How to Select a Good Mold Manufacturer in China?

After a new product is designed, a lot of companies would be ready to develop the plastic injection mold for small-scale trial production, to see how the market will respond before making a decision on massive production. At this point, it will be of great importance to select a proper plastic mold manufacturer. Personally, I’ve been working in the mold making industry for many years, and have known no less than 100 mold manufacturers, big or small. In this article, I am going to sum up my years of understanding and experience in the profession and would be happy to share it with all of you.


(1)Which type of mold manufacturer is a better choice for cooperation?

Currently, there is a diversity of mold manufacturers available in the China market, including foreign-funded companies, Taiwan invested firms, Hong Kong enterprises, and domestic ones. The pros and cons comparison between each type of mold makers are stated as below:

  1. Foreign-funded mold manufacturers are characterized by standardized management, robust processes, strong service awareness, as well as assured quality and service, but their overhead costs and selling expenses are pretty high, meaning that mold development charge will be accordingly higher. If you have a strong financial capacity and would like to seek a company that is able to provide assured product quality, a large size foreign-funded mold manufacturer will be your choice of cooperator! However, in practice, large sized foreign-funded companies often have regular clients, so their prices will often be in line with the international market, which is not easy for many enterprises to accept. A set of large precision automotive mold usually costs hundreds of thousands or even millions dollar – the direct cause that discourages many buyers.
  2. Chinese privately owned mold manufacturers are mostly established by the first batch foreign enterprise employees who first accessed the mold industry. Dubbed up-and-coming stars, one of the biggest inherent advantages of such enterprises is the relatively low material, overhead and selling costs. The mold and product prices they offer are definitely lower than those of foreign, Taiwan and Hong Kong mold manufacturer, so they are always able to attract the attention of a large number of mold buyers.


(2)What size mold manufacturer is a good choice for cooperation?

Large, medium or small sized mold manufacturer?

Personally, I think the choice in this regard is dependent on the circumstances. From the perspective of factory size, everyone will preferentially choose the largest one, because the large ones are usually fully equipped and thus more competent! Potentially, their sizes also reflect the level of customer’s choice. But, on the other hand, the larger the size, the higher the overhead costs will be, so the price will not be as competitive. In particular, the prices offered by large-sized foreign-funded mold manufacturers are literally unacceptable for most buyers. It is of critical importance that whether the mold manufacturer you select treat you as a VIP, i.e. whether they respect you. As a matter of fact, looking for a supplier is like choosing a partner, a suitable match is required. If your company does not have a strong financial capacity and your purchase volume is not big enough, looking for cooperation with large-sized mold manufacturers will only lead to the result that you are not respectfully treated despite the money you spend because you are not able to meet their requirements, be it order amount or volume. That is to say, they cannot benefit from your orders.

Under such circumstances, most people will choose SMEs. As a matter of fact, it is now a common practice in the industry, after all the mold and product prices appear to be more acceptable. However, you will soon find other concerns arising – can the mold be produced punctually with assured quality, how will the mold perform in the process of mass production and what is the life cycle of the mold? At this point, I would suggest you pay attention to the general knowledge that you have to select a fully equipped mold manufacturer, i.e. it has to be equipped with supporting precision CNC machines, slow wire cut machines, EDM, high-speed high-precision injection machine and 3D/2D measuring instruments that are necessary for mold processing, production, and testing. This is the point that should be taken as the key element for inspection, evaluation and final selection of a mold manufacturer.


(3)Meet mold manufacturer face to face, or ask for some reference customers before place order.

When you search “China mold manufacturer “or” Chinese mold maker” in Google, you will get a lot of contacts from mold manufacturers in China, I suggest you send RFQs to 5 or more mold makers, then you can compare their price, they react speed, their professional support. Normally the mold manufacturer arranges an engineer or a sales who have the engineering background to contact you, it will be much better than a sales girl without any engineering experience. because mold making is a customize and professional service, mold experience will help a lot in communication and avoid misunderstanding in engineering. it will save lots of time and cost. when you decide to work with one mold manufacturer, I suggest to visit them on site, then you can check their facility and talk to them face to face. if time or budget is limited, then you can ask for some reference customers from your own country who are dealing with them, the people who live in the same country and culture are always more trustworthy.

Sositar mould is a professional plastic injection mold manufacturer in Shenzhen China. We have more than 100 customers all over the world. most of them come from European and US.

Let us share some interaction between Sositar mould and customers all over the world.

Meet our customer in London UK

customer in London UK


Visit our customer in Birmingham UK

customer in Birmingham UK


Visit our customer in Kleve Germany

customer in Kleve Germany

Visit our customer in Rotterdam Netherland

customer in Rotterdam Netherland


Visit our customer in Milan Italy

customer in Milan Italy

Meet our customer in Florida USA and trial the product we made


Meet customer in Des Moines USA

The customer from Australia visit us in Shenzhen

The customer from Canada visit us in Shenzhen

Hot runner types and their application in plastic injection mold

1,How the Hot Runner Works?

The cold runner of a plastic injection mold refers to the section between the mold inlet and the gate. The molten plastic maintains its fluidity inside the runner by virtue of the injection pressure and its own heat. However, as a part of the molding material, the runner is not a product. As a result, when designing a mold, we need to consider not only the filling effects, but also the material-saving effect produced by shortening or downsizing the runner; but in actuality, it is not easy for us to have the best of both worlds.this is why we need hot runner mold

Also known as the runner-less system, the hot runner is a system in which the molten plastic is not solidified after each injection, so when releasing the plastic part, the gate inside the runner does not have to be released. Since the plastic inside the runner is not solidified, the runner still stays unblocked when the next injection takes place.

2,The advantage of Hot Runner Molds compare to cold runner molds

Why has the hot runner system been developed? What benefits can the hot runner system bring us? Engineers familiar with the plastic injection molding process all know that the traditional injection molding is always associated with the following disadvantages:

  1. Hard to fill; b. thin-walled large size parts easy to deform; c. material waste in the runner; d. inconsistent plastic part quality when using a multi-cavity mold, and so forth.

The appearance of the hot runner system has provided an almost perfect solution for these problems. Generally speaking, the hot runner system offers the following benefits:

■ Shorter cycle time

As it is not limited by the runner cooling time, the plastic part can be ejected in a timely manner after being molded and solidified. For the production of thin-walled small parts, the molding cycle can be kept within 5s.

■ Material saving

In a complete hot runner mold, material waste will not occur thanks to the absence of a cold runner. This is of a greater significance to the application projects of the more costly plastic materials. As a matter of fact, global major hot runner manufacturers had witnessed leap forward development during the years when world crude oil and raw plastic materials were expensive. This is because the hot runner technology is an effective way to reduce waste materials and thus lower raw material costs.

■ Reducing defective products, and improving product quality

The temperature of the molten plastic inside the runner system can be accurately controlled during the hot runner injection molding process, so that the plastic is able to flow into each cavity in a more consistent manner, thus guaranteeing consistent quality products. The hot runner gate is of a higher quality, so after mold release, the part produced will have a smaller internal stress, leading to a smaller part deformation. Therefore, many high quality products in the market are all produced with the hot runner mold.

■ Eliminating following steps, helpful for production automation

After being molded with a hot runner mold, the part will be the final product, so there will be no follow-up steps like gate trimming, recycling and runner cooling, thus helpful for production automation. Many manufacturers from both home and abroad have combined the hot runner system with automation, so as to greatly improve production efficiency.

3,The Hot Runner types

  1. The mold that employs a hot sprue for direct or indirect material feeding, is referred to as a hot sprue mold.


2,The mold that is designed with a hot runner manifold and a secondary hot sprue, is referred to as a hot runner mold.

Examples of hot sprue molds

(1) The hot sprue mold structure that feeds materials through the point gate. It is only suitable for the single-cavity mold, and is limited by the gate location.

2) The hot sprue mold structure with the end surface engaged in injection molding. It is suitable for the single-cavity mold, but sprue marks will be left on the part surface. The end surface of the hot sprue is machinable.

(3) The hot sprue/cold runner mold structure with a bit of the traditional runner features. This structure is able to produce multiple plastic parts at the same time, but some cold slugs will be produced.

Besides open gate hot runner as above,Now people start to choose valve gate hot runner for high class product.

What is valve gate hot runner?

Mount the hot sprue and the manifold into the injection mold, and then by way of heating, keep the fluidity of the plastic material after it is out of the barrel. The result is, the product gate seems to be able to contact with the nozzle directly. So, when releasing the product, the sprue is closed under the action of the needle valve. The application of one or more cold runners is thus avoided, realizing wasteless production.

The advantages of the valve gate hot runner include:

  • Shorten production cycle, improve production efficiency;
  • Since the cold runner is replaced, cold slugs will not be produced, thus saving a large amount of raw materials;
  • Improve product consistency, free from obvious gate marks;
  • Remarkably improve product appearance;
  • Help reduce product stress, minimize product deformation and structural brittleness, so the production capacity and life cycle of complex products are improved;
  • Offer more process control, allow accurate adjustment to the injection molding process;
  • Not limited by the change of plastic injection molding materials.

Why the venting is so important for a plastic injection mold?

The venting slot serves two purposes: 1. Expel air from the plastic injection mold cavity during the injection process of the molten plastic material; 2. Get rid of the various gases produced during injection molding processing. The setup of venting slots is of great importance, especially for thin-walled products or the locations far away from the gate. In addition, close attention should also be paid to venting slots for the production of small-size or precision parts, because they are able to protect the products from surface burns, insufficient filling, as well as other defects.

So, what is sufficient air venting for plastic injection molding? Generally speaking, if no burn marks are left on the product at the highest molten plastic injection speed, then the venting effect of the mold cavity will be considered sufficient.


Venting Methods:

There are lots of ways to vent the mold cavity, but each of them has to guarantee that: while performing venting functions, the size of the vent slot needs to be able to prevent material from entering the slot, as well as clogging at the same time. However, if there are too many slots, it will do more harm than good, because if the clamping force against the mold cavity parting surface area without vent slots is too high, the cavity material will be prone to cracks, which is very dangerous. In addition to being designed on the parting surface of the mold cavity, the vent slot can also be machined in the end section of the runner system. The clearance around the ejector can also act as a way to let the trapped air out. If the height, width and position of the vent are not appropriately defined, flash will be caused, thus affecting product aesthetics and precision. As a result, the clearance design should be able to prevent flash from occurring around the ejector pin. In particular, it is worth noting that molded parts like gears expect no flash at all. Therefore, the following venting methods should be employed:

  1. Completely eliminate the air in the runner;
  2. Apply peening treatment to the parting surface with the 200# silicon carbide abrasive, and open vent slots in the end section of the runner system, mainly referring to machining slots in the end section of the sub-channel, of which the width should be equal to that of the sub-channel while the height may vary from material from material.

Design Approach:

Based on the years of injection mold design and mold trial experience, this article is aimed to generally explain the design principles of several slot types. For parts with complicated geometric shapes, the vent slot positions should be identified after several mold trials. If a mold design adopts the integrated structure, poor ventilation will be its biggest disadvantage. So, for molds with integrated cavity and core, the following venting methods can be adopted:

(1) Make use of the slot or insert location in the mold cavity;

(2) Make use of the lateral insert crevice;

(3) Machine the local part into the spiral shape;

(4) When it is extremely difficult to expel the air out of the mold, an insert should be adopted. If it is not easy to machine a vent slot in some locations of a mold, such as in the corners, the insert molding process may be appropriately applied on condition that product appearance and precision are not affected. This method not only helps with venting, but is also able to lower the difficulty level for machining, and convenient for maintenance, too.

Vent Slot Design Dimensions:

The width of the vent slot ranges from 1.5 to 6mm, while the depth design should be able guarantee that the plastic material will not get into the slot to cause flash. Its value is dependent on the viscosity of the molten plastic, but usually its applicable range is from 0.013 to 0.05mm.



Appropriately designed venting slots are able to drastically reduce injection pressure, injection time, pressure holding time and clamping force, thus making the plastic injection molding process much easier by improving production efficiency, lowering production costs and saving the energy consumed by machine.

ABS Plastic Injection Molding Process

ABS plastic injection molding process

Analysis of the ABS Plastic Injection Molding Process


Thanks to its outstanding mechanical strength and excellent overall performance, the ABS plastic plays an important role in many industries that include plastic product manufacturing, electronics, machinery and building materials. Usually, in plastic product manufacturing, plastic decorative pieces that require electroplating also adopt the ABS plastics.


Generally referred to as the Acrylonitrile-Butadiene-Styrene Terpolymer, ABS is a terpolymer made by polymerizing styrene and acrylonitrile in the presence of polybutadiene. Due to the different proportions of the three compounds, ABS may show different performances and melting temperatures. When mixed with other materials or additives, the purpose and performance of ABS can be further expanded to a wider range, such as impact/heat resistance, flame retardancy, transparency, reinforcement and electroplating. Plastic product manufacturing often involves electroplating. Ranging between that of PS and PC, the fluidity of ABS has an influence on injection temperature and pressure, with the latter more greatly influenced. As a result, during the injection molding process, a higher injection pressure is often employed to reduce the viscosity of the molten ABS and thus improve the mold filling performance.


Injection Molding Performance of ABS: Generally, the melting point of ABS is about 170℃ and its decomposition temperature is 260℃, with a wide range of adjustable injection temperature. During the injection molding process, the temperature often ranges between 180 – 240℃. Due to the existence of rubber, it is able to absorb a small amount of water, so it needs to be dried during production – dried for 1 – 2h in temperatures between 80 – 90℃; at the same time, due to its poor thermal stability caused by the existence of rubber, it tends to decompose, so it is better that the raw material does not stay in the barrel for too long during injection molding; though the viscosity of the molten ABS is higher than that of PS, it is able to completely fill the mold with average gates and sprues; the product often generates static electricity, which tends to attract dust and dirt; shrinkage rage is 5‰; and overflow limit value 0.05mm.

(1)Dry the ABS Material


Due to it moisture absorption feature and sensitivity to moisture, the ABS plastic needs to be fully dried and preheated before processing,
which not only helps eliminate the firework-like bubbles and silver lines on the surface caused by moisture, but also facilitates plasticization, thus reducing color shading and cloud pattern on product surface. The moisture content in the raw ABS material needs to be kept under 0.13%. Pre-injection drying conditions: dry under 75 – 80℃ temperature conditions for 2 – 3h in dry seasons or winter; and under 80 – 90℃ temperature conditions for 4 – 8h in summer or rainy days. It will require a long time to dry the material, i.e. up to 8 – 16h, if the product requires excellent glossiness or features a complex structure. Due to the fact that the cloud pattern on product surface caused by the small amount of moisture is often ignored, it is better to convert the hopper of the machine into a hot wind drier, so as to prevent the dried ABS material from absorbing moisture again in the hopper. However, this type of hopper needs enhanced moisture control, as well as overheat prevention during occasional production interruption. The proportion of recycled materials cannot exceed 30%, while the application of recycled materials is not allowed for electroplating grade ABS.


(2)Injection Temperature

For the ABS plastic, the relationship between temperature and melt viscosity is different from that of other amorphous polymers. In the melting process with the temperature keeping rising, the melt viscosity actually drops in a very limited range. Once reaching the plasticizing temperature (proper processing temperature range, such as 220 – 250℃), if the temperature is kept rising blindly, the ABS material with poor thermal stability will definitely experience thermal decomposition, which may increase the viscosity, thus making it more difficult for injection molding, while affecting product mechanical properties. Therefore, though the injection temperature of ABS is higher than that of some other plastic materials, like the PS, it does not enjoy a more extensive temperature range as the latter does. For some injection machines with poor temperature control, when certain number of ABS products have been manufactured, yellow or brown coke granules, more or less, tend to be found on the product. Also, it is very difficult to resolve this problem by addition of new materials or means of empty shots. The reason is that the ABS plastics contain butadiene, so under high temperature conditions, when some plastic pellets firmly stick to some not-easy-to-cleanse surfaces, like screw slots, they will be prone to decomposition and carbonization if they are heated for too long. Since operation in higher temperature conditions may cause problems to ABS, it is necessary to control the temperature of each barrel section. Of course, different mold temperatures are applicable to ABS materials of different types and compositions. The temperature of a plunger injection machine is kept between 180 and 230℃, while that of a screw type injection molding machine is between 160 and 220℃. It is worth mentioning that the processing temperature of ABS is quite high, while it is very sensitive to the changes of various process factors, so it is of great important to control the temperature at the barrel front end and the nozzle part. It has been proved that any slight changes in these two parts can be reflected on the product. The more drastic the temperature change is, the more defects will be caused, including weld lines, poor glossiness, flash, product sticking and discoloration.


(3) Injection Pressure

The viscosity of ABS is higher than that of PS or MPS, so a higher injection pressure is required during the injection molding process. Of course, not all ABS products require the application of a high pressure. A lower injection pressure may be applicable to small, simple-structured or thick products. During injection molding, the in-cavity pressure level at the moment of gate closing often plays a critical role in defining the surface quality and the silver line defect level. A too-low pressure will lead to a high plastic shrinkage rate and create a bigger opportunity for the plastic to be released from the cavity surface, thus causing cloud pattern on product surface. If the pressure is too high, the friction between the plastic and the cavity surface will be increased, thus causing mold adhesion.


(4) Injection Speed

For the ABS material, a moderate injection speed will bring about a great effect. If the injection speed is too fast, the plastics will be burnt or decomposed to separate out gasification products, so defects like weld lines, poor glossiness and plastic around the gate turning red, etc. will occur. However, when producing thin-walled or complex products, a high injection speed needs to be ensured, or insufficient filling will be caused.


(5) Mold Temperature

The injection molding temperature of ABS is relatively higher, and so is the mold temperature. Usually, the mold temperature is adjusted between 75 and 85℃. When manufacturing products with a large projection area, the cavity temperature is required to be between 70 and 80℃, while that of the core is between 50 and 60℃. When injection molding large, complex-structured or thin-walled products, it should be considered to heat the mold specially. In order to shorten molding cycle and maintain mold temperature stability, after the product is ejected, hot bath, cold bath or other mechanical molding methods need to be applied to compensate for the original cooling and molding time of the cavity.


(6) Shot Size Control

Usually, when injecting the ABS plastics, each shot size only reaches 75% of the standard shot size. To improve product quality, as well as dimensional stability, surface glossiness and an even color, the shot size is required to be 50% of the standard size.


(7) Selection of Injection Machine

May select the standard injection machine (screw length-to-diameter ratio is 20:1, compression ratio greater than 2, and injection pressure greater than 150MPa). If masterbatch is used or the product has a high requirement for appearance, the screw of a smaller-size diameter can be selected. Clamping force should be identified as per 4700 – 6200t/m2, depending on the specific plastic grade and product requirements.