What is Plastic Injection Mold?

Time: 2018-01-24
Summary: Know more about the mold and know more about the Chinese mold line at present.

What is Plastic Injection Mold?

Plastic injection mold is a tool for manufacturing plastic parts, also makes the plastic parts have full structures and precise dimensions.


What's Shrinkage and Warpage?

Plastic part designers often use design for manufacturing and assembly(DFMA) guidelines to reduce the number of parts in an assembly. For this reason,plastic part designs can be extremely complex with many features and tight tolerances.The delivery of plastic moldings that satisfy the dimensional requirements is a joint responsibility of the mold designer,molder,material supplier,and part designer. The part designer should provide a design with uniform thicknesses and achievable specifications. The material supplier should provide consistent polymer resin and useful guidance regarding material properties.The molder should select suitable and consistent processing conditions for operation of the mold.The mold designer should provide a mold with balanced melt filling and cooling,and for which the mold cavity dimensions were engineered for an appropriate shrinkage.

For the laptop bezel, the parting line can be radiated outward to form the parting surface. it can be observed that all of the out of plane feature along the parting line now become complex surfaces on the parting plane. These surfaces pose two significant issues during mold operation.First,any misalignment between the sharp features on core and cavity inserts will cause wear between the sliding surfaces if not an outright impact between the leading edge of the core and the mating cavity surface.


Mold material selection

As part of mold base and materials procurement process,the various types of materials must also be selected.Just as there are many different plastics suitable for injection molding,there are many ferrous and non-ferrous metals that are suitable for use in injection molds

Some of the more common metals and their properties are provided in Appendix B.Of all these metals,AISI P20 is the most common due to its favorable combination of properties.However,P20 is sometimes improperly specified in many molding applications since other metals would provide better performance,lower mold making cost,or lower injection molding costs.


Mold base Sizing

The size of the mold base is determined primarily by the area required to accommodate the designed cavity layout.A primary issue, however,is the potential for conflict between the placements of the cavities and other mold components(such as leader pins,guide bushings,and others).Furthermore,there is the potential for conflict between cavity support systems(such as cooling lines,ejector pins,support pillars,etc.)and other mold components(such as leader pins,guide bushings,and others.)Due to these conflicts, mold bases are often sized larger than what would first be considered.


Parting plane for bezel

For the laptop bezel, the parting line can be radiated outward to form the parting surface. it can be observed that all of the out of plane feature along the parting line now become complex surfaces on the parting plane. These surfaces pose two significant issues during mold operation.First,any misalignment between the sharp features on core and cavity inserts will cause wear between the sliding surfaces if not an outright impact between the leading edge of the core and the mating cavity surface.


Cavity and Core insert Creation

With the definition of the parting plane and all necessary shut-offs,the core insert and cavity insert have been completely separated.To create the cavity and core inserts,the length,width,and height of the inserts must be defined.

All of these requirements suggest making the core and cavity inserts as large as possible.For smaller molded parts,increasing the sizing the core and cavity inserts may have little added cost. 

However,the cost of larger core and cavity inserts can become excessive with increases in the number of cavities or molded part size.

The mold layout design assumes that the number of mold cavities and type of mold has been determined.To develop the mold layout, the mold opening direction and the location of the parting plane are first determined. Then, the length, width,and height of the core and cavity inserts are chosen. Afterwards,a mold base is selected and the inserts are placed in as simple and compact a layout as possible.It is important to develop a good mold layout design since later analysis assumes this layout design and these dimensions are quite expensive to change once the mold making process has begun.

Mold base Sizing

The size of the mold base is determined primarily by the area required to accommodate the designed cavity layout.A primary issue, however,is the potential for conflict between the placements of the cavities and other mold components(such as leader pins,guide bushings,and others).Furthermore,there is the potential for conflict between cavity support systems(such as cooling lines,ejector pins,support pillars,etc.)


and other mold components(such as leader pins,guide bushings,and others.)Due to these conflicts, mold bases are often sized larger than what would first be considered.


Overview of injection molding process

An operating injection molding machine is depicted in figure 1.1.Injection molding is called a net shape manufacturing process because it forces the polymer melt into an evacuated mold cavity,after which it cools to find desired shape.


While molding processes can differ substantially in design and operation,most injection molding process generally include plastication ,injection,packing,cooling, and mold resetting stages.During the plastication stage,the polymer melt is plasticized from solid granules or pellets through the combined affect of heat conduction from the barrel of the molding machine and into the mold,The molten resin travels down a feed system ,through one or more gates and throughout one or more mold cavities where it will form one or more desired products.


Product Design Details

If the project is approved and a bidget is allocated,then the product development process continues,usually with additational resources to perform further analusis and design,During this second stage,each component may inclide the cinsidering of aesthetic ,structural,thermal,manufacturing,and other issues.

·Mold Design - We tend to get better designed molds in the USA. If the injection mold can be designed in the US and build in China, this has produced superior molds for us.
·Lead Times - Maybe we just had a bad experience, but one injection mold seemed to take the slow boat across the ocean.
The lead time wasn't competitive with a stateside mold.·Mold Trial - Generally not possible to be on hand for the initial injection mold trial in China.
·Mold Modifications - We have gotten better/faster injection mold modifications when the mold was able to be sent back to the original mold maker in the USA. Sending the mold to China to be modified is cost prohibitive, so a 3rd party mold maker has to make the injection mold modifications.


How to cooperate with China injection mold manufacturers?

Over the years we have been taken advantages of with the low cost of plastic injection mold manufacturing in China. You get what you pay for. I have heard of an injection mold cracking in half just last year from China, water poring out of it in the press. As an injection mold maker back in the 80's and 90's we started to see import steel making its way to our machines. We could tell on theCNC's or polishing we were not getting what we bargained for. After heat treat OMG, warp. We use to say they put old "bed springs" in the alloy mix. When drilling a hole in this steel it would not be constant feed rate by hand, you could feel the drill push easy then hard etc. So we had to be very precise in ordering our plastic injection molds with certification. We ordered 4 large injection molds from a Chinese manufacturer in 2004, the heat treat was horrible, we did not get our certification, it looked fake. Someone was trying to pull a fast one. I hardness tested several blocks from the plastic injection molds, not good. We spent approx $450,000 for those injection molds back in 2004 and we tried to outsource and handle the engineering real close. We tried to do everything real and it back fired. These injection molds were for electrical components and tolerances on my parts were critical, 4 to 8 cavity molds, the quality was just not there. Glass filled nylon wore down these plastic injection mold in 8 months, we had to retool.


Chinese Injection Mold Making Companies Development Trends

Many Chinese injection mold making companies has also made significant progress in the CAE application, and achieved good results. CAE technology can greatly save the cost of test mode, shorten the development cycle, and has become an important means to ensure injection mold quality. CAE technology is gradually changing the mold design from experience to scientific.


At present, the China auto parts of high strength steel plate is also limited to structural parts and beam parts, the tensile strength ofmaterials used in the 500MPa or less, therefore, quickly master the high-strength steel stamping technology is Chinese automotive mold industry to be resolved important questions.


The fact of China plastic injection mold makers

Chinese injection molds cut major corners on water, mold steels, unless you really get after them to use good mold steels. I have seen Chinese plastic injection molds that literally warp in the press because their injection molds are the softness of lead. I have seen well-made Chinese molds, but again, the materials used have been sub-par. The delay in communications is also an ongoing problem. That's one reason I use America, UK or Italy over China -- never have to worry about any of the things previously mentioned. The only reason that Chinese injection molds remain more competitively priced is the fact that the country pegs its currency to the dollar, so you never see the true cost. In the near future, the Chinese will not be able to continue pegging their currency to US dollar especially when the dollar is getting trashed by our own government. Because of this weak dollar policy, it makes everyone except China very expensive. Yet, for our European friends, it makes US made stuff cost competitive. However, even with our virtually worthless currency, I still get 25-30% less costly tooling in Portugal.


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