Injection Molding 101

You will have to examine your application and choose accordingly, but I will try to explain briefly some of the advantages to a hot runner solution. As I currently work for a hot runner company I see many parts that benefit from this application. Here are a few basic reasons to look into hot runners:

• Less waste -which in turn reduces contamination
• Controlled melt temperature - reduces warping of part as well as plastic degrading
• Great gate vestige - Reduces the need for second operations
• Lower injection pressures - allows for more cavities per mold
• Shorter cycle times - improves production efficiency

A hot runner injection mold is a mold in which plastic resin in the manifold plates is kept at the melt temperature during injection molding process. Compare with traditional injection mold, hot runner mold does not have runner channel, so the hot runner system is also called runnerless molding system. Actually, the hot nozzle system is an extension of the nozzle and work as a distributor for melt to each cavity.

Hot runner injection mold does not have to wait the runner to cool down, it shorten injection cycle time significantly, many hot runner mold can mold thin-wall parts at 5 seconds or less per cycle.

Amorphous resins such as ABS, Polycarbonate, and Polystyrene have much lower shrinkage values than the polyolefins. The higher shrinkage of polyolefins is due to the fact that, in their molten state, they take up more volume than in the solid state because polyolefin resins are semi-crystalline. When the resin solidifies, the chains in the crystalline regions pack tightly together resulting in a reduction in volume. In general, the polyolefins can be ranked for shrinkage:

HDPE ≥LLDPE ≥LDPE ≥PP

Once a resin has been selected, shrinkage can be controlled, to some extent, through mold design and processing conditions.

We have to some correction of water flow and temperature of water with mold temperature control unit, but it is extra cost and setting it to optimum level with minimum extra energy cost is critical and time consuming.

At corners, forces are in 3d for shrinkage, some corners require the design to be compromised or if not to compromise an another pre made filler plastic insert to added while molding and/or extra cooling at corners externally and/or by giving feeding material (hot), ie an extra gate point with, and its direction of feeding material, towards the shrink pulling.

Quick link of the Troubleshooting Guide:

1) Dark brown spots 2) Brittle fracture 3) Bubbles (Or Steelyard balance : stranded gas) 4) Burn marks 5) Plastic injection molding machine of mucosa 6) Grain crack 7) Plastic degradation 8) Injection molded parts deforms 9) Injection molded cloak 10) Injection molded parts does not shrink 11) Surface roughness 12) Silver streak

13) Surface wrinkled skin texture 14) Color distribution 15) Fusion line location decline 16) Insufficient filling 17) Injection molded parts size differences 18) Systolic trace 19) Dirt marks 20) Sprue adhesion 21) Empty hole 22) Curved injection molded parts 23) Spraying trace 24) Surface layer (peeling)

After we finish assembling a new mold, we need to launch it and do a mold tryout.

The mold designer and mold maker sometimes made mistakes, any small mistakes could lead to major damage of the injection mold, the purpose of mold tryout is to find out any potential defects and ensure the mold perform stable and efficiency during molding production. If the tryout process did not follow reasonable procedures and make the appropriate records, then we cannot guarantee smooth production, this could cause serious problem, it means much higher cost or even delay for mass production.

A gate is a designed small opening to allow the melted plastics into the mold cavity, a successful gate design is determined by gate type, dimensions, location, it's deeply related with the materials been used, the type of mold plates, and economic factors. Gate design is one of the most important elements to influence injection mold quality and productivity.

Single gate has always been selected as a priority unless the distance of the mold flow is too big and beyond the limits of the injection molding pressure. Multiple gates always cannot avoid the problem of weld line where the flows meet from different gates. This weld line are not allowed sometimes specially when the injection molded parts have critical appearance or strength requirement, single gate also have good performance on packing, part strength, lower scrap rates, which gating way to employ will be determined by the structure, dimension appearance requirement of the molded parts.

The injection molding is a process to form the plastic resin into the desired shape. Injection molding machine press the melted plastic into the mold, and cool down by cooling system into solid designed shape, almost all thermoplastics using this method, comparing with other processing way, injection molding has advantage of accuracy, productivity, it has higher requirement for equipment and mold cost, so it is mainly for high-volume production of molded parts.

An injection mold is a precision tool and also must be strong enough to withstand thousands of high pressure molding cycles, so the material of the injection mold must be carefully; it's normally special and expensive steel, The way of manufacturing injection mold have been changed along with modern technology developing, main steps are CNC milling, drilling, EDM, wire-cutting, CAD/CAE/CAM technology utilized in a wide range make the lead-time of mold making shorter and shorter.

Let's face it, those of us in the plastics molding industry typically have to deal with sink marks with most parts. We all understand that the Geometry, Variations in Wall stock and Interior features sometimes go beyond design guidelines and result in sink marks. For reducing sink marks there are some process adjustments that may resolve the problem. If you have Quality Tooling and an accurate water temperature controller, you might be able to "Freeze Off" the Cavity side of the part while keeping a good hold pressure on the resin. This technique would usually add time to the total cycle but perhaps worth it. I have also used the RHRC (Rapid Heat Rapid Cool) process which is not used at many molders since it requires a Steam System that is piped to the Molding Presses