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Building a plastic injection molding tool
A perfect, precision part begins with the mold. Building the tool takes time and a great deal of accuracy. It can also represent the largest investment in the manufacturing process, so getting it right is critical to the success of a project. If your goal is to manufacture parts with a high degree of precision in large-volume, the tooling becomes even more complex.
When plastic meets the mold
The tool and the molding process are customized based on the type of plastic. Plastics that are amorphous are less free-flowing and tend to shrink less than crystalline or semi-crystalline plastics, which offer better flow, but higher shrinkage. For this reason, many projects call for engineering resins that offer a better melt and less shrinkage. Plastic suppliers provide information on the shrinkage rate of their resins along with temperature and melt flow rate recommendations.
Machining to exact standards
Resin shrinkage impacts the design and machining of the tool cavities. The cavities must accommodate for the amount of shrinkage that can occur. Using modern CAD software, the design engineer will create cavities that are larger than the actual finished part. The size of this allowance is based on the specific properties of the resin. Some of this shrinkage can be addressed by regulating the packing and holding rate in the mold, but all plastic shrinks as it cools, even after the part is ejected from the mold. Worse case, warpage can occur when a part has molded-in stress. This stress can be a result of issues with pressure, temperature, flow rate, gate location or venting.
A strong mold design leads to high-volume quality
Precision parts can only be accomplished by meeting exacting standards not only in the cavities, but in the design of the mold components. Gates must be properly placed to allow for proper melt flow and pressure. The appearance of the final part can be improved by positioning the gates in an inconspicuous location on the part. The size of the gate is also an important consideration. The gate must be large enough to provide for proper packing of the material without extending the cycle time. If the gate is too small the packing may be insufficient to fill the cavity (also called a short shot) or the part may display other defects.
The design of the mold must also include vents. Vents allow the air that is displaced by the melted resin to escape the tool. As with gating, the size and position of the vents are key factors in producing a quality part. Vents that are too large can allow the plastic material to escape and cause flashing. Vents that are too small may not release enough of the trapped air and gas. These gas bubbles can cause an improper fill (short shot) or worse. The gas could combust and cause burn marks on the part.
Proper cooling is key
An efficient and effective cooling system is the hallmark of a quality injection mold. The mold needs to maintain a consistent temperature to avoid shrinkage and warping while minimizing the cycle times to maximize production output. This delicate balance is achieved with a well-designed cooling system.
The final step in the molding process is releasing or ejecting the parts from the mold. The part geometry, type of resin, and mold finish are all considered when designing the ejector system. The placement of the ejector pins, the type of ejection mechanism and the cycle times need to be calculated with precision to avoid any defects in the part. This is generally accomplished with a series of carefully placed ejector pins, the size and position of which are determined by the shape, size and wall thickness of the part.
Running samples improves quality
Avoiding any unnecessary rework of a mold cavity will save time and money in the long run. Experienced molders create a sample mold that is used to produce a test run of the part. This step is vital in determining if any adjustments to the mold, the resin selection, or molding parameters such as temperature and flow rate are needed. If there are any quality issues, the project team will work together to determine the cause and re-sample the parts until they meet the customer’s and the molder’s standards.
A quality precision mold, built to last for years is an investment in your company. Below we review the variables that impact the cost of a plastic injection mold.
The core metal
For shorter production runs, some mold makers will use molds made from aluminum. Aluminum is a perfectly reasonable choice if you will not need the mold to perform long-term. However, if a project requires that a mold lasts for several years, an aluminum mold may cost more in the long-run. High-volume precision molds are made from hardened steel. Steel can withstand the pressures of a long production run while still meeting close tolerances.
The number of cavities
It is pretty intuitive when you think about it. Fewer cavities in a mold require less tooling work, time and ultimately less cost. A reputable, experienced molder will be able to maximize cavitation in the mold to maintain the highest level of productivity. In general, most molders recommend creating one mold per part versus creating a family mold. Family molds are created with various cavities for assorted parts. They tend to produce inferior products and have more downtime due to maintenance issues.
The mold base
Think of the mold base as a case that holds all of the mold cavities, inserts and components together. The cost of the base is estimated based on the size of the mold and the type of steel used to make the base as well as the customization required. Most mold bases come in standard sizes and are further machined to meet the requirements of a specific project.
All molds must also be customized. Customization includes the placement of cores, cavities, ejectors, cooling lines, etc. The steel used in the tool also impacts cost. Hardened steel molds last the longest and are more expensive to machine. Once done, however, they have a long production life.
Just as the number of cavities plays a role in determining the cost of the mold, so does part complexity. This includes the surface finish of the final part as well as the number of undercuts required. Parts, which demand tight tolerances, also contribute to the mold complexity.
Turnkey or vertically integrated injection molders
Some mold builders also manufacture the parts. This type of integration can help defray the mold building cost. Often full-service molding manufacturers will subsidize a portion or all of the cost of the mold based on the full term and value of the manufacturing contract. They will amortize the cost of the mold so they can maintain profit margins while providing the lowest possible per piece cost to their clients.
Summing up, it should be stated that as building a tool is a large investment in time and money, getting it right is critical.
Source: Paula Hynes, The Rodon Group
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