In mold manufacturing, mold exhaust is a very important link. Mold exhaust can represent quality of overall mold structure to the greatest extent. Even if a good mold does not have a good exhaust, it is impossible to produce a product of stable quality. A good mold should have good airtightness and ensure that gas is freely discharged, rather than being highly compressed.

First of all, we must understand that unreasonable exhaust will produce following disadvantages:
1. Gas accumulation will erode mold surface and parting line;
2. Color problem: chromatic aberration occurs, which makes color black or bright; bite surface is bright and foggy
3. To increase mold polishing: waste time, money and make steel easily corroded;
4. It is impossible to make product reach standard required by customer;
5. There will be too many bonding lines (this combination is neither strong nor beautiful);
6. It will waste too much injection pressure and increase difficulty of injection molding machine;
7. To use too high barrel temperature (increase difficulty of injection molding machine work, abnormal temperature control, deviation of material performance, increase cycle time);
8. It will reduce level of materials;
9. It will reduce roundness of product and cause product deformation;
10. Struggling force of high-pressure gas in mold cavity will increase cycle instability, and it will often need to adjust machine to solve quality problem;
11. Make work of technicians, QA, operators and mold room personnel more difficult.

 

Finally, we come to understand reasonable exhaust technical requirements:
It is necessary to make a full-circle exhaust structure as much as possible, and depth is within plastic overflow value. Full-circle exhaust is the best, but it is necessary to leave 1/2 of steel on clamping line to compensate for pressure.
If PL surface is irregular and cannot be exhausted all around, better way is to use a 12mm wide exhaust slot to master mold. Gas must eventually be exhausted into atmosphere. Use flat-bottomed and chamfered milling cutters as much as possible to machine exhaust grooves. If you must use a spherical milling cutter, you must calculate clearly that processing depth is consistent with flat-bottomed depth before it can be used. Length of exhaust sealant is one of the most easily overlooked by most people, and is related to proper exhaust problem;
On this issue, there are a lot of different dimensions. Specifications of material supplier range from 3 to 5mm. Experience has shown that if length is greater than 3mm, it will no longer play a good role. Therefore, for safety, we set standard as 3mm, exhaust depth is determined by molding material. If in doubt, please refer to processing specifications.
One of the most common misconceptions is that too much exhaust leads to occurrence of flashes. In fact, opposite direction is correct. Unreasonable exhaust requires excessive pressure. When gas is allowed to exhaust, rubber will go out with gas, only cause of excessive exhaust and occurrence of flashes is that depth of air groove is too deep.
Usually try to use all thimble to exhaust. This is particularly important for large plastic parts. Of course, this is also determined by diameter of thimble. If this is not possible, it is necessary to grind surface to exhaust, use a suitable length and depth, reduce clearance of thimble. Runner, especially large or long runner, must be exhausted.
Another common problem is that when parts cannot be exhausted all around, last filling position and second last filling position must be considered, and the two positions must be exhausted. Deep bone position, column position and small needle dispensing position or thin glue position must be specially considered for exhaust, or use flat thimble, semi-needle structure or exhaust steel, or even vacuum exhaust device.

 

A part cannot be exhausted too much, although parting line can be done, this will bring about following problems related to exhaust: not enough steel to withstand pressure on parting line, exhaust is pressed on parting surface ；
The best exhaust will eventually disappear. After a certain material cycle, if there is not enough steel to compensate pressure on parting line, mold will snap together and air groove will wear out. Parting line should should avoid reduction of mold clamping part, which is one of the biggest problems encountered by large molds. For front and back mold inserts, front and back mold core materials cannot be closed before mold base parting surface is closed. Correct method is to use pressure-resistant blocks to close molds between AB plates of mold base at the same time.

Lifter is a common ejection element, which has two functions: ejecting undercut and ejecting plastic part. When lifter is designed, it will often interfere with side tube or insert, as shown in Figure 1. (A) Distance between lifter and main tube is very close, which will cause interference; at this time, it is necessary to lengthen broken surface of top of lifter to keep lifter away from main tube, as shown in Figure (b). (C) Distance between lifter and insert is very close, (d) fixed plate is designed at the bottom of insert to shorten insert and solve interference.

 

Figure 1 Interference problem of lifter design
In different countries and different companies, there are some customary design methods for stability of lifter. In molds exported to Europe, America, Japan and other developed countries, customers will have certain requirements for lifter design. Generally speaking, customers require lifter to be designed directly on thimble board. That is, simple lifter structure shown in Figure 2 cannot be used. Advantage of simple lifter is that it can avoid interference of llifter with thimble or other components in some situations, and occupy less space on thimble board. When lifter is ejected, guide pin and guide sleeve is designed on ejector plate, which can greatly increase stability of lifter.
In export mold, conventional lifter design should connect lifter to push plate, simplified lifter can be used in special situations such as excessively long lifter, limited space, exceeding range of ejector plate and mutual interference. Use of simplified lifter in molds exported to Europe, America and Japan should be confirmed by customers.
Note when using simple lifter: In mold of fixed-molded inclined elastic slider, it is not suitable to use simple lifter, so as to avoid interference between inclined elastic slider and lifter, and lifter cannot be reset.

 

Figure 2 Simple lifter structure
Extremely small lifter means that when plastic part is relatively small, size and stroke of lifter are very small due to structure of plastic part. At this time, strength of lifter is very weak, so shorten length of lifter as much as possible, adopt a reliable lifter guide mechanism and nitriding lifter parts. Figure 3 shows structure of extremely small lifter of mobile phone shell. Height of lifter seat 5 is increased, lifter seat 5 is slidingly fitted in guide block 4, and length of lifter 2 is shortened. Lifter 2 and lifter seat 5 are matched with an arc surface.
Another important problem of lifter ejection is anti-loosening problem. If screws of lifter connection are loose, lifter mechanism will fail, which will seriously affect life of mold. For extremely small ilifter shown in Figure 3, a spring washer 9 is added under screw to prevent loosening.
In order to increase wear resistance of small lifter, nitriding is often done. Current new DLC technology can effectively extend life of small lifters. DLC is a substance composed of carbon, similar in nature to diamond, and at the same time having a structure composed of graphite atoms. Diamond-like carbon film (DLC) is an amorphous film. Due to its high hardness and high elastic modulus, low friction coefficient, wear resistance and good vacuum tribological characteristics, it is very suitable as a wear-resistant coating, DLC film not only has excellent wear resistance, but also has a very low friction coefficient, generally lower than 0.2, which is an excellent surface anti-wear modified film. Friction coefficient of DLC varies with preparation process and composition of film, friction coefficient can reach as low as 0.005. DLC technology increases life of small lifters by reducing frictional resistance.

 

Figure 3 Structure of a very small lifter
1. Core 2. Lifter 3. Movable mold plate 4. Inclined top guide block 5. Inclined top base 6. Push rod fixing plate 7. Push plate 8. Screw 9. Figure 10. Movable mold base plate