j) Compression: compression with pre-pressure at 200°C and pressures from 20 to 50 bars, followed by slow cooling. Mostly used with high molecular weight PE with a melt flow index of 0.01, which does not crack, has good wear and sliding properties; thus used for gears, seals, filter plates.

k) Rotational molding: suitable for producing large seamless containers or vessels. Special PE powder is used, e.g., LLDPE. Wall thicknesses are limited.

III. Examples of Use

a) Machinery and vehicle components: seals, closure caps, handles, corrosion protection, battery housings, interior linings, textile bobbins

b) Electromechanics: insulation for high-voltage cables, power lines, installation pipes, distributors, motor housings, bobbins.

c) Construction elements: pipes for drinking and wastewater, heating pipes, fittings, cover films, sealing films, tanks for hot oil, artificial grass

d) Transport elements: transport carriers, bottle crates, various containers, packaging films, bottles, tubes, cans, garbage bins, various carrying films, tying films e) Miscellaneous: monofilaments for nets and ropes, textile industry bobbins, toys of all kinds, household containers

IV. Special Types of PE

a) PE in powder form: with defined grain size for rotational processes, powder sintering, and coating. Applications: rotationally formed hollow bodies, electrostatic or powder sintering coating.

b) LLDPE: has higher strength and stiffness at the same density as LDPE. Particularly suitable for thin, about 5mm thick blown and laminated films (for packaging), as well as rotationally formed parts (large hollow bodies, such as containers, sailboards).

c) High molecular weight PE: also ultrahigh molecular weight PE, used for special purposes such as bearings, gears, coatings, abrasion-resistant, requiring high impact and notch toughness, and good wear properties. Processing involves pressing powdered raw materials into semi-finished products, which can only be further processed by machining.

d) Cross-linked PE: HDPE cross-linking occurs by injection molding using peroxide and at tool temperatures of 200-230°C or with energy-rich radiation. Properties: cross-linking increases long-term stability, impact toughness at lower temperatures, and resistance to stress cracking. Short-term usage temperatures are up to 200°C. Only elastic softening of the material occurs at high temperatures due to cross-linking. Applications: automotive and electrical components. PE can also be cross-linked during extrusion (energy-rich radiation). Applications: hot water supply, underfloor heating systems, coatings for high-voltage cables.

e) Ethylene Vinyl Acetate Copolymer (EVA): copolymerization of ethylene with vinyl acetate changes some properties. Increasing VA content makes the mass more flexible and gives it rubber-like properties; toughness at low temperatures, resistance to thermal shocks, flexibility, stress crack resistance, transparency, weather resistance, and adhesion increase; however, hardness, stiffness, melting point, tensile strength, and temperature resistance decrease.

I. Properties

a) Density: 1.05 g/cm³
b) Structure: Amorphous thermoplastic with low water absorption capability.
c) Color: Clear with high surface gloss; transparent and capable of full coverage in all colors.
d) Mechanical properties: Hard, rigid, brittle, and very impact and notch sensitive. Exhibits low tendency to creep. Glass fibers are sometimes used for reinforcement.
e) Electrical properties: Good electrical resistance properties nearly independent of moisture content; surface moisture affects electrical properties. Excellent dielectric properties independent of frequency. Prone to static electricity buildup, hence antistatics are added.
f) Optical properties: Used for indoor optical purposes. Surface gloss decreases and yellowing occurs with outdoor use.
g) Temperature properties: Usable up to 70°C, thermally resistant types up to 80°C. Highly flammable, with a strong sooty flame.
h) Chemical resistance: Resistant to concentrated and diluted mineral acids (except oxidizing acids), bases, alcohols (except higher alcohols), water; highly resistant to aging. Not resistant to organic solvents such as gasoline, ketones (acetone), aromatics (benzene), and chlorinated hydrocarbons, essential oils. Sensitive to UV radiation, hence UV stabilizers are sometimes added.
i) Physiological properties: Physiologically non-hazardous.
j) Susceptibility to stress cracking: Strong tendency to stress cracking, especially in the open air.

II. Processing

a) Injection molding: Most commonly used processing method. Material temperatures range from 180 to 250°C, tool temperatures from 30 to 60°C. Shrinkage during processing ranges from 0.4 to 0.7%, with practically no subsequent shrinkage. For achieving high surface gloss and transparency, pre-drying of granules (1-2 hours at 70 to 80°C) is recommended.
b) Extrusion: Possible to extrude products with higher Vicat softening temperature. Extrusion temperatures range from 180 to 220°C, PS packaging films are biaxially stretched.
c) Thermoforming: Rarely used due to the formation of stresses during shaping, leading to more frequent material cracking. Thermoforming temperatures range from 130 to 150°C, with vacuum forming with pneumatic or mechanical pre-stretching.
d) Adhesion: Most commonly used bonding method. Solvent-based adhesives (e.g., toluene, dichloromethane, butyl acetate) are mainly used, in which up to 20% of polystyrene can be dissolved. Bonding to other materials is done with two-component or adhesive adhesives.
e) Welding: Welding is done with heat elements, heat impulses, and ultrasonically. High-frequency welding is not possible due to low dielectric losses.
f) Removal: Possible; cooling of cut locations with water or air is recommended.
g) Special processing methods: Blow molding of smaller products for packaging, and decorative processes for final products, such as printing, vacuum metallization, and hot stamping.

III. Examples of Use

a) Packaging: Products for packaging with high surface gloss and transparency; e.g., for cosmetics, disposable items, pens, small food packaging.
b) Lighting: Lights of all kinds with a crystal glass effect, for indoor use only.
c) Precision engineering and electrical engineering: Instrument covers, magnetic and film reels, insulation foils, relay parts, reels.
d) Miscellaneous: Household boxes, workshop and hobby boxes, cases, disposable syringes, simple toys, disposable tableware and utensils, fashion accessories, toothbrushes, household items, containers, cake covers, egg containers.
e) Special PS types: Represent alloys of polystyrene and polyolefins, mainly used in the packaging industry. Despite having impact-resistant polystyrene, these plastic masses have lower rigidity and poorer color-covering ability. However, despite the polyolefins, they can be processed in the same way as polystyrene (on the same extruders and thermoforming machines). They have lower water vapor permeability than pure PS, with improved resistance to stress cracking and temperature resistance.

Bio-based materials are made of renewable sources such as plants or microorganisms. Their key advantage is reducing dependence on fossil fuels and potentially decreasing greenhouse gas emissions. However, bio-based nature alone does not necessarily guarantee that the material is biodegradable or environmentally friendly. In most cases, the properties of these materials are similar to those obtained from oil, as the manufacturer can obtain the same type of material from both oil and renewable sources.

Examples of the use of bio-based materials include packaging for food and beverages, such as plastic bottles, storage containers, and bags. Additionally, bioplastics can be used to make cosmetic packaging, medical devices, clothing, and even automotive parts.