plastic injection molding

plastic injection molding s

Plastic injection molding

There are many online companies now offering injection molding services and turn around of parts within 15 days. These companies are well established and offer quick prototyping solutions. However a prototype mold is not the same as a production tool and while it does help to first produce a prototype tool it is not always a simple transition from prototype to hard tooling for larger production volumes . Most of these rapid prototyping companies will simply make an aluminium block and inject plastic into that aluminium block. This is a very simple mold with no cooling channels in the core and cavity block. So the cycle times are longer than typical production tools.

If you need help in designing your plastic part for injection molding or if you need assistance with tool design please contact me at. colm@pro-plastics.com

Like wise, if you need any advice on sourcing or specifying your molding requirements I can assist you in finding suitable injection molders and toolmakers for your next project . Please contact me at. colm@pro-plastics.com

Why use plastic injection molding for your project?

Firstly, you need to consider how many parts you actually need for your project if it is in the thousands then injection molding is probably your best solution. as plastic injection molding offers good repeatability, consistency an inability to design complex features into your part. however if your part is very large , anything more than 15 inches by 15 inches will be a challenge two manufacturer with injection molding , due to the large size of mold needed as well as the large size molding machine that will be needed . not to say it can’t be done however the availability of such large machines (Over 600 tonnes) can be difficult to find within the market.

Rapid tooling all your part is a service many companies provide on the Internet; these tools are usually made from soft metals such as aluminium and can be used for up to a few thousand parts. However, the heat conduction properties of aluminium are not the same as harder steel such as stainless steel which most production tools would be made from. 

If you need any advice on sourcing or specifying your molding requirements I can assist you in finding suitable injection molders and toolmakers for your next project . please contact at. colm@pro-plastics.com

What is injection molding all about?

Plastic injection molding is a process whereby a material such as term or plastic thermoset or a thermoplastic is first melted within a heated barrel, then injected into a closed mold under pressure. the plastic within the mold is allowed to cool while the next shot in the heated barrel is been prepared. wants to part has been cooled sufficiently, the mold is opened, and the part ejected. essentially this is what happens in most typical injection molding processes. however there is a wide variety of additional processes that can be done during the cycle for example parts can be moved within the tool such as sliders and lifters to allow undercuts in your part so does these sliders and lifters can move before ejection of the part . we can also have threaded features within our part so that within the mold we have a rotating core to allow depart to be ejected.

If you need help in designing your plastic part for injection molding or if you need assistance with tool design please contact me at. colm@pro-plastics.com

Plastic injection molding and how it works.

Generally, the plastic is fed into the molding machine in pallet form. These pellets are approximately 5 millimetres long and 4 millimetres in diameter. The pellets are loaded into a hopper that is fixed to the top of the barrel of the molding machine. A Helix screw is rotated within the heated barrel causing the pellets of material to be drawn into the heated barrel and up to the top of the Helix screw. The plastic at the front of the screw becomes pressurised thus forcing the screw backwards. A slight back pressure is applied to the screw to compress the melt at the front of the screw. When the screw has been fully charged the molten plastic, shot is ready for injection. At the top of the screw there is a cheque ring which during injection closes causing the screw to act like a plunger. Therefore, as the screw moves forward the plastic is injected out of the nozzle of the machine and into the mold.

The mold is clamped closed in the injection molding machine as the injection pressure of the molten plastic can be quite large. Once the plastic is in the mold it starts to cool very quickly. Cooling times can be anything from 5 to 45 seconds depending on the material and the thickness of the part. The cooling phase of the injection molding cycle is typically the longest part of the cycle and can typically account for 60 to 70% of the injection molding cycle.  Once the part is fully cooled the mold is opened and the part is ejected, either into a bin below the mold or a robot takes the part from the tool face.

If you need any advice on sourcing or specifying your molding requirements I can assist you in finding suitable injection molders and toolmakers for your next project . please contact at. colm@pro-plastics.com

Plastic injection mold material considerations

plastic injection molding is essentially a heat transfer process , where hot molten plastic is injected into a cold mold . where the heat is removed from the molten plastic and transferred into the mold , where are cooling channels within the mold removed the heat from the mold . it is therefore important for the cooling cycle time that the material that the mole is made from has a good thermal conductivity . Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal conductivity. For instance, metals typically have high thermal conductivity and are very efficient at conducting heat, while the opposite is true for insulating materials like Styrofoam. While we can see the terminal conductivity of aluminium is more than four times that of p20 stainless steel.

Thermal conductivity of p20 steel   29 BTU*in/(hr*ft²*ºF)

Thermal conductivity of Aluminium 136 BTU*in/(hr*ft²*ºF)

However, aluminium is a much softer metal and will not produce good parts for as many cycles as a harder tool steel such as P 20 or H 13 which most production tools would be made from. That said aluminium is usually used for prototyping tools.  

Hardness – Rockwell B of p20 steel   104 – 110

Hardness – Rockwell B of Aluminium 60.

Mold Surface finish.

Another consideration when deciding on what material to use for your mold, the surface finish and the Polish of aluminium can generally not achieve as high a surface finish as a higher carbon content P 20 or H 13. Therefore, the surface finished on an aluminium tool is generally not as good as the surface finish that can be achieved on harder stainless-steel tools. The surface finish of the core and cavity has a major effect on how difficult the part is to eject from the tool. Generally, the rougher the surface finish the more difficult it is to eject however very high surface finish such as a mirror finish can also be very difficult to eject. Therefore, it may be necessary to use larger draught angles and more ejector pins on an aluminium tool.

If the parts are not able to be ejected from the tool automatically then an operator will have to stop the molding machine open the door and manually removed apart from the mold. For large scale production this is a large cost and should be avoided if possible. These are just some of the considerations you should consider when choosing what type of prototype mold you should make.

 As a small representative tool in the same material as your production tool of either a P20 or harder H 13 material should be used. In so doing you would have a better representative mold that would be easier to transfer the learnings from the small prototype tool to the larger production tool. Also, there is not a huge difference in tooling costs as P 20 material cost is not significantly larger than aluminium and the machining time will be the main cost of any mold.

If you need help in designing your plastic part for injection molding or if you need assistance with tool design please contact me at. colm@pro-plastics.com

Types of plastic injection molding.

here are many different types of plastic injection molding however some of the main types would be insert injection molding where a part, usually metal is inserted into the mold and plastic is injected around it. An example would be when a screw feature within a part needs to be strengthened, a brass part is inserted into the mold and plastic is injected around it, giving the traded feature more strength.

However, plastic can also be inserted into the mold and inject plastic around it however there is always the risk of melting the inserted plastic so there should be a significant difference in the melting temperatures of the two materials. It is recommended that the inserted plastic to have at least 30 to 40 degrees higher melting temperature.  However, if a melt bond is needed between the inserted plastic and the injected plastic for adhesion then the melt temperatures of both materials should be closer.

Another common type of injection molding is over-molding. This is where a substrate such as a pre molded plastic part is inserted into the mold and a soft rubber like plastic is injected around it. This can commonly be seen in handle or grip covers in many hardware tools. This over molding process for cost reasons and low production runs can be done manually. Where an operator would insert the part into the mold every shot. For larger runs a more expensive tool can be made that rotates within the molding press. The first shot is molded on one side of the tool then the tool is rotated 180 degrees and a second shot of a softer material is injected in over the initial plastic part. The over-molded part is then ejected, and the tool rotates back 180 degrees for the next shot. These tools are generally more expensive however for larger runs the automation saving means it is more economical.

If you need any advice on sourcing or specifying your molding requirements I can assist you in finding suitable injection molders and toolmakers for your next project . please contact at. colm@pro-plastics.com

How to reduce course with plastic injection molding.

  1. Choose a commodity plastic resin such as polypropylene, HDPE, HIPS or for parts that require more strength used ABS or polycarbonate.
  2. Reduce the amount of material in the part by designing parts with a uniformed wall thickness. A common wall thickness for molded parts is somewhere between 1-3 millimetres.
  3. Watch out for thick areas in your part at corners, or where features such as bosses and ribs are joining onto the main wall of the part.
  4. Know where you are going to split the tool, so as to understand where your split line for your part will be.
  5. Try to eliminate undercuts in your part, so that the part can be easily ejected from the moving half of the tool.
  6.  Specify a machine finish on the non-cosmetic side of the part and specify a low SPI B3 Polish finish on the cosmetic side.
  7. Allow at least 1 draft, and preferable 3-5 degrees draft in the direct of ejection. This will make it easier for the part to be ejected.
  8. If your part is to be assembled, try to design in self mating parts that make the assembly process easier.
  9. Consider your gate position and aesthetics look of the gate mark. You will need to consider where you can easily inject material and on a non-customer facing surface preferable.
  10. By using interchangeable core and cavity inserts, you can reuse mold bases and just swap out the core and cavity inserts as well as the ejector block. Checkout Hascos’ range of quick changing mold bases.
  11. Where possible use multicavity our family molds.
  12. Try to keep the part size as small as possible by considering assembling large components. Larger plastic parts require larger moles and larger machines. By keeping the part size small you can reduce manufacturing costs.

If you need help in designing your plastic part for injection molding or if you need assistance with tool design please contact me at. colm@pro-plastics.com

What can injection molding be used for?

Well a simple walk through any supermarket will show you the range of items that are being produced with injection molding. Everything from bottle caps to food containers, electronic devices, automobile parts and medical devices. Plastic injection molding is very big industry employing hundreds and thousands of people worldwide. There are new materials being developed every year that have new and improved properties. Many parts that have been traditionally manufactured with cast metal are now being manufactured with fibre reinforced plastics using traditional injection molding process.

If you need help in designing your plastic part for injection molding or if you need assistance with tool design please contact me at. colm@pro-plastics.com

If you need any advice on sourcing or specifying your molding requirements I can assist you in finding suitable injection molders and toolmakers for your next project. please contact at. colm@pro-plastics.com

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