Introduction: A complete set of molds includes multiple related systems, which complement each other. Only when various systems achieve the best combination, can normal use and accuracy of mold be guaranteed. Because structure of mold must meet requirements of product, different products have different requirements, product has variability and diversity, which determines diversity of mold. Generally speaking, typical plastic molds include: pouring system, ejection system, cooling system, exhaust system, core pulling system, etc. This article will briefly explain each system of mold.

 

1. Parting surface
In order to remove product from mold, mold must be divided into two parts on male and female mold sides. This interface is called parting surface. It has function of parting and venting, but due to difference of mold accuracy and molding, it is easy to produce burrs and knots, which will hinder appearance and accuracy of product. Pay attention to selection of parting surface:
It should not be located in an obvious position to affect appearance of product. When opening mold, product should be left on the side with demoulding mechanism. Located in a place where mold processing and product post-processing are easy. For products that require high coaxiality, cavities should be designed on same side as much as possible. Avoid long core pulling, consider placing it in opening direction of male mold. If necessary, set core pulling mechanism on the side of male mold as much as possible. 
Generally, arc part is not used for parting, which will affect appearance of product. For plastics that have good fluidity and are easy to overflow, insert and break type to prevent burrs. For products with high height and small demolding angle, middle parting can be used, cavity is divided into two sides to facilitate demolding.
2. Demoulding angle
In order to make product easy to be ejected from mold, mold must be set with a demolding slope. Size depends on product shape, plastic, mold structure, surface accuracy and processing methods. Generally, it is 1-3°. The larger draft angle, the better without affecting appearance and performance of product.
1. Box and lid:

2. Grid:
Different grid shapes, sizes and meat thicknesses should have different demoulding angles. Empirical formula is as follows:
0.5(A---B)/H =1/12—1/14
A=big end size B=small end size H=height
When grid pitch is less than 4mm, demolding slope is about 1/10, and grid meat thickness is more than 8mm. Slope should not be excessively increased. You can split mold at position where there are more glue positions on the side of master mold. If glue position of grid section is increased, slope may be increased.
3. Reinforcing ribs (can improve material flow, prevent stress and deformation, play a reinforcing role)
a. Longitudinal rib: 0.5(A-B)/H=1/500—1/200; b. Bottom rib: 0.5(A-B)/H=1/150—1/100; c. Convex column: 0.5 (large end diameter-small end diameter)/H = 1/30—1/20 (inner and outer holes are on same side of mold)
Female side: 0.5 (large end diameter-small end diameter)/H = 1/50—1/30; Male mold side: 0.5 (large end diameter-small end diameter)/H = 1/100—1/50 (inner and outer holes are on both sides of mold)
Note: Demolding angle of female mold side can be larger than that of male mold side to facilitate demolding.
3. Thick flesh
Thickness of product will directly affect molding cycle and production efficiency, will cause shrinkage and stress due to uneven thickness. When designing mold, pay attention to thickness of product:
Whether mechanical strength of product is sufficient. Can impact and demoulding force be dispersed uniformly without cracking? When there is an embedded part, it must be prevented from cracking, whether it will affect strength due to thinness of joint line. Make flesh as thick as possible to prevent shrinking and sinking. Whether thickness is too thin will cause insufficient filling or hinder material flow.
Following is a reference for common plastic standard meat thickness:

4. Convex column
Generally, it is a protruding cylinder on product, which can enhance strength of periphery of hole, assembling hole and increasing local height. It is necessary to prevent shrinkage caused by increase in thickness and insufficient filling or scorching caused by accumulation of air. Attention should be paid to design:
Height should not exceed twice its own diameter, otherwise it is necessary to add stiffeners. Position should not be too close to corner or side wall to facilitate processing. Round shape is preferred to facilitate processing and material flow. For example, bottom can be 0.3-0.5mm higher than bottom surface.
5. Hole
There are holes in most products, and there are three main ways to obtain them: Direct molding on product Form a reserved hole on product first, then finish machining. Drilling is completely machined after forming.
Pay attention to following points when designing: Distance between hole and hole must be more than 2 times hole diameter. Distance between hole and edge of product should be more than 3 times hole diameter. Thickness should be increased around hole. Distance between hole and side wall of product should be more than 0.75 times hole diameter. When diameter of hole is less than 1.5mm, it is easy to bend and deform. Please note that hole depth should not exceed 2 times hole diameter. For through hole with parting surface in the middle, in order to prevent eccentricity, aperture on unimportant side can be enlarged.
6. Thread
For assembly purposes, there are sometimes thread designs on product, which can be formed directly or machined after forming. For threads that are frequently disassembled or subjected to large forces, metal thread inserts are used, following principles should be paid attention to when designing: Avoid using threads with a pitch of less than 0.75mm, use threads with a maximum pitch of 5mm. Due to plastic shrinkage, avoid directly forming long threads to prevent pitch distortion. Avoid use when thread tolerance is less than plastic shrinkage. If internal and external threads are matched, a gap of 0.1-0.4mm must be left. Threaded part should have a demoulding angle of 1--3°. Screw thread cannot be extended to the end of product, a polished rod part of about 0.8mm must be set to facilitate mold processing and thread life. On some similar bottle cap products, it will have some vertical knitting patterns, spacing should be large, minimum is 1.5mm, generally 3.0mm, and a flat part of at least 0.8mm is provided on parting surface.
7. Inserts
In order to prevent product from cracking, increase mechanical strength or use it as a medium for conducting current and for decoration, inserts are often embedded when product is formed. Points to note: To ensure reliability of insert, glue layer around insert should not be too thin. When fitting inserts and insert holes, they must be tight enough to not affect pick-and-place. In order to make insert and plastic tightly combined, embedded part is often designed to be rough or concave-convex (embossing, drilling, punching and bending, notching, undercutting, etc.).
8. Other points
Reinforcing ribs should not be too thick, generally not more than half thickness of flesh, to prevent shrinkage. As long as appearance and function are not affected, smooth surface should be changed to bitten surface as much as possible, which can reduce difficulty of mold processing, increase aesthetics, and prevent shrinkage. At the periphery of convex column, part of flesh thickness can be removed to prevent shrinkage and subsidence. For holes with thinner meat, edge and height of holes should be increased to facilitate reinforcement. Core tip is affected by contraction force, product is easy to break when ejected. A convex edge can be set to withstand ejection force. Setting R at corner can improve strength and prevent stress concentration, which is beneficial to material flow. Avoid acute angles, thin meat parts may cause insufficient filling of materials. For products with ripples on the outside, it can be changed to strengthen edge for convenience of post-processing. When parting surface has a stage shape, it is not easy to process the mold. Consider changing to a diagonal or curved parting. Penetrating core-pulling is prone to failure, so it is better to change core-pulling to both sides. Because round shape is easier to process than other shapes, it can reduce cost and is preferred. When adding etched characters or patterns on product, if there are no special requirements, try to design concave characters to facilitate mold processing.

Gating system is an important part of plastic mold design, it is often divided into ordinary and runnerless gating systems. It is closely related to shape, size, machine and parting surface of plastic products used.

 

Pay attention to following principles when designing: Runner should be as straight as possible and as short as possible to reduce bending, finish is between Ra=1.6-0.8um. Consider number of mold cavities and design according to mold cavity layout, as symmetrical to center line of mold as possible. When projected area of product is large, avoid opening a gate on one side to prevent uneven injection force. Gate location should be easy to remove, leaving no obvious marks on product, and not affecting appearance of product. When designing main runner, avoid direct impact of plastic on small cores or small inserts to avoid bending or breaking. Main runner first reserves processing or correction margins to ensure product accuracy.
1. Main runner design
Main runner is a passage connecting nozzle of machine to entrance of runner. It is the first place where plastic passes when it enters mold cavity. Its size are closely related to flow rate of plastic and length of filling time. Too big causes too much recycled cold material, longer cooling time, and more trapped air. It is easy to cause bubbles and loose tissues, t is easy to produce eddy currents and insufficient cooling;
If flow path is too small, heat loss will increase, fluidity will decrease, injection pressure will increase, causing difficulty in molding. Under normal circumstances, main runner will be manufactured as a separate sprue bushing and set on female mold plate. However, some small molds will directly open main runner on female mold plate without using sprue bushing. Main points of main runner design:  Inner hole of sprue sleeve is conical (2--6°), and finish is Ra=1.6-0.8um. Taper must be proper, too large will cause pressure to decrease, produce swelling, easy to mix into air to produce pores, too small taper will increase flow rate, causing injection difficulties. Diameter of sprue bushing should be 1-2mm larger than nozzle diameter of machine to avoid accumulation of residual material, resulting in pressure drop and sprue breakage. Generally, rounded corners (R=1—3mm) are set at large end of sprue sleeve to facilitate material flow. Contact point between main runner and nozzle of machine is designed as a hemispherical pit, depth is usually 3-5mm. Special attention should be paid to radius of sprue bushing which is 1-2mm larger than radius of nozzle. Generally, R=19-22mm to prevent glue overflow. Main runner should be as short as possible to reduce cold material recycling, reduce pressure and heat loss. Try to avoid block structure as far as possible for main runner to prevent plastic from entering joints and causing difficulty in demolding. In order to avoid damage caused by repeated contact and collision between main runner and high-temperature plastic and nozzle, sprue sleeve is generally processed by high-quality steel and heat treated. There are many kinds of forms, which can be selected according to different mold structures. Generally, they will be fixed on mold plate to prevent sprue sleeve from rotating or being taken out during production.
2. Shunt design
Runner is connecting part of main runner, between runner and gate, plays a role of splitting and turning. Runner must send molten plastic to gate at a faster rate to fill mold with minimum pressure loss. Because of same cross-sectional area, square has the longest perimeter and round is the shortest. If area is too small, it will reduce plastic flow rate, extend filling time, and easily cause product to be short of material, burnt, silver wire, and shrink;
If it is too large, it will accumulate too much gas, increase cold material, prolong production cycle, and reduce production efficiency. For different plastic materials, runner will be different, but there is a design principle: It is necessary to ensure that ratio of surface area of manifold to its volume is minimized. That is, when length of runner is fixed, ratio of surface area or side area of runner to its cross-sectional area is required to be the smallest.
There are many types of runners, which vary depending on structure of plastics and molds. Commonly used types include round, semi-circular, rectangular, trapezoidal, U-shaped, and regular hexagon. Basic principles of design are: If conditions permit, cross-sectional area of runner should be as small as possible and length should be as short as possible. When runner is long, a cold slug hole should be set at the end to contain cold slug and prevent air from entering, and cold slug hole is generally equipped with a pull rod to facilitate release of glue channel. In a multi-cavity mold, runners should be as consistent as possible and length should be as short as possible. Cross-sectional area of runners should be greater than sum of cross-sectional areas of runners. Surface is not required to be excessively smooth (Ra=about 1.6), which is good for heat preservation. If there are many runners, consider adding a split cone to prevent molten plastic from directly impacting cavity, also to avoid sharp turning of plastic to make plastic transition smoothly. Runners are generally distributed in a balanced way. In special cases, an unbalanced way can be used. It is required that each cavity is fed in a balanced manner at the same time, with a compact arrangement and a short process to reduce size of mold. When designing runner, the smaller size should be selected first, so that there is a correction margin after mold trial.
General runner diameter (size)

3. Gate design
Gate refers to connecting part between end of runner and cavity, which is last part of gating system. Its function is to make plastic enter and fill cavity more quickly. It can quickly cool down and close. Prevent hot glue that has not cooled down in cavity from flowing back. Product size, cross-sectional area size, mold structure, molding conditions and plastic properties must be considered when designing. Gate should be as short as possible, easy to separate from product, without causing obvious traces, its types are diverse, mainly including:
Types of gates and their characteristics

Design Points: Glue port should be opened in thick part of product to ensure smooth and complete filling of mold. Position should be chosen to make plastic filling process the shortest to reduce pressure loss and facilitate mold exhaust. Through mold flow analysis or experience, judge whether joint line of product due to gate position affects appearance and function of product, it can be solved by adding a cold slug hole. Avoid opening gates near the slender core to prevent material flow from directly impacting core, causing deformation, dislocation or bending. For shaped or flat products, it is recommended to use multi-point pouring to prevent product warpage and material shortage. Volume is set at the place that does not affect appearance and function of product, can be at the edge or bottom. Gate size is determined by product size, geometry, structure and plastic type. You can select a smaller size first and then modify it according to condition of trial mold. When mold is multi-cavity, same product adopts symmetrical pouring method. When different products are formed in same mold, the largest product should be placed near main runner first. At the end of cold material hole near gate, a drawing rod is often set at the end to facilitate demolding of runner.
4. Hot runner
At present, an important direction for development and improvement of gating system is development of hot runner molds. Main difference between it and general injection mold is that during injection molding process, plastic in gating system will not be cooled and solidified, nor will it form a runner to be demolded with product. Therefore, it is also called runnerless mold, it has become more and more widely used in the design of large and precise molds.
It has following advantages: Short molding cycle, eliminating need for cutting gates, trimming products, crushing and recycling processes, saving manpower and material resources, improving production efficiency. No cold glue, which can reduce material consumption. Temperature is strictly controlled during production, which significantly improves product quality and reduces production of defective products. Plastic in injection system is always in a molten state, which is conducive to pressure transmission, can reduce injection pressure, and is conducive to molding. There is no runner, so mold opening stroke can be shortened, which is beneficial to life of mold and machine.
However, structure of hot runner mold is complex, temperature control requirements are strict, and a precise temperature control system is required. Manufacturing cost is high, and it is not suitable for small batch production.
According to different plastic characteristics, there are different requirements for hot runner molds, see following table:

After product completes a molding cycle, mold is opened. Product will be wrapped on one side of mold and must be removed from mold. This work must be completed by ejection system. It is an important part of entire mold structure, generally composed of three parts: ejection, reset and ejection guide.

 

1. Divided by power
1. Manual ejection: When mold is opened, ejection system is manually operated to eject product. It can simplify mold structure, smooth demoulding, and prevent product from deforming. However, labor intensity of workers is high, productivity is low, scope of application is not wide. It is generally used when screwing out thread core manually.　
2. Motorized ejection: Ejection mechanism is pushed out by power of injection machine or an additional motor to eject product. It can push ejector plate through ejector pin on machine to achieve purpose of ejection. It is also possible to install fixed-distance tie rods or chains on male and female templates to drive ejection mechanism to eject product by mold opening force. When adjusting mold, you must pay attention to controlling mold opening stroke. It is suitable for mold with ejection system on the side of female mold.
3. Hydraulic ejection: Install a special oil cylinder on mold, injection machine controls action of oil cylinder. Ejection speed and time can be adjusted by hydraulic system. Ejection system can be returned before mold is closed.
4. Pneumatic ejection: Use compressed air to set air passages and small ejection holes on mold to directly blow out product. No ejection marks are left on product, suitable for thin or long cylindrical products.
2. According to mold structure
Primary ejection mechanism, secondary ejection mechanism, master mold ejection mechanism, pouring system ejection mechanism, thread ejection mechanism, etc.
Design Principles: When selecting parting surface, try to keep product on the side with demoulding mechanism. Ejection force and position are balanced to ensure that product does not deform or burst. Thimble must be located at a place that does not affect appearance and function of product. Use standard parts as much as possible, which is safe and reliable for manufacturing and replacement.
There are various types of ejector systems, which are related to shape, structure and plastic properties of product. Generally, there are ejector pins, ejector pipes, push plates, ejector blocks, air pressure, composite ejection, etc.
3. Ejector
It is the simplest and most common form of ejector mechanism, and its cross-sectional area forms are mainly as follows:
1. Round shape is the most widely used in production due to its convenient manufacturing, processing and repairing, and good ejection effect. However, circular ejection area is relatively small, which is prone to stress concentration, top penetration, and top deformation. Try to avoid using it in tube-shaped and box-shaped products such as small drafting angle and large resistance. When ejector rod is relatively slender, it is generally set in a stepped shape with a supporting ejector pin to strengthen rigidity, avoid bending and breaking.
Design Points: Ejection position should be set at a place with high resistance, and not too close to insert or core. For box-shaped constant-cavity molds, side resistance is the largest, top and side surfaces should be ejected at the same time to prevent product from deforming and breaking. When resistance of product is balanced, ejector rod should be set symmetrically to balance force. When there are thin and deep reinforcing ribs, top rod is generally set at the bottom. If there are inserts on mold, effect of setting thimble on it is better. Avoid setting a thimble at product inlet to avoid breakage. When ejection marks are not allowed on the surface of product, set ejection ears and then cut them off. For thin products, set a thimble on runner to take product out. Thimble is matched with thimble hole, generally a clearance fit. If it is too loose, it is easy to produce burrs, it is too tight to cause jamming. In order to facilitate processing and assembly, reduce friction surface, a mating length of 10-15mm is generally reserved on mold core, remaining part is expanded by 0.5-1.0mm to form an escape hole. In order to prevent thimble from rotating during production, it must be fixed on thimble plate. There are various forms, which must be specifically determined according to size, shape and position of thimble, which will not be listed here. After ejector system is carried out in next cycle of production, mold must be returned to its original position. Main forms include forced return, pull rod return, spring return, and oil cylinder.
4. Pipe jacking
It is also called a cylinder or sleeve thimble, which is suitable for ejecting products with a ring-shaped cylinder or with a center hole. Because it is in contact all the way, force is even, it will not deform product, and it is not easy to leave obvious ejection marks, which can improve concentricity of product. However, avoid using products with thinner surrounding meat to avoid processing difficulties and weakened strength, which may cause damage.
5. Push plate
This form is suitable for all kinds of containers, box-shaped, cylindrical and slender thin products with a center hole. It ejects smoothly and evenly, with large ejection force and no ejection marks. Generally, there will be a fixed connection to prevent push plate from being pushed down during production or when supporting mold. But as long as guide post is long enough to strictly control stroke of supporting mold, push plate may not be fixed.
Fit between push plate and core must be smooth to prevent friction or jamming, and also must prevent plastic from penetrating into gap. When product is a blind hole, it will be difficult to demold and product will be deformed due to vacuum adsorption. Generally, a mushroom valve is set on male mold, mushroom valve is opened when it is ejected, and air enters to make demolding smooth. It can be returned to position with a spring, or it can be connected with ejection device to double as an ejector rod.
6, eject block
For some products with flanges or larger sizes, in order to facilitate processing and demolding, they are often designed to be ejected in the form of ejector blocks. Most of planes are parting surfaces, there are two or several larger diameter ejector rods connected below, ejection area is large and stable. It is widely used in molds with forming surfaces and larger sizes.
7. Air pressure ejection
When product is a deep-cavity thin meat piece, it is easy and effective to eject it with compressed air. Male mold core can be provided with some small air inlets or mushroom-shaped rods. After mold is opened, 5-6 atmospheres of compressed air is introduced, spring is compressed to open valve, high-pressure air enters between product and male mold core to release product. However, for box-shaped products, sidewalls will be pulled laterally due to gas entering, air will leak out. At this time, it should be used in conjunction with push plate.
8. Compound ejection
Affected by shape of product, most molds use more than two ejection methods in order to achieve ideal ejection effect. Specific form must be determined according to product and mold structure, will not be described in detail here.
9. Other ejection methods
9.1 Point-shaped glue inlet runner will fall off automatically
Point gate is on the side of master mold. In order to remove glue channel, a parting surface must be added. After mold is opened, glue channel is usually taken out manually, which causes trouble in operation and lower productivity. In order to adapt to automatic production, it is best to design an automatic fall-off device to make glue channel automatically fall off when ejecting.
a. Undercut pull-off. Drill an oblique hole at the end of runner, pull out glue channel after opening mold, and eject it from center mandrel.
b. Material pulling rod is broken. Rubber channel is pulled out by material pulling rod. After mold is opened for a certain stroke, limit rod drives push plate to push rubber channel down.
c. When master mold push plate is pushed out of mold, female mold plate and master mold push plate are separated first, glue channel stays in female mold plate and moves together with female mold for a certain stroke. Limit rod restricts movement of push plate, push plate separates from mold plate. Rubber track is pulled off and falls off automatically.
d. Ejector pin is broken. For slender and deep cavity molds, an ejection system can be set in master mold. After mold is opened, ejector pin is ejected out of glue channel with limit rod stroke. Product is ejected by push plate. It is related to itinerary, and application is special.
9.2 Side ejection method of female mold
Normal products will be ejected from male mold side, but for some products due to special shapes or product special requirements, ejector device must be located on female mold. Because master mold is a fixed machine, ejector rod cannot act on the top plate and must be completed with help of mold opening force or external force. Common ones are oil cylinder, electric, pull hook and so on.
9.3 Thread ejection
Due to special shape of thread and general products, it must be ejected by rotation or demoulded sideways. According to complexity of product and output, there are generally two ways of manual and motorized.
1) Forced unthreading
For plastic with strong elasticity (PP. PE), its elasticity can be used for forced demoulding without damaging screw teeth. Use flexible rubber to make thread core. When opening mold, use spring to exit ejector pin in core to make rubber core shrink inward, then use ejector pin to eject product. This method can simplify mold structure, but rubber core has a shorter life and is only suitable for small batch production. Some threads can be formed by semicircular sliders or shaped rings. Two halved sliders are combined to form a complete thread or product is ejected by hand.
2) Motor will unscrew thread.
When screw thread comes out, it must be relatively rotated, mold must have a rotation stop device to ensure it.
a. External stop: Female mold is equipped with a rotation stop pattern, product can fall off automatically when male mold core rotates.
b. Internal stop. For products with internal threads, a stop-rotation form is set on the top surface of male mold core. When demolding, stop mold core rotates and thread can be ejected axially. Note that thread pitch of stop mold core must be consistent with product thread pitch.
c. Product end face stop. Set a small stop bump on the end face of product, push plate will push product out when core rotates.
When small products have side gates, product can be taken out only by ejecting glue channel, but avoid using it for soft plastics.
Commonly used core rotation drive methods include manual, electric, hydraulic cylinder, hydraulic motor and large pitch screw nut drive. Generally speaking, when rotating mechanism is designed, product has a few threads, thread core must make a few turns.

Design rules of cooling system. Purpose of designing cooling system is to maintain proper and efficient cooling of mold. Cooling channels should use standard sizes to facilitate processing and assembly. When designing cooling system, mold designer must determine following design parameters according to thickness and volume of plastic part: location and size of cooling channel, length of channel, type of channel, configuration of channel, and design rule of cooling system.

 

1. Location and size of cooling pipeline
To maintain an economical and effective cooling time, it is necessary to avoid excessively thick plastic parts. Cooling time required for a plastic part increases rapidly as its flesh thickness increases. Thickness of plastic parts should be as uniform as possible, such as design shown in Figure 6-56. Cooling channels are preferably arranged in male module and female module. Cooling channels arranged outside module are not easy to accurately cool mold.
Generally, distance between cooling channel of steel mold and mold surface, cavity or core should be maintained at 1 to 2 times diameter of cooling channel. Experience requires that steel cooling channel should be maintained at a depth of 1 times diameter, 1.5 times depth of beryllium steel alloy. Depth of diameter is twice depth of diameter for aluminum. Spacing between cooling channels should be maintained at 3 to 5 times diameter. Diameter of cooling channel is usually 10~14 mm (7/16~9/16 inches), as shown in Figure 6-57.
2. Flow rate and heat transfer
Temperature difference between two sides of plastic part should be kept to a minimum, temperature difference of tight fitting plastic part should be kept within 10℃. When coolant flow changes from laminar flow to turbulent flow, heat transfer effect becomes better. Laminar flow transfers heat only by heat conduction between layers; turbulent flow uses mass transfer in radial direction, plus heat transfer and heat convection. As a result, heat transfer efficiency increases significantly, as shown in Figure 6-58. Care should be taken to ensure that coolant in all parts of cooling pipeline is turbulent.
When coolant reaches turbulent flow state, increase in flow rate has a limited effect on improvement of heat transfer. Therefore, when Reynolds number exceeds 10,000, there is no need to increase flow rate of coolant. Otherwise, heat transfer will only be slightly improved, but it will cause high pressure in cooling pipeline and require higher pump costs. Figure 6-59 illustrates that once coolant becomes a turbulent flow, a higher flow rate of coolant cannot improve heat transfer rate or cooling time, but pressure drop and pump cost are significantly increased.
Coolant will flow to path of least resistance. Sometimes you can try to use restrictor plugs to guide coolant to cooling channels with higher thermal loads. Air gap will reduce heat transfer efficiency, so try to eliminate air gap between embedded part and mold plate, as well as air bubbles in cooling pipeline.
Cooling analysis of mold flow analysis software can assist in discovery and correction of static cooling lines, shortcut cooling lines, and high pressure drops in cooling lines.

Venting of injection molds is an important issue in mold design. Especially in rapid injection molding, requirements for venting of injection molds are more stringent.

 

1. Source of gas in injection mold: Air in pouring system and mold cavity. Some raw materials contain moisture that has not been removed by drying, and they vaporize into water vapor at high temperatures. Due to high temperature during injection molding, some unstable plastics will decompose and produce gas. Gas generated by volatilization or chemical reaction of certain additives in plastic raw materials
2. Poor venting of injection mold will bring a series of harms to quality of plastic parts and many other aspects. Main manifestations are as follows: During injection molding process, melt will replace gas in cavity. If gas is not discharged in time, it will be difficult to fill melt, resulting in insufficient injection volume to fill cavity. Gas that is not smoothly removed will form high pressure in cavity and penetrate into plastic under a certain degree of compression, causing quality defects such as pores, loose tissue, voids, and silver streaks. Because gas is highly compressed, temperature in cavity rises sharply, which in turn causes surrounding melt to decompose and burn, cause local carbonization and scorching of plastic parts. It mainly appears at confluence of two melts, dead corners and gate flanges. Unsmooth removal of gas makes speed of melt entering each cavity different. Therefore, it is easy to form flow marks and fusion marks, reduce mechanical properties of plastic parts. Due to obstruction of gas in cavity, filling speed will be reduced, molding cycle will be affected, production efficiency will be reduced.
3. Key points of exhaust slot design: Exhaust groove is placed on the side of concave mold of parting surface as much as possible to facilitate manufacture and cleaning of mold; Try to set it at the end of material flow and the larger part of wall thickness of plastic part; Exhaust direction should not face operator, should be processed into a curved or bent state to avoid scalding workers during gas injection; Width of exhaust groove is usually 1.5-6mm, depth of groove is 0.02-0.05mm. It is advisable that plastic does not enter exhaust groove.
4. Way of exhaust system:
1. Open exhaust slot. Venting groove is usually opened on one side of cavity, around cavity or at last part where melt is filled. Exhaust channel size: exhaust channel A depth: 0.01~0.02mm width: 3~5mm length: generally 3~5mm. Exhaust duct B Depth: 0.05~0.08mm Width: 3~5mm or more Length: According to needs. Exhaust channel C Depth: 1mm may be taken, Width: may be greater than 5mm, Length: connected to boundary of mold plate. Parting surface exhaust. Mold runner exhaust
2 Vacuum exhaust. This method requires that parting surface of mold be mild, gas in mold cavity is pumped out through air hole. However, it needs to be equipped with vacuum equipment, which increases cost of mold, and is generally not used.
3 Use gap to exhaust. Gap between mating surfaces of inlaid parts, such as cavity and core insert. Gap between side core-pulling parts. Fit clearance of ejected parts (push rod, block). When parting surface gap (normal roughness) is used to exhaust gap, use time is long, gap may be blocked, and it should be cleaned regularly to keep it unblocked.
4 Exhaust using porous metal. In recent years, a newly developed metal material with a uniform interconnected pore structure---porous metal has a very good effect on exhaust of mold cavity. When it is difficult to vent some parts of cavity, porous metal can be used to make cavity inserts, venting effect is very obvious. When using mold, pay attention to maintenance and cleaning to keep air holes unblocked.
5 Mixed exhaust. Usually it is a mixture of exhaust channels and gap exhaust. Overflow value o plastic and exhaust gap, exhaust system should ensure that gas escapes smoothly, plastic melt cannot flow out. Overflow value of plastic materials can be divided into following three types: Low viscosity material does not produce medical clearance: 0.01~0.03mm; Medium viscosity material does not produce medical clearance: 0.03~0.05mm; High-viscosity material does not produce medical clearance: 0.05~0.08mm
Mold exhaust gaps of commonly used materials are as follows:

When side wall of plastic product has through-hole grooves and bosses, plastic product cannot be directly removed from mold. Molded parts of molding holes, grooves and bosses must be made movable, which is called a movable core. Mechanism that completes active extraction and reset is called extraction mechanism.

 

1. Classification of core pulling mechanism
1. Motorized core pulling. When opening mold, relying on mold opening action of injection inspection, core pulling machine is used to bring movable core and extract core. Motorized core pulling has advantages of large demolding force, low labor intensity, high productivity and convenient operation, and is widely used in production. According to its transmission mechanism, it can be divided into following types: oblique guide column core pulling, oblique slider core pulling, gear rack core pulling and so on.
2. Manual core pulling. When mold is opened, movable core is drawn out directly or by transferring parts. Disadvantage is that production, labor intensity is high, and because of restrictions, it is difficult to obtain a large core pulling force. Its advantages are simple mold structure, convenient manufacturing, short manufacturing mold cycle, suitable for trial production of plastic products and small batch production. Due to limitation of characteristics of plastic products, manual core-pulling must be used when motorized core-pulling cannot be used. Manual core pulling can be divided into following types according to its transmission mechanism: threaded core pulling, rack and pinion core pulling, movable insert core, other core pulling, etc.
3. Hydraulic core pulling. Movable core depends on hydraulic cylinder. Advantage is that core hydraulic device can be replaced according to size of demolding force and length of core pulling distance. Therefore, a larger demolding force and a longer core pulling distance can be obtained. Transmission is stable due to use of high-pressure liquid as power. Disadvantage is that it increases operating procedures and also has a complete set of core-pulling hydraulic devices. Therefore, its use range is limited, and it is generally used very small.
2. Design principle of inclined guide column core pulling mechanism:
Movable core is generally small and should be firmly installed on the slider to prevent it from loosening and slipping during core pulling. Connection between core and slider has certain strength and rigidity. Slider should slide smoothly in guide chute, avoid jamming or jumping. Slide limit device must be reliable to ensure that slide block stops at a certain level after mold is opened and does not slide arbitrarily. Locking block should be able to withstand pressure during injection, and a reliable connection method should be used to connect to mold plate. Locking block and mold plate can be made into one body. Oblique angle θ of locking block is generally taken as θ1-θ>2°-3°, otherwise oblique guide post cannot drive slider to move. After slider completes core-pulling movement, it still stays in guide chute, length of guide chute should not be less than -4, 3 of full length of slider. If not, slider is easy to tilt and damage mold when it starts to reset. To prevent sliding block from being set on fixed mold, in order to ensure that plastic product stays on fixed mold, lateral core must be drawn out before mold is opened. It is best to use a directional fixed-distance tensioning device.
3. Design of core-pulling mechanism of inclined slider
Concave or convex on side of plastic product is shallow, required core pulling distance is not large, but when required demolding force is large, inclined slider core pulling structure can be selected. Feature of this kind of inclined slider core pulling structure is that when push rod pushes inclined slider, push rod and core pulling (or parting) actions are carried out at the same time.
Because oblique slider has good rigidity and can withstand greater demolding force, oblique angle of oblique slider is slightly larger than oblique angle of oblique guide column. Generally, oblique angle of oblique block cannot be greater than 30°, otherwise it is prone to failure. Protruding length of inclined slider is generally not more than 2/3 of guide length. If it is too long, it will affect sliding of inclined slider. Because oblique block has a simple core-pulling structure, it is safe and reliable, and it is more convenient to manufacture. Therefore, it is widely used in plastic injection molds.
Sliding guide and combination form of inclined slider. According to shape of sliding part, it can be divided into rectangle, semicircle and dovetail. Combination form of inclined slider. Combination of inclined slider should consider core pulling direction, and try to keep appearance of plastic product without leaving obvious marks on the surface of plastic product. At the same time, consider that combined part of slider has sufficient strength. If shape of plastic product has a turning point, stitching line of oblique slider should coincide with fold line of plastic product.
In conclusion: Above is only a brief summary description of precautions of each system in mold design. Content is not very comprehensive, specific details of design calculation are not involved. It only gives beginners a preliminary understanding and hopes to be helpful to everyone.