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How to select a Plastic Injection Molding Machine

Oct 12.2022

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Buying an injection molding machine is not a small investment. Buying a machine that is too large is a waste of money, while a machine that is too small is not suitable. Therefore, choosing the right machine for the product is the most important thing for the buyer. The supplier recommends the right machine and suggests how to choose the machine is the first lesson the supplier provides to the buyer.

In order to understand the various steps of selecting an injection molding machine more systematically, the following content and detailed instructions are for this purpose.

Determine the injection weight of the injection molding machine

injection molding machine

Nowadays, there are too many injection molding machines on the market. It is only considered that the injection machine is too simple and the clamping force is not comprehensive. It is necessary to combine the two to make a choice.

Injection weight is an important parameter of an injection molding machine, and it is the most useful and critical parameter for selecting an injection molding machine. Expressed in gram or ounze, although this parameter is simple and easy to understand, it can also be easily misunderstood. the reason is simple. When the plastic material is determined, the weight of the plastic part can be determined. Therefore, it is easy for them to use the injection weight to select an injection molding machine. In fact, the definition of injection weight refers to the maximum plastic injection weight that the injection device can reach when the screw performs the maximum injection stroke under the screw. Air shot state (not injected into the mold); this parameter reflects the processing capacity of the injection molding machine to a certain extent; it can be measured or calculated theoretically (theoretical value and measured value may be different). The material used in the test is usually ordinary PS with a density of 1.05.

Steps to determine the injection weight

  1. Calculate the product weight first: W = product weight + gate system weight, where: W is the weight required for injection. In actual situations, there are two situations to consider.
  2. When the injection molded product is polystyrene (PS), the injection volume of the injection molding machine should be Wps; Wps = (1.3 ~ 1.1) × W (product weight + total gate system weight), when the product requirements are high, The coefficient in the above formula should take a large value, otherwise, it can take a small value.
  3. When the product is the imitation method of other plastics (named X plastic), first calculate the theoretical injection volume of the plastic should have Wx; Wx = (1.3 ~ 1.1) × (product weight + total weight of the gate system); then according to the plastic The density (X) is converted into the actual weight of the PS material Wps, the conversion formula is: Wps=Wx×1.05/Vx
  4. According to the calculation results of the Wps and LOG manuals, select the appropriate injection molding machine.

Remarks: The relationship between injection weight and injection volume

  1. The injection weight is more than just the injection volume multiplied by the density of PS. Measure the injection weight, and the injection volume is a theoretical value. Because the plastic material will flow into the gap between the barrel and the screw during injection, and the non-return ring needs to move backwards to reach the closed position, the injection volume is doubled because the density of PS is greater than the injection weight.
  2. Products with low requirements, such as toy dolls: the total weight should be 85% of the injection weight; in high-demand injection molding; such as crystal products, 75%.
  3. Do not use an injection molding machine with an excessive injection weight

    The total weight of plastic and runner plastic should be between 35% and 85% of the injected weight. The lower limit is due to the following three considerations: the bending of the plate; the residence time of the plastic in the barrel and the power consumption per kilogram of injection molded parts. The use of small molds for small injection molded parts will cause excessive bending of the pressure plate, resulting in warping (affecting product quality), and even cracking of the pressure plate. Use an oversized injection molding machine to inject small plastic parts. If the residence time of the molten material is too long, it may cause the melted plastic in the barrel to decompose.

  4. Injection speed

    For ultra-thin-walled products, high fluidity materials are used to make the molten plastic flow at an ultra-high speed to ensure that the thickness l of the surface layer formed by cooling and solidification after the molten material contacts the mold wall is as thin as possible, which is to delay the generation time of the solidified skin as much as possible. So it involves the issue of injection speed. In terms of experience, the injection speed can be 1 to 1.5 times the flow length ratio.


How to Calculation of clamping force of injection molding machine

There are two important factors to calculate the clamping force: 1. Projection area; 2. Cavity pressure.

  1. Projected area (S) is the largest area seen from the line of sight of mold opening and closing.
  2. Cavity pressure determination (P)
  3. The cavity pressure is affected by the following factors
  4. Number and location of doors
  5. Door size
  6. Product thickness
  7. Viscosity characteristics of materials used
  8. Injection speed

2.1 Grouping of thermoplastic flow characteristics

  • Group 1 PS PE PP
  • Group 2 PA6 PBT PET
  • Group 3 PU soft PVC
  • Group 4 ABS as POM
  • Group 5 PMMA PC/ABS PC/PBT
  • Group 6 PC PEEK rigid UPVC

2.2 Viscosity grade

Each group of plastics mentioned above has a viscosity (flowability) grade. The relative viscosity grades of each group of plastics are as follows

Group Multiplication Constant (K)

The first group ×1.0

The second group ×1.3~1.35

The third group ×1.35~1.45

The fourth group ×1.45~1.55

The fifth group ×1.55~1.70

The sixth group ×1.70~1.90

2.3 The cavity pressure is determined by the wall thickness, and the ratio of the flow rate to the wall thickness

P=P0•K (multiplication constant)

2.4 Determination of clamping force (F)


electric injection molding machine

Determine the type of injection molding machine

There are many different types of injection molding machines, and choosing the right type can bring greater benefits to your business.

  • Hydraulic injection molding machine

    Hydraulic presses have historically been the only option available to molders until Nissei Plastic Industrial Co., LTD introduced the first all-electric injection molding machine in 1983.

    Hydraulic machines, although not nearly as precise, are the predominant type in most of the world, with the exception of Japan.

  • Electric injection molding machine

    The electric press, also known as Electric Machine Technology (EMT), reduces operation costs by cutting energy consumption and also addresses some of the environmental concerns surrounding the hydraulic press. Electric presses have been shown to be quieter, faster, and have a higher accuracy, however the machines are more expensive.

    Read this article to learn more: Electric VS Hydraulic Injection Molding Machine

  • Hybrid injection molding machines

    Hybrid injection (sometimes referred to as “Servo-Hydraulic”) molding machines claim to take advantage of the best features of both hydraulic and electric systems, but in actuality use almost the same amount of electricity to operate as an electric injection molding machine depending on the manufacturer.


Choose right injection molding machine manufacturers

You can cooperate with a large number of injection molding machine manufacturers in the market.

But, have you ever tried to choose a Chinese injection molding machine supplier? You may be influenced by what others say, but injection molding machines from China are actually the best, most reliable, and most trustworthy.


Give you two reasons to choose a Chinese manufacturer

  1. China has the most complete industrial chain in the world. The industry has developed rapidly for decades and has already reached a world-class level.
  2. China has a huge cost advantage, which is not only reflected in labor costs. At the same time, China has a large number of talents, a complete supply chain and excellent production technology, which brings huge cost advantages to Chinese manufacturing.

Besides you can get it with them at the best and most competitive price, you can also establish relationships with other suppliers in the future.

hydraulic injection molding machine

How to find a Chinese injection molding machine manufacturer

It is very important to find a suitable and excellent injection molding machine manufacturer. It affects your business development, including your cost, production efficiency and product quality.

To find a injection molding machines manufacturer in China, you can use the following methods:

B2B platform

  • Alibaba

    China's largest foreign trade export website, where you can choose injection molding machines suppliers and manufacturers in different regions.

  • Made in China

    Made-in-China.com specializes in the promotion of Chinese products and only accepts Chinese suppliers to release relevant product information. It is currently the only large-scale B2B platform whose sole mission is to promote Chinese products.

  • Global Sources

    Focus on foreign trade of domestic small and medium-sized enterprises, and provide professional services for foreign buyers.


Generally, the brand manufacturer websites that you can search through Google are quite professional. And they respond more quickly and professionally than B2B platforms.


How to choose the right Chinese injection molding machine manufacturer

In order to be able to choose a suitable injection molding machine manufacturer, you can refer to the following points:

  • Check reviews and customer feedback

    One way to determine whether a supplier of injection molding machines is reliable is to check what their previous customers have said about them. Most (if not all) injection molding machine suppliers will post customer feedback on their website.

  • Check the "About Us" section

    It is very important to check their website and navigate to the "About Us" section they have. If you think they are good at doing what they do best, don't hesitate to call them. On the other hand, if you can’t find what you need, try to contact them and start a conversation.( About Us)

  • Contact the manufacturer of the injection molding machine

    Unless you contact them, you actually don't know anything about the supplier.

    By sending them an email or phone call, you can determine how they will conduct their business. You can check and evaluate their urgency by paying attention to their response speed to messages and inquiries; you can feel the feeling of working with them.

By performing these three things, you can reduce the time for inspection and everything, and reduce many detours.


Write at the end

Purchasing an injection molding machine is a big investment, and choosing the right injection molding machine is very important. Through the above steps, you can get the specifications of the injection molding machine you need and the injection molding machine manufacturer suitable for you. Each manufacturer will provide a different solution. Please carefully ask the injection molding machine supplier for all details to ensure that you get the product you want. Please select the most suitable manufacturer after horizontal comparison. Hope this guide is helpful to you.

Finally, we recommend Log-Machine, China's top injection molding machine manufacturer. We provide high-quality injection molding machines and low prices. The log machine will be your ideal partner. Click here to learn more.

Contact us for more information.

Victoria Addington
I liked how this post shared that customer feedback will help determine a companys reliability. My friend wants a plastics injection process to achieve their desired product. I should advise him to look for a company that with positive customer feedback. 
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