This checklist can be used as a general reference guide for injection mold design engineers. It is divided into 3 parts of a mold design process.
Part 1 – Requirements to start your mold design:
Check the injection machine where the mold is to be mounted. This will help you decide the size and structure of the mold for ease of installation and other factors. Important notes:
Locating ring size (or other positioning method)
Nozzle size
Method of clamping (Auto or manual)
Temperature control system
Determine the number of cavities and volume requirements. This will help you decide the material that you are going to use and other mold components that you will choose for cost effective design.
Determine the gate location and size.
Determine the location where ejector pin marks are prohibited.
Part 2 – Mold base layout:
Place cavities close to the center of the mold to minimize base size and runner length.
Ensure that the molded part remains on the movable half (ejector half) upon opening of PL to facilitate proper ejection.
Waterlines should be placed as evenly as possible to the contours of the cavity.
Use support pillars underneath the cavity pockets.
Use ejector guides for molds with small ejector pins and rectangular ejector pins.
Provide eye-bolt hole for ease of mounting and dismounting.
Install mold opening prevention locks on the operator side.
Establish pry bar groove on the corners of the mold parting line to facilitate ease of mold opening during assembly and maintenance.
By this time you may ask for the mold layout approval from the customer.
Part 3 – Cavity/core details:
Check material shrinkage. Locate portions (corners) for possible significant deflection and deformation.
Maintain uniform wall thickness.
Draft angle should be within dimension tolerance.
Divide core blocks to simplify machining and provide gas vent path.
Gate, small cores, and cores with shut-off fittings are better designed as insertable components for easy modification and repair.
Watch out for possible deformation of core pins.
Position the ejector pins on the ribs and other high strength locations. Ensure ejector balance.
Detailing/part drawing: Include all parameters needed for processing -material, quantity, surface finish/texture, dimensions, tolerances and many more. Do not assume the machinist understands everything.
Any design change and amendments to the mold must be re-approved by the customer or mold owner.
Few extras that could make your mold one step further in terms of quality:
Bevel edges. Whenever possible use machine to bevel the edges.
Minimize scratches on the mold base. Keep the work table clean.
Injection molds can be classified into three main categories: single-cavity, multi-cavity, and family.
Single-Cavity Injection Molds
Single-cavity injection molds have a single hollow and can be used to produce one product at a time. They are an efficient, cost-effective option for production operations with low order volumes or parts that are oversized or complex. Single-cavity molds allow operators to devote more attention to each individual product to ensure there aren’t air bubbles, unfilled portions of the mold, or other potential flaws. These molds are also less costly than multi-cavity injection molds of the same part.
Multi-Cavity Injection Molds
Multi-cavity injection molds have multiple identical hollows. They enable manufacturers to inject molten plastic into all of the hollows at once and create multiple products simultaneously. As a result, they offer shorter lead times for batches of goods, which increases production efficiency, reduces delays, and decreases costs for large-volume or expedited orders.
Family Injection Molds
Family injection molds are very similar to multi-cavity molds. However, rather than having multiple identical hollows, each hollow has a different shape. Manufacturers can use these molds to produce prototypes or different products that are sold together in a single variety pack. This type of mold is convenient for producing different products made of the same elastomeric material. However, the hollows need to be carefully arranged and sized; if the family mold is imbalanced, the fluid won’t be injected evenly and may cause production flaws.
Before initiating mold production, it is crucial to conduct a comprehensive review of the injection molding product and its mold design. This process is known as “Design for Manufacturing” (DFM).
Since mold production strictly follows the design blueprints, ensuring the rationality and accuracy of the design is an essential step.
Product Design Review
The inspection of product design includes, but is not limited to, uniformity of wall thickness, rib design (considering the thickness and height of ribs), and draft angles. These elements directly impact the overall quality of the product and the ease of production.
Mold Design Review
For mold design, aspects that need detailed examination include the type and location of the gate, the type and position of ejector pins (components used to eject the finished product), the location of parting lines, and the design of sliders. These details are directly related to the efficiency of mold production and the quality of the final product.
Although these inspections cannot completely guarantee the absence of flaws in the design and some adjustments and repairs might still be needed during the later stages of mold making, thorough pre-checks can significantly reduce the occurrence of design defects, thereby enhancing product quality and reducing production costs.
Injection molding is a highly-engineered process that demands careful planning and attention to detail if the finished product is to achieve its full potential. The first step in designing any injection molded part is answering some basic questions:
What market(s) will it serve? (automotive, industrial, medical, housewares, food containers, etc.) What kind of design project is it? (A new part or product? A part redesign?
Who are the “stakeholders” in this part design project? Included in this group are all suppliers in the chain of production, original equipment manufacturers (OEMs), the sales and marketing teams, and other companies that add value to the part. Each link in this stakeholder chain will often have its own set of requirements for the part’s design. If you can identify these early, you will save time, money, and rework later.
An injection mold design checklist enables a mold designer to ask the right questions so that any new mold build will go according to plan and be producing parts in the shortest possible lead-time.
Mold design Checklist:
Gate location / Ejection method / Parting lines (also called split line) / Venting positions / Estimated cycle time / Basic cooling design / Interlocking method between fixed and moving sides of mould / Heat treating requirements such as nitriding on moving mould components.
With the special information, the ratio of the maximum flow length to wall thickness is be calculated. The maximum flow length is the distance from the gate to the furthest parting line. This ratio is used to estimate the cavity pressure inside the mould tool during first stage filling. The higher it is the more robust the mould design must be.
Additional items to include in injection mold design checklist:
Hot runner or cold runner
Pitching distance between cavities
Pre alignment between fixed and moving halves – usually guide pins and bushes
Plate thicknesses – especially back plates for mould rigidity
Sufficient support pillars for mould rigidity
Mould design for easy mould assembly
Mould design for easy maintenance in the moulding machine: for example cleaning blocked gates or removing cold runners by hand
Mould design for easy machine installation and removal
Corrosion protection coatings such as electroless nickel plating on P20 bolster plates
mold design checklist tips
Check the design. I once worked with a mold maker from Germany who claimed that he had never seen a mistake on a print the entire time he worked at Volkswagen. He insisted it was because 7 different people checked each print before it was released for manufacturing. Likely his story is exaggerated, but the point remains: check the print. Just because Siemens NX or Solidworks was used doesn’t mean it can’t have errors!
Are draft angles included? Mold designers who have never worked as a mold maker tend to overlook this little feature, and it can subsequently cause many molding problems. Anyone who has had to dig out a stuck plastic part from a highly detailed cavity block can appreciate the value of drafted side-walls.
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