Whether experienced or a novice, every engineer can benefit from tips gleaned by the experience of others. Here are several that can make plastic injection molding more productive and profitable for you.

Protomold plastic injection molded parts.

Design considerations
For easier ejection of an injection mold part from a mold, it is important to provide sufficient draft on all vertical faces of at least 0.5 degrees. Draft not only makes it easier to remove a part from a mold, but it is particularly important in rapid injection molding where the molds are manufactured using three axis CNC mills. These mills have a limited ability to machine high aspect ratio mold features such as those required to produce part features such as tall, thin ribs. For part designs that include undercuts (features that require mold side actions to produce), the draft on those features needs to be considered as well for the same reasons.

As a general guideline, when you add draft to aluminum molds, the draft can range from a minimum of 0.5 degrees for shallow features and up to 0.3 degrees for deep features. A good rule of thumb is to add one degree per inch of feature depth.

When choosing the best gate location, it is best to gate into thicker areas of the part. Gating into thick regions ensures that the part can be packed out sufficiently for much lower and more uniform volumetric shrinkage. This layout leads to greater dimensional stability and helps to decrease the possibility of short shots due to the flow front freezing off as it moves from the thin regions to the thicker areas of the part.

Be aware of gate orientation when gating into the thick regions to avoid jetting. You can prevent jetting by placing the gate so that the flow is directed against a cavity wall.

When creating a 2D part drawing for quotation, designers should focus on the annotations to communicate the final part design intent to the quoting suppliers. At minimum, the following items should be shown or specified for a successful process:
• Finishes for cavity and core according to Society of the Plastics Industry (SPI) standard, textures type and number
• Visible or cosmetic surfaces and non visible surfaces
• Material type, manufacturer, and grade number or flexural modulus required
• Material color, Pantone number, or other paint manufacturers
• Tolerances as per SPI Standards and Practices of Plastics Molders and critical dimensions and tolerances to be held
• Suggested gate location
• For a cosmetic or mechanical part, where ejector pins, weld or flow lines are not acceptable
• Acceptable location for part and revision number, molding date insert
• Acceptable flatness deviation
• With which parts and how this part will be assembled or mated

Time-to-market pressures can dictate the need to use mold filling analysis software. Gone are the days when mold filling simulation was used to fill a tricky part or when the shop floor was producing unacceptable parts.

A good rule of thumb is to conduct a filling simulation to review and optimize gate locations, flow lines, and weld line placement.

One practice is to run mold filling analysis early when the part has been created as a computer model. Using the analysis later in the process when the part is cut in tool steel accelerates the risk of lost time and increased labor if there are problems with the mold.

Focus on annotations to communicate the final part design intent to the quoting suppliers.

In addition, before submitting a mold quotation, it is recommended that all new plastic parts go through a filling simulation to review and optimize gating location, flow lines, and weld lines placement. To communicate the gating location, supply images of filling results to the moldmakers and molders. You can also validate the results from the virtual filling analysis with actual mold trial short shots which can be requested from the molder. In today’s tight production schedules, this practice can be a good insurance policy to make sure parts fill properly and reduce tool rework. 

There are many important issues to include in new mold request for quotation documents.

As the plastic solidifies in molds, it freezes from the outside near the mold surface toward the inside. In thick sections, this process results in inward pulling stresses due to contraction that can cause sink marks in the outer surfaces of the part. In addition, because thinner sections will freeze faster than thicker sections, there is also the possibility of stresses building up between thick and thin sections, resulting in part warpage. Hence, in the design of parts to be injection molded, it is a good practice to maintain consistent wall thickness and avoid thick areas whenever possible.

Warpage, due to stresses in step transitions between wall thicknesses, can sometimes be reduced through the use of a ramp. The use of gussets can be helpful to provide support in corners to avoid warping. Thicker and non-uniform wall thicknesses can often result in sink marks in the part due to the same solidification physics. Therefore, thinner, more uniform wall thicknesses help avoid sink.

Warpage can sometimes be reduced through the use of a ramp.

Selecting the proper material is crucial not only for part function and affordability, but also to part manufacturability.

Application-specific requirements will always drive the need for particular material properties such as tensile strength, impact resistance, or ductility. But you should consider the molding properties of the resin, as well as its mechanical characteristics and cost. For instance, key molding criteria include the ability to fill the mold, tendency to flash, ease of part ejection, and the potential for warp, sink, or void creation. Careful design of the part geometry can address some of these issues. However, the resin choice is very important.

Progress reports help all parties involved in the process ensure target delivery dates are met.

Progress reports for better tracking the mold build
Once a purchase order has been issued along with the final 3D CAD files, 2D drawings, and mold design drawings have been reviewed and approved, progress reports become an important tool to have in place with mold suppliers to ensure the target delivery date is met. The weekly updated document monitors and tracks the mold build process to keep moldmakers focused on all the moldmaking tasks and activities required during the mold build timeline while communicating and showing its status to the mold buyers. Each report should show the percentage of completion of each activity with the goal of having all the activities completed at 100% at the end of the established timeline. Digital photos of the progress on the core and cavities and related mold components are good tools to determine if the progress report actually matches the physical state of the mold build. Small mpg videos are also easy to receive and review with the weekly progress report.

Avoid jetting by placing gates so that plastic flow is directed against a cavity wall.
 
Engage suppliers early
It is a good practice to involve all suppliers early in the process to identify and discuss resin candidates, for instance, with material suppliers.

The same is true for contacting moldmakers and molders. Having everyone involved early will usually help the new design evolve into the best and most cost- effective with reduced tooling costs. In addition, it may result in eliminating mold complexity, reducing parts count, and facilitating easier part assembly. Another rule of thumb is to use suppliers’ technical resources as part of the design team.

The following are a few suggested points to be included in new mold RFQ documentation:
• Detailed mold design drawings featuring a full bill of materials, assembly drawings, and detail drawings for each mold to be submitted for review approval.
• Mold to have an identification plate bearing the following info: part number, mold number, and “Property of XYZ Company.”
• Type of mold required; 2 plates, 3 plates, and hot runner
• Type of mold steel and hardness for mold base, plates, and core/cavity inserts
• Expected yearly part volumes requirements
• Type of mold classification required according to SPI Standards (101, 102, 103)
• Mold life and number of cycles guarantee
• Mold maintenance responsibilities
• Mold insurance

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Design guidelines for subtractive rapid prototyping

First Cut Prototype uses multi-axis CNC machining to produce functional parts from solid blocks of real plastics. Unlike additive processes that can create geometries of almost unimaginable complexity, subtractive rapid prototyping is limited to shapes that can be cut from a block of solid material using CNC milling machines.

The First Cut Prototype process currently uses three-axis CNC mills capable of producing parts within a 10-in. by 7-in. by 3.75-in. deep envelope. The maximum depth that can be milled from either side of the part is two inches. Parts smaller than 0.25-in. by 0.25-in. by 0.25-in. are hard to hold while machining.

Very small features, particularly if they are deep, may not be machinable. Recessed text should have a minimum stroke width of 0.020 in. The spacing between characters on raised text should be 0.020 in. or greater. Interactive FirstQuotes® will present the available list of thermoplastics and will identify radiuses and features that cannot be machined.

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Information in this article was provided by the following companies:

Protomold
www.protomold.com

Evolve Design Solutions, Inc.
www.evolve-designsolutions.com

Autodesk/Moldflow
www.moldflow.com

MPF