Injection molding materials and mold design professional knowledge sharing (Part 1)

Time:2024-07-31 08:23:31 / Popularity: / Source:

Plastic mold design process requirements

To design an advanced plastic mold, we first need to have a high-level design idea, and we must also study product processability, characteristics and uses of plastic materials, selection of mold steel, processing methods, mold structure design, molding schemes and injection machine models.
Among them, it is very necessary to study processability of mold design from perspective of mold design and injection molding. Purpose is to reduce trouble caused by poor mold processability to mold manufacturing and molding.

Plastic materials and characteristics

Plastic refers to a material with polymer synthetic resin as main component, which has plasticity and fluidity at a certain temperature and pressure, can be molded into a certain shape, maintains same shape under certain conditions. Commonly used plastics are divided into two categories: thermosetting plastics and thermoplastics. Characteristic of thermosetting plastics is that they can solidify into insoluble materials under heat or other conditions. Characteristic of thermoplastics is that they can be repeatedly heated to soften or cooled to solidify within a specific temperature range.
In terms of performance, plastics have advantages of light weight, good strength, corrosion resistance, good insulation, easy coloring, products can be processed into any shape, high production efficiency, and low price.

1. Properties of plastics

Wide application of plastic products is inseparable from its own properties and characteristics. Following introduces composition characteristics of plastics.
(1) Molecular structure of plastics
Main component of plastics is resin, which is divided into natural resin and synthetic resin.
(2) Composition of plastics
Resin: Main function is to bond other components of plastics and determine main properties of plastics, such as mechanical, physical, electrical, and chemical properties. Proportion of resin in plastics is generally 40% to 65%.
Filler: Also known as additives, correct selection of fillers can improve performance of plastics and expand its scope of use.
Plasticizer: Some resins have very low plasticity and poor softness. In order to reduce melt viscosity and melting temperature of resin, improve its molding and processing performance, improve flexibility, elasticity and other necessary properties of plastic, high-boiling organic compounds that are compatible with resin and not volatile are usually added. This type of substance is called a plasticizer.
Colorant: Also known as colorant, it mainly plays a role in beauty and decoration, including coating part.
Stabilizer: Any substance that can delay deterioration of plastics is called a stabilizer, which is divided into light stabilizers, heat stabilizers and antioxidants.
Lubricant: Improve fluidity of plastic melt, reduce or avoid friction and adhesion to equipment or molds, and improve surface finish of plastic parts.

2. Plastic abbreviations and Chinese comparison

Table 1-1 Thermoplastic abbreviations and Chinese comparison
Abbreviation code Name Abbreviation code Name
ABS Acrylonitrile-butadiene-styrene copolymer MDPE Medium density polyethylene
A / S Acrylonitrile-styrene copolymer PA Polyamide (nylon)
A / MMA Acrylonitrile-methyl methacrylate copolymer PAA Polyacrylic acid
A / S / A Acrylonitrile-styrene-acrylate copolymer PC Polycarbonate
CA Cellulose acetate (cellulose acetate) PAN Polyacrylonitrile
CN Cellulose nitrate PCTFE Polychlorotrifluoroethylene
EC Ethyl cellulose PE Polyethylene
FEP Perfluoro(ethylene-propylene) copolymer (perfluoroethylene-propylene) PEC Chlorinated polyethylene
GPS General purpose polystyrene PI Polyimide
GRP Glass fiber reinforced plastic PMMA Polymethyl methacrylate
HDPE High density polyethylene POM Polyoxymethylene
HIPS High impact polystyrene PP Polypropylene
LDPE Low-density polyethylene PPC Chlorinated polypropylene
PPS Polyphenylene sulfide PPO Polyphenylene ether (poly-2,6-dimethyl-1,4-phenylene ether), polyphenylene ether
PPSU Polyphenylene sulfone PVCC Chlorinated polyvinyl chloride
PS Polystyrene PVDC Polyvinylidene chloride
PSF Polysulfone PVDF Polyvinylidene fluoride
PTFE Polytetrafluoroethylene RP Reinforced plastics
PVC Polyvinyl chloride S/AN Styrene-acrylonitrile copolymer

3. Introduction to commonly used plastics

Plastics of different compositions reflect their different use values. Table 1-2 shows performance and uses of commonly used thermoplastics.
Table 1-2 Characteristics and uses of commonly used thermoplastics
Name Properties Applications Design considerations
Polyvinyl chloride PVC Hard - poor mechanical strength, stable chemical properties, low price, flame retardant
Soft - good wear resistance, low price, slightly toxic, not suitable for tableware and packaged food
Hard - suitable for making plates, pipes, doors and windows, wire troughs, electric switches, sockets, etc.
Soft - suitable for making plastic sandals, films, raincoats, floor glue, wallpaper, artificial leather, hoses, etc.
Poor fluidity - short and thick flow channels, fewer turns in branch channels
Chlorine corrosion mold - molded parts such as cavities and cores should be electroplated
Polyethylene PE Non-toxic, soft, cheap, light, softer than water High-pressure LDPE - suitable for making films
Low (medium) pressure HDPE - making daily necessities (rice buckets, washbasins, kettles, fresh-keeping boxes, etc.)
Large shrinkage - design calculations and manufacturing should fully consider shrinkage
Large deformation - pay attention to anti-deformation measures in plastic part structure design
Polystyrene PS High applicability, hard and brittle, best colorability, bright colors, non-toxic, low price, good electrical insulation Candy bowls, triangles, tapes, CD boxes, toys, stationery, etc., electrical components and shells Transparent - forged steel should be used for cavity and core to ensure a smooth surface
Hard and brittle - demoulding slope α should be large, generally α≥2°
Styrene-butadiene-acrylonitrile copolymer ABS High surface hardness, stable dimensions, good colorability, can be chrome-plated, heat-resistant, stamping-resistant, non-toxic Household appliance shells, tableware, kitchenware, etc. There should be sufficient demoulding slope α≥ 5°——Prevent "eject angle"
Pay attention to gate position——Prevent and reduce weld marks
Polypropylene PP Good overall performance, excellent toughness, opacity, non-toxicity, small specific gravity (0.9), can float on water Hinged products (glasses cases), plastic bags, ropes, woven bags, films, water life-saving equipment, aircraft equipment, home appliance shells Large shrinkage deformation - design calculation should consider impact of shrinkage on product matching, and there should be a structure to prevent deformation. For hinged products, pay attention to location of gate
Good fluidity - matching surface of mold has high requirements to prevent overflow
Polyamide (Nylon) PA Good wear resistance, acid resistance, alkali resistance, pressure resistance, and water resistance. Good self-lubrication, opaque Nylon wire, gears, bearings, faucets, seals, etc. Best fluidity - requires high precision of mold parting surface and mating surface to avoid overflow
Large shrinkage, unstable size - impact of shrinkage should be considered in both size calculation and mold manufacturing
Large demoulding resistance - demoulding slope should be larger
Polyoxymethylene (POM) It is an ideal plastic to replace steel and aluminum. It has excellent comprehensive performance, good self-lubrication, better than nylon, and bright colors. Mechanical properties are comparable to steel, but price is relatively expensive Gears, (home appliances), bearings, wheels, springs, fan blades, game console buttons Poor fluidity, difficult molding - mold must have a heating device, main channel is "short and thick", and branch channel should have fewer turns
Formaldehyde is toxic - cavity and core should be electroplated for corrosion protection
Polycarbonate PC Excellent stamping resistance, transparent, non-toxic Bulletproof glass, lenses, car lampshades, coffee pots, household blenders, gears, parts of refrigeration equipment, impact drill shells Poor fluidity - mold should have a heating device, main channel should be short and thick, and branch channel should have few turns
Transparent - demoulding slope α≥2°
Core and cavity should be made of forged steel for polishing

4. Plastic characteristics and identification

Table 1-3 Characteristics and identification of commonly used plastics
Plastic name Usage characteristics Identification
ABS Acrylonitrile-butadiene-styrene copolymer Good overall performance, good chemical resistance and electrical properties, super easy processing, appearance characteristics, low creep, excellent dimensional stability and high impact strength Flame color when burning is yellow and black smoke, and burning smell is rubber
PA polyamide (nylon) Tough, wear-resistant, fatigue-resistant, oil-resistant, water-resistant, mildew-resistant, but strong water absorption
Nylon 6-good elasticity, high impact strength, strong water absorption
Nylon 66-high strength, good wear resistance
Nylon 610-high strength, good wear resistance, but weak water absorption and rigidity
Nylon 1010-translucent, weak water absorption, good cold resistance
Flame color when burning is yellow, and burning smell is a special smell
PC polycarbonate It has particularly good impact strength, thermal stability, gloss, antibacterial properties, flame retardant properties and anti-pollution properties, good creep resistance and electrical insulation, and very low shrinkage, generally 0.1% to 0.2%, with good mechanical properties, but poor flow properties Flame color when burning is yellow black smoke, and burning smell is a special smell
PE polyethylene High-pressure polyethylene has good softness, transparency, elongation and impact strength
Low-pressure polyethylene has high melting point, rigidity, hardness and strength, weak water absorption, outstanding electrical properties and good radiation resistance
Color of flame when burning is yellow at the top and cyan at the bottom. Burning smell is paraffin.
POM polyoxymethylene It has good creep resistance, geometric stability and impact resistance, good ductility and fatigue resistance, low water absorption, and a very low friction coefficient, but poor thermal stability, easy to burn, and long-term exposure to atmosphere will age. Color of flame when burning is yellow at the top and blue at the bottom. Burning smell is formalin.
PS polystyrene Excellent electrical insulation, colorless and transparent, light transmittance second only to plexiglass, poor coloring, water resistance, good chemical stability, general mechanical strength, but brittle and prone to stress cracking, not resistant to organic solvents such as benzene and gasoline Flame color when burning is orange-yellow black smoke, and burning smell is styrene.
PSF polysulfone Excellent heat and cold resistance, creep resistance and dimensional stability, acid resistance, alkali resistance, high temperature resistance and high temperature steam resistance. Polysulfone has high hardness and impact strength, can be used for a long time at -60~+150℃, and can still maintain good insulation in water, humid air or high temperature, but is not resistant to aromatic hydrocarbons and halogenated hydrocarbons. Polysulfone has good heat resistance and cold resistance, can be used at -240~+260℃, has high hardness and radiation resistance.  
PP polypropylene It has a low heat distortion temperature (100℃), low transparency, low gloss, and low rigidity, but has strong impact strength, excellent moisture absorption resistance, acid and alkali resistance, corrosion resistance, and solubility resistance Flame color when burning is blue, and burning smell is diesel
PVC Polyvinyl Chloride Hard PVC has high mechanical strength, excellent electrical properties, strong acid and alkali resistance, good chemical stability, but low softening point
Soft PVC has large elongation, low mechanical strength, lower corrosion resistance and electrical insulation than hard PVC, and is easy to age
Color of flame when burning is yellow at the top and green at the bottom, and smell of burning is chlorine
PPE It has strong chemical stability, weak moisture absorption, good geometric stability, electrical insulation characteristics and very low thermal expansion coefficient Flame color when burning is yellow black smoke, and the burning smell is rubber Glue smell
Fluoroplastic Excellent corrosion resistance, aging resistance and electrical insulation, low water absorption  
Cellulose acetate Good toughness, oil resistance, dilute acid resistance, transparent and glossy, good dimensional stability, easy to paint, dye, bond, cut, impact resistance and tensile strength weakened at low temperature  
Polyimide Good overall performance, high strength, creep resistance, good heat resistance, can be used for a long time at -200 ~ +260℃, high wear resistance, excellent electrical insulation, radiation resistance, corona resistance, dilute acid resistance, but not alkali, strong oxidants and high-pressure steam  
PMMA polymethyl methacrylate It has excellent optical properties and weather resistance. PMMA products have very low birefringence, room temperature creep properties and impact resistance Color of flame when burning is yellow at the top and cyan at the bottom. Burning smell is chocolate.

Parting surface

In order to facilitate removal of plastic parts from closed mold cavity, and to facilitate placement of inserts or removal of pouring system, mold must be divided into two or more parts. Surface that can be separated from mold to remove plastic part is usually called parting surface. At the same time, with parting surface as boundary, mold can be divided into two major parts, namely movable mold and fixed mold part. The other surfaces are called separation surfaces or parting surfaces. Injection mold has only one parting surface.
Selection of parting surface is a relatively complex issue because it is affected by geometry of plastic part, wall thickness, dimensional accuracy, surface roughness, insert position, demolding method, molding position of plastic part in mold, ejection method, design of pouring system, and mold exhaust method.
Forms of parting surfaces are generally as follows: (a) in Figure 1-1 shows a horizontal parting surface; (b) shows an oblique parting surface; (c) shows a stepped parting surface; (d) shows a curved parting surface. Therefore, for mold designers, correct selection of parting surfaces has a vital impact on mold manufacturing and operation.
mold structure design 
When selecting location of parting surface, parting surface is generally not located on decorative outer surface or at a corner with a circular arc. Parting surface must be set at the largest cross-section of product, and it is convenient for plastic part to remain on the side of movable mold after mold is opened to ensure that ejection mechanism can be demolded smoothly. Following principles should be followed when selecting parting surface.

1. Mold retention method

In order to facilitate demolding of plastic parts, plastic parts should be retained in lower mold as much as possible when mold is opened. Since ejection mechanism of plastic part is usually set in lower mold, especially mold used for automated production, it is more important to correctly select mold retention method for plastic part.
Whether mold retention method is selected correctly will directly affect product quality and production efficiency.
As shown in Figure 1-2 (a), since core is located in fixed mold, plastic part will shrink and wrap core tightly after mold is opened, leaving plastic part on fixed mold side. Therefore, it is more difficult to demold and mold structure is complicated. If structure shown in Figure 1-2 (b) is used instead, it will be more reasonable.
mold structure design 

2. Appearance of plastic parts

Parting surface should be selected as far as possible in a position that does not affect appearance of plastic part, and flash generated at parting surface should be easy to trim. As shown in Figure 1-3, if plastic part without a circular arc spherical surface on one side adopts structure of Figure (a), surface quality of plastic part will be damaged, while structure of Figure (b) is more reasonable.
mold structure design 

3. Coaxiality requirements of plastic parts

Figure 1-4 shows a pair of gear molds. Gear rim and outer circle of step part have coaxiality requirements. If parts with coaxiality requirements are molded in movable mold and fixed mold respectively, as shown in Figure 1-4 (a), it will be difficult to ensure their coaxiality requirements due to inaccurate mold clamping; if structure shown in Figure 1-4 (b) is used instead, so that parts with coaxiality requirements are all molded in movable mold, coaxiality requirements can be met.

4. Direction of flash on plastic parts

When selecting parting surface, location of flash on plastic part should be considered according to use requirements of plastic part and plastic used. If plastic part is not allowed to have horizontal flash, structure shown in Figure 1-5 (a) can be used, which is conducive to demoulding. Especially for nylon with good fluidity, this structure can also reduce generation of flash. However, structure shown in Figure 1-5 (b) is not appropriate.
mold structure design 
mold structure design 
Gap between parting surface after mold closing is generally not more than 0.01~0.03mm, otherwise flash will be formed, so parting surface must be ground by a "surface grinder".

5. Demolding slope of plastic parts

When selecting parting surface, consider reducing size difference between large and small ends of plastic part caused by demoulding slope. For plastic part shown in Figure 1-6 (a), if cavity is set on one side of mold, size difference of plastic part will be large due to demoulding slope. When plastic part is not allowed to have a large demoulding slope, use of this structure will inevitably make demoulding difficult. If plastic part has no strict requirements on appearance, parting surface can be selected in the middle of plastic part, as shown in Figure 1-6 (b). It can adopt a smaller demoulding slope, which is conducive to demoulding.
mold structure design 

6. Venting function of parting surface

Venting function of parting surface can discharge part of high-temperature gas inside cavity out of cavity, ensuring that there are no pores on the surface of product, which is conducive to improving appearance quality of product.
Generally, a venting groove with a depth of 0.025-0.1mm and a width of 1.5-6mm is opened on one side of parting surface concave mold. Ejector, cavity, and core insert can also be used for venting.

7. Mold manufacturing

Parting surface should divide mold into parts that are easy to process to reduce difficulty of mechanical processing.

Casting system

Casting system refers to flow channel of plastic melt in mold from nozzle to cavity. Function is to smoothly fill various parts of cavity with plastic melt, transmit injection pressure to various parts of cavity during filling and solidification process.
Pouring system consists of main channel, branch channel, cold material hole and gate, as shown in Figure 1-7.
mold structure design 

1. Design principles of pouring system

Design of pouring system is an important part of injection mold design, which has a direct impact on efficiency of injection molding and quality of plastic parts;
Therefore, following principles must be paid attention to when designing pouring system:
Understand molding characteristics of plastics, that is, temperature and shear rate of plastics.
Prevent deformation of cores and plastic parts, avoid material flow from rushing out of small diameter cores or fragile metal inserts, and prevent deformation of plastic parts due to excessive shrinkage stress at gate.
Good exhaust to ensure smooth, fast and non-turbulent flow of fluid.
Reduce process and plastic consumption to shorten molding cycle, improve molding effect, and reduce amount of plastic.
Position and shape of feed port should be determined in combination with shape and technical requirements of plastic part.
When one mold has multiple cavities, prevent parts with different sizes from being placed in one cavity.
Convenient trimming to ensure appearance quality of plastic parts.

2. Type and design of runners

Runner is part between main runner and gate, which plays the role of diversion. Requirement is that heat and pressure loss of plastic melt during flow is minimized, and amount of plastic in runner is minimized. Shape of runner is shown in Figure 1-8. Shape and size of runner must be determined based on factors such as molding volume of plastic part, wall thickness and shape of plastic part, process characteristics of plastic, injection molding speed, and length of runner.
mold structure design 
Key points of runner design:
Surface of runner does not need to be very smooth, and surface accuracy is generally 1.25. Because surface of runner is not too smooth, it can fix cooling skin of molten plastic, which is conducive to heat preservation.
When runner is long, a cold material hole should be opened at the end of runner to accommodate cold material generated at the beginning of injection molding to ensure quality of plastic part.

3. Type and design of gate

Gate refers to small channel between end of runner and cavity. Main function is to increase flow rate and temperature of plastic and prevent side flow of plastic into cavity.
(1) Pin-point gate
Pin-point gate is also called water point gate. It can be automatically demolded by using a three-plate mold frame. Mold structure is relatively complex. Gate is automatically pulled off during production process. It is suitable for automated injection production. It can be used in a one-cavity mold or a multi-cavity mold. It can inject both small and large products, especially plastic parts with patterns. It does not affect appearance. Figure 1-9 shows pin-point gate structure.
(2) Submerged gate
Submerged gate is also called submerged gate. Feeding position is selected in a more hidden place of product to avoid affecting appearance of product. Runner and plastic part are automatically separated during ejection, so a larger ejection force is required. Submerged gate is not suitable for overly tough plastics. Figure 1-10 shows submerged gate structure.
mold structure design 
(3) Side gate
Side gate is also called edge gate or side water gate. It is generally opened on parting surface and feeds from edge of plastic part. Its shape is rectangular or nearly rectangular. Processing is convenient and simple, and application is flexible. It can be fed from outside or inside of product. Figure 1-11 shows side gate structure.
(4) Direct gate
Direct gate is also called large water gate or center gate. There is no branch channel. Plastic enters cavity directly through main channel. Therefore, it has advantages of short plastic flow, small flow resistance, fast feeding, small kinetic energy loss, and good pressure transmission. However, it is difficult to cool gate, and plastic parts have obvious gate marks. Because heat is concentrated near gate, condensation is late there, internal stress is large, and it is easy to produce defects such as bubbles and shrinkage holes there. Figure 1-12 shows direct gate structure.
mold structure design 

4. Design principles of gate position

When designing casting system, gate position should be selected first. Whether gate position is correct or not will directly affect molding quality of product and whether injection process can proceed smoothly.
Following principles should be followed when selecting gate position:
Gate position should be selected on parting surface as much as possible to facilitate mold processing and gate cleaning.
Distance between gate position and each part of cavity should be as consistent as possible, and its flow should be as short as possible.
Gate position should ensure that when plastic flows into cavity, it faces wide and thick-walled part of cavity to facilitate flow of plastic.
Avoid plastic from directly hitting cavity wall, core or insert when flowing into cavity. Plastic should flow into each part of cavity as quickly as possible to avoid deformation of core or insert.
Try to avoid welding marks on product or melting marks on important parts of product.
When plastic flows into cavity, it should flow evenly in direction parallel to cavity, and it is conducive to discharge of gas in cavity.
Gate should be set at part on product that is easiest to remove, and at the same time, it should not affect appearance of product as much as possible.

Ejection mechanism

Mechanism that removes plastic part from mold is called an ejector mechanism or demolding mechanism. Direction of action of ejector mechanism is consistent with direction of mold opening. A good ejection mechanism requires that plastic part is not deformed or damaged during demolding, and ejection mechanism should be located in an inconspicuous part of part.
Ejection mechanism can be summarized into three categories: mechanical ejection, hydraulic ejection and pneumatic ejection. Following principles should be followed when designing ejection system:
In order to prevent product from being deformed due to ejection, thrust point should be as close as possible to core or part that is difficult to demold, such as slender hollow cylinder on product, which is often ejected by a push tube (standard parts are usually a sleeve). Arrangement of thrust points should be as uniform as possible.
Thrust point should act on part of product that bears the greatest force, that is, part with good rigidity, such as ribs, flanges and wall edges of shell-shaped products.
Try to avoid thrust point acting on thin surface of product to prevent product from breaking and perforating. For example, shell-shaped products and cylindrical products are mostly ejected by push plates.
In order to avoid the traces of ejection affecting appearance of product, ejection device should be located on hidden surface or non-decorative surface of product. For transparent products, special attention should be paid to selection of ejection position and ejection form.
For further reading, please refer to Injection molding materials and mold design professional knowledge sharing (Part 2).

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