Quick understand on plastic injection molding

Injection Molding

Injection molding is referring to a process to inject a melted resin into metal blocks. After the resin is cooled down, it will become a solid plastic product. An injection tooling is to be made with a gap among steel blocks to form a desired shape. You will need at least two steel blocks so that you can take out a product after molding. 

For an injection machine, it is designed with one end melting and injecting plastic resin and the other end moving back & forth to open and close a tooling. The moving side (i.e. called “Core”) will also provide ejecting mechanism to eject the product out from the tooling after open. On the other hand, the injecting side (i.e. called “Cavity”) cannot eject the product, but only provide a gap for taking/drop out the residual of the molding (i.e. called “Runner”). 

injection molding machine structure 

  

 

Tooling design

Since it can be very difficult to take out the product from the Cavity side because of no ejecting mechanism, a tooling needs to be designed in a way that the product is always being hold at the Core side after open. To do so, the Cavity side is generally smooth whereas the Core side is relatively rough. A slope no matter obvious or unobvious will also be applied to surfaces to reduce the friction to the Cavity side. Such slope is called draft angle. Without sufficient draft angle or smoothness of the surface at the Cavity side, it will lead to quality issues such as deformation, scratches, cracks or product is being hold at the Cavity side and difficult to be taken out. 

For a proper designed tooling, a product will be hold at the Core side when the tooling is opened. For automation and to take out the product easily, the Core side will provide additional ejecting action to push the product out from the tooling. Ejector pins are commonly used, which are small metal rods to perform the pushing action. There are other advance ways to push out the product including moving a whole platform or using gas to blow out the product.   

cnc milling on copper before edm processing

CNC Milling

In many cases, a product can be complicated in shape and structure to provide desired function and style. However, this may involve lots of undercuts, which are product areas that conflicts or jams the tooling’s open action. To overcome this, tooling will be designed with slides or side cores, which are additional steels to provide a side action. Those slides will be moved side way first before the main tooling is opened. For a complicated product, it may involve many side cores, multi-movements, multi-directions and even curvy motion. The more the side cores and the deeper of a undercut, the bigger the tooling to allow the necessary side actions, the longer time to mold the product, the less reliability, the shorter tooling life, the more complicated in tooling design and workmanship to avoid conflicts and to provide an efficient cooling. As a result, it may cost a lot more. 

 

Sometimes, a product may have different versions with minor changes. Raising a new tooling may be very costly and using metal insert is a good option. Insert is a piece of steel inside a tooling that can be replaced. Insert has many applications such as changing wordings or logo, spare for critical part or weak locations or a correction on mistake. Since insert is meaning two pieces of steel joining together, there will be a tiny line at the jointing position. Similarly, the joint between cavity and core will also create a line on the product, which is called parting line. 

 

For a tooling, it is not necessary to mold only one piece at a time. You may mold a lot more in one shot by having more cavities in a tooling. Each cavity can be designed as identical or different depends on the needs. For example, you can put a case top and case bottom in a tooling and you can also put 4 identical cups in a tooling. Some suppliers can made even more than 128 cavities. 

Running an injection machine is expensive. The more the cavities in a tooling, the less cost to be shared by each piece and higher daily output. However, it also implies a bigger tooling and bigger machine to do the molding. Therefore, it is suggested to have more cavities if you have good order. Otherwise, fewer cavities will be better off for small order. 

During the molding process, very hot resin will be injected to the tooling at high pressure and then being cooled down quickly. Therefore, the steel has to be robust and durable. There are many different grade of steel to be selected depending on what resin to be used. Some resin will be corrosive such as PVC. Some resin scratches the tooling serious such as those mixing with glass fiber. Heat treatment and even special coating may be applied to extend the tooling life. 

There is a significant wall thickness of a tooling to overcome the high pressure during injection molding. Therefore, there is a long plastic residual from the machine’s injecting point to the product edge (i.e. called “gate”). Such residual include sprue (the main channel) and runner (the sub channel), but generally being referred to runner only as a simplified term. Such sprue and runner have a significant weight and can be costly, if it cannot be recycled. Therefore, such sprue and runner should be minimized to save cost. A hot runner system is an effective way to eliminate the sprue and runner by extend the resin melting mechanism to the gate. Since hot runner system is expensive, it will be used when the order is with big volume or automation is important or a good looking injection gate is needed. 

There are different gate designs. Large gate can provide bigger flow of resin to shorten the molding time. However, the large gate will be uneven and ugly after cut. A small gate will have a better appearance, but will involve longer time to mold or need to have a higher pressure to compensate the slower flow of resin. Edge gate is to locate the gate at an edge of the product so that it is not obvious. A banana gate is more or less a u-shape gate to relocate the gate from behind to provide a perfect product surface. Tooling can have more than one gate to quicken the molding process, to ensure a complete fill up on all areas or to avoid deformation by providing a good balance on injection forces. The gate position is important not only to ensure quicker molding time, but also sensitive to a good finishing. A change in gate position may create, avoid or move flow marks, which are lines on a product that are caused by resin flowing. 

There are many types and brands of resin in the market, such as ABS, Clear ABS, HIPS, PS, Acrylic, PE, HDPE, LDPE, PVC, Nylon, PC, PBT, POM, Rubber, TPE, TPR, TPV, PP, Transparent PP and PET. Each behaves differently in many ways including strength, flexibility, deformation, appearance, transparency and shrinkage. The difference leads to different mold design and molding condition to make a desired product.  

Spraying ink on plastic parts

Spraying

Over Molding

Some designers would like to combine two different plastics, like many handles are designed with a soft grip on top of a hard plastic. It can be done by two-shot mold or insert molding. Two-shot molding or twin shot molding is a tooling that allows injection of two materials. After the first injection, the tooling will be rotated immediately for a second injection. Insert molding split the process by two sets of tooling. One set is to mold a base or called bare unit. The bare unit will then be put into a second tooling manually to mold a second plastic. The manually putting up the bare unit is an inserting process and therefore is called insert molding. Two-shot molding has strong bonding and more precision over insert molding. However, it needs to be molded by specially designed multi-shot injection machine. In general, insert molding is less costly than two-shot molding. 

Advanced Molding Methods

Insert molding is also used to mold plastic on a metal part. That is to put metal parts such as screws or metal pins into a tooling manually before a molding process. The metal part will then be hold strongly by the plastic. Plastic will always shrink when it cools down. If the walls or rids are thick, it will lead to incompletely filled product such as dimples. Therefore, we shall avoid wall and rids to be too thick in tooling design. In some cases, we have to provide a very thick rid or wall to stand for strong forces. In such a case, gas-assist molding will be used to inject nitrogen gas in the middle of the rib and will create a tunnel in the rib to eliminate shrinkage. For connectors or piping, it may need to use screw and unscrew structure. We can specially design a core or an insert which can provide a rotating action to create the screwing and unscrewing connection. It is difficult to make thin parts as resin flowing is slow. If it needs to make extremely thin part, compression injection will be used. It is to open and compress the tooling at the last molding process so that it forces the resin to fill up the very thin surface. 

Common quality issues

There are many types of quality issues. Polish and proper draft angle will help on scratches and shiny look. Use thin rids to reduce shrinkage. Adding rids to prevent deformation. Relocation and change type of gates to improve flow marks. Design round corners or move away parting line to avoid sharp edge. Improve house-keeping to avoid dirt. Better mold fitting and good mold maintenance to eliminate flash. Improving cooling system inside the tooling, add pins or add cutting line to let hot gas coming out to eliminate burns. Change resin to improve looks and performance. Add texture to hidden minor defects. 

Since tooling is made of physical steels, it is difficult or impossible to change after it is finished. A good communication of your applications and concerns to a reliable well experienced supplier before tooling start is critical. 

pad printing

pad printing

346 Responses to Quick understand on plastic injection molding

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  2. Chase Taddeo says:

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