Mould Design Sharing-Design of Exhaust System

Position of exhaust system in mold is very important. Although exhaust system design is not complicated, it plays a vital role in normal injection of mold. Exhaust gas is gas that can be exhausted from mold during injection moulding. Model diagram is shown in Figure 3.130.

exhaust system design

 When injection is started, material flows from water inlet into mold cavity. When it reaches point A, air from point A to point C can no longer be discharged, and it becomes “dead”, “sleepy gas”. When material continues to flow in, air is compressed under effect of pressure. When air is constantly being compressed, temperature is constantly increasing, back pressure is constantly increasing, so that rising temperature is sufficient to scorch plastic. Moreover, no matter how much pressure is added, trapped air cannot be compressed to zero. In other words, no matter how much injection pressure injection molding machine increases, it will never flow material to point C, that is, size of product is inaccurate, products with high precision are absolutely not allowed. This is phenomenon of “dead gas” and “sleepy gas”, which is harmful to injection moulding. Design should try to eliminate and solve it. Above illustrations use air as an example. In fact, so-called “dead gas” and “sleepy gas” contain a variety of gases, such as:
① Air in cavity is in natural state.
② Water vapor evaporated from plastic materials.
③ Volatile gas decomposed from liquid plastic.
④ Reaction and volatile gas of auxiliary additives in plastic. These “dead gas” and “sleepy gas” have great harm and impact on product. Its formation will affect appearance, mechanical strength, shrinkage, dimensional accuracy, injection molding production time of product. Therefore, sufficient attention should be paid to exhaust problem of product.

How to determine direction of exhaust in mold

Judging direction of exhaust is actually judging place where “dead gas” and “sleepy gas” may be. This is a very important prerequisite. If judgment is wrong, exhaust position we designed will be wrong. All processing will be obsolete, and problem cannot be solved. Therefore, it is important to determine direction of exhaust. To accurately determine direction of exhaust, it is necessary to understand process and direction of flow material.

Flow process and direction of flow material during mold injection are shown in Table 3.20. Flow process and direction of flow material during mold injection are analyzed.
exhaust system design

This is a rectangular box type product. Way of water inlet is through water inlet. When L2> L1, position of fusion line should be at position I. Generally speaking, product’s fusion line is where product produces dead gas and sleepy gas during injection.This is also a rectangular box type product. Way of water inlet is ordinary large water inlet. Material will flow in direction shown by lines 1, 2, and 3 until it fills entire cavity. Position of fusion line should be in II position, that is, place where dead gas and sleepy gas is produced.

exhaust system design

This is a button type product. Way of water inlet is ordinary large water inlet or fine water inlet. No matter whether it is ordinary large water inlet or fine water inlet, flow material fusion line or fusion position will be at A and B, this is where product produces dead gas and sleepy gas during injection molding.This is a rectangular box product. Inside of product also has deep bones and deep rubber columns. Product is filled with water through an ordinary large nozzle. Material will flow along lines 1, 2, and 3 during injection until it fills the entire cavity. Position of fusion line should be at A, B, and C. These three places are where product is dead and trapped. Especially at B and C, this is vertical material position, gas trapped by flowing has no way to go.

exhaust system design

This is a ring-shaped product. Way of water inlet is ordinary water inlet. Melting line of product during injection molding should be on diagonal line where product touches perforation, that is, A line, place where dead gas and sleepy gas is produced.This is a rectangular box product. Water inlet method is ordinary water inlet. During injection molding, flow material will flow along 1, 2, and 3 directions. Fusion line should be at A, B, and C, especially at A and B, because of vertical glue position, when material flows in, trapped gas has no way to go.This is a long cylindrical product. Way of water inlet is ordinary water inlet from parting surface. During injection molding, flow material will be first filled with ring, then flow upward in entire ring layer until entire cavity is filled. Position of fusion line should be on the top of product, that is, A place, which is also the place where dead gas and sleepy gas is produced.

Division of exhaust area, primary and secondary exhaust levels 

Exhaust system are a general term for exhaust method of entire set of molds. Exhaust system often does not make a detailed distinction, so these primary and secondary exhaust locations are often ignored. In fact, division of exhaust area, division of primary and secondary exhaust levels are very important. If we don’t know exhaust area, how can we exhaust it? If we don’t know main exhaust position and secondary exhaust position, how can we design it? Following is explained with Figure 3.138.
(1) Dividing exhaust area: It can be clearly seen from figure that this product has three trapped air levels, namely A, B, and C, which means that this product has three exhaust area, namely A trapped air level exhaust, B trapped air level exhaust and C trapped air level exhaust.
(2) As can be seen from explanation of flow process, flow material is used to complete injection moulding of product through mainstream directions of lines 1, 2, and 3. For example, if the first side is not designed to vent, second side, third side, and fourth side can be leaked. If second side and fourth side are not designed to vent, third side can be leaked. However, if third side is not designed to vent, then “stuck air” will have no way to leak and can only be flushed back. When it meets with material flowing from line 3, fusion line will be generated, which will also produce “Trapped C”. From above, it can be seen that first side is not main exhaust level, second side and third side are not main exhaust levels, but it is more important than first side, third side is the most important exhaust level.
When flow enters along 4th line, there is no way to run out of “sleepy air”. Therefore, exhaust of sleepy air level is also main exhaust level.
When flow enters along 5th line, there is no way to run out of “trapped gas”. Therefore, exhaust of B trapped gas level is also main exhaust level.
It can be known from Figure 3.138 and its water entry point:
① This product is divided into three exhaust areas, namely A, B, C three exhaust areas.
② Exhaust on third side, A-position exhaust, and B-position exhaust are main exhaust areas. Exhaust with inserts is shown in Figure 3.139.
③ Second and fourth side are secondary exhaust levels.
④ First side is the least important exhaust level.

How to design exhaust

There are several common exhaust methods:
① Open exhaust grooves on parting surface of front and rear molds to exhaust.
② Exhaust with a ready-made thimble.
③ Exhaust with ready-made inserts.
④ Exhaust with craft inserts.
⑤ Exhaust with breathable steel.
Of these several exhaust methods, the first four are used the most and are also the most commonly used ones.

Exhaust grooves on the front and rear mold parting surfaces

It is the most common method to open exhaust grooves on parting surface of front and rear molds. It can be said that each set of molds must use this exhaust method, because this type of exhaust method has the largest exhaust volume, the easiest processing, and the most obvious effect. Each mold has a parting surface, this exhaust method is completed by slotting between front and rear panel inserts, as shown in Figure 3.140.
Exhaust groove should be on opposite side of water inlet side, on left and right sides of water inlet side. When product is large and material level is thick, exhaust groove will also be opened on water inlet side. There are two concepts for opening exhaust slot, as shown in Figure 3.141 and Figure 3.142 in Table 3.21.

Exhaust System

Analysis
This is way of exhausting a small number of exhaust grooves, but a wide groove.
When it is small and medium-sized products, plastic burr value is above 0.05mm,
A: It is best to be between 0.015-0.02mm, minimum should not be less than 0.01mm, and maximum should not be greater than 0.025mm.
B: It is best to take between 0.15-0.20mm, minimum should not be less than 0.10mm, and maximum should not be greater than 0.30mm.
C: It is best to take between 2.0-2.5mm, minimum should not be less than 1.5mm, and maximum should not be greater than 3.0mm.
M: Usually take 6-20mm.
N: Usually 40-80mm.
When it is a large product and plastic burr value is above 0.05mm,
A: It is best to take between 0.02-0.03mm, minimum should not be less than 0.015mm, and maximum should not be greater than 0.035mm.
B: It is best to take between 0.3-0.4mm, minimum should not be less than 0.2mm, and maximum should not be greater than 0.5mm.
C: It is best to take between 3.0-3.5mm, minimum should not be less than 2.5mm, and maximum should not be greater than 4.0mm.
M: Usually take 12-20mm.
N: Usually 50-90mm.
When it is small and medium-sized products, plastic burr value is 0.02-0.04mm,
A: It is best to take between 0.01-0.015mm, minimum should not be less than 0.01mm, and maximum should not be greater than 0.02mm.
B: It is best to take between 0.15-0.20mm, minimum should not be less than 0.10mm, and maximum should not be greater than 0.30mm.
C: It is best to take between 2.0-2.5mm, minimum should not be less than 1.5mm, and maximum should not be greater than 3.0mm.
M: Usually take 6-15mm.
N: Usually take 40-60mm.
When it is a large product with a plastic burr value of 0.02-0.04mm,
A: It is best to take between 0.015-0.02mm, minimum should not be less than 0.015mm, and maximum should not be greater than 0.03mm.
B: It is best to take between 0.25-0.35mm, minimum should not be less than 0.2mm, and maximum should not be greater than 0.4mm.
C: It is best to take between 2.5-3.0mm, minimum should not be less than 2.0mm, and maximum should not be greater than 4.0mm.
M: Usually take 12-20mm.
N: Usually take 40-60mm.

Exhaust System

Analysis
This is way of exhausting a large number of exhaust grooves, but a relatively small groove.
When it is small and medium-sized products, plastic burr value is above 0.05mm,
A: It is best to take between 0.015-0.02mm, minimum should not be less than 0.01mm, and maximum should not be greater than 0.025mm.
B: It is best to take between 0.15-0.20mm, minimum should not be less than 0.10mm, and maximum should not be greater than 0.30mm.
C: It is best to take between 2.0-2.5mm, minimum should not be less than 1.5mm, and maximum should not be greater than 3.0mm.
M: Usually take 3.0-6.0mm.
N: Usually take 20-30mm.
When it is small and medium-sized products, plastic burr value is 0.02-0.04mm,
A: It is best to take between 0.01-0.015mm, minimum should not be less than 0.01mm, and maximum should not be greater than 0.02mm.B: It is best to take between 0.15-0.20mm, minimum should not be less than 0.10mm, and maximum should not be greater than 0.30mm.
C: It is best to take between 2.0-2.5mm, minimum should not be less than 1.5mm, and maximum should not be greater than 3.0mm.
M: Usually take 3.0-6.0mm.
N: Usually take 20-30mm.
Large products do not generally use this form of exhaust.

Ejector ejection with ready-made ejector pin.

Generally, when designing ejector pin, one more consideration is to let ejector pin have both ejection and exhaust functions. It is a conventional exhaust design. Cylinder exhaust, flat ejector exhaust, and straight exhaust also belong to this category, as shown in Table 3.22.

Exhaust System

This is a perspective view of product.Bone in the picture is processed by spark machine and directly punched on rear mold insert. During injection, 4 thimbles on bone side only have a bone effect on large plastic position, but have no venting function on bone position. At point A of bone, there will be trapped air, which cannot be exhausted.Bone in the picture is also processed by a spark machine and directly punched on rear mold insert. However, it is ejected with a flat thimble. Flat thimble is directly placed on the bottom of bone. In this way, during injection, flat thimble under bone has function of venting. Air will be discharged through fitting position of flat thimble head, which eliminates phenomenon of trapped air. This is exhaust design of flat thimble.

exhaust levels

This is a perspective view of product.This is column processed by spark machine directly on rear mold insert. During injection, two thimbles on the side of column only have function of exhausting large gate position, but there is no function of exhausting itself. At point C of column, there will be trapped air, which cannot be exhausted.This is design of barrel ejection mechanism. Barrel and barrel needle are combined into a column and set at the bottom of barrel. In this way, barrel and barrel needles under column have function of venting during injection. When material flows to point D, air will be discharged through fitting position of head of barrel needle, eliminating phenomenon of trapped air. This is exhaust design of barrel and barrel needle.

Vent with ready-made inserts.

So-called “ready-made inserts” means that insert structure must be designed when designing mold without considering injection venting. Regardless of whether it has function of venting, mold must be designed according to insert structure, see Table 3.23.

exhaust levels

When L is more than 15mm, when designing mold, even if need for exhaust is not considered,  structural design of insert must be designed in such a deep bone position, otherwise it is difficult to polish. During injection molding, when material flows to point A at the bottom of bone, air will be discharged out of mold along fitting of insert to eliminate phenomenon of trapped air. This is ready-made insert exhaust design.From point of view of product structure, even if need for exhaust is not considered, mold will be designed into structure of front and rear mold inserts. Because front and rear die positions are too difficult to machine and polish. During injection molding, when material flows to points B, C, D, E, air will be discharged out of mold along fitting of insert to eliminate phenomenon of trapped air. This is ready-made insert exhaust design.

Vent with process inserts.

So-called “process inserts” means that mold will not design structure of insert insert without considering injection exhaust, and design that should be designed as an insert structure for exhaust is called “process insert” . Design of process insert is exhaust design method usually used when product structure is special, exhaust requirements are high during injection molding, product size requirements are high. See Table 3.24.

exhaust levels

When L is less than 12mm, it is analyzed according to general mold structure design. These bone positions and small rubber columns do not need to be designed as insert structures, but injection molding process is quite difficult and reject rate is quite high. In this case, we must try to exhaust gas. To exhaust gas from these two positions, two front mold gate positions should be inlaid and vented separately. When material flows to points A and B, trapped air will be discharged out of mold along fitting position of head of small insert to eliminate trapped air. This is exhaust design of process insert.

exhaust levels

There are many bone positions on back mold. Although value of h is not high and h is less than 10mm, there are too many bone positions. It is very serious to be trapped during injection. If you do not try to exhaust air, sometimes mold cannot produce at all. In order to vent air, insert structure is made at rear mold bone. During injection, when material flows to points C, D, E, and F, gas will be discharged out of mold along fitting position of head of small insert to eliminate phenomenon of trapped air, which is also exhaust design of process insert.

exhaust levels

Front mold has a high convex gate. If there is no exhaust level, trapped air is inevitable, and it is also quite serious. If it is not good, it will burn plastic like black carbon. Two small recesses on the top of convex gate should be made into an insert structure. In this way, when material flows to G point during injection, trapped air will be discharged out of mold along fitting position of head of small insert to eliminate trapped air phenomenon.

exhaust levels

Front mold has a high piece of convex rubber strip. If there is no exhaust, air trapping will occur, making mold impossible to produce at all. In this case, we must try to exhaust air, so front mold must be designed with an insert. When material flows to H point, trapped air will be discharged out of mold along fitting position of insert to eliminates phenomenon of trapped air. which is also design of process insert discharge.

Vent with breathable steel.

When product structure is very special, process inserts are not allowed to be vented in the place where air is trapped. In short, in the place where air is trapped is not allowed to have a little scar or other defects, only using breathable steel as a mold insert can solve such problems. Breathable steel is a steel with a mesh structure. Just like there are many small tunnels inside steel, gas can be connected in all directions. Compressed air is used to blow this steel, and there will be bubbles on other side, which shows that air permeability is very strong, but this steel is very expensive, it should be used with caution when designing mold.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s