Designing a mold tool is all about designing it to produce the part required as efficiently as possible with maximum output and minimal tool down time. Unfortunately, few mold makers in China apply best practices, and few importers are knowledgeable enough to catch design issues.
A few common errors to avoid are shown below. They can be categorized in two groups: tooling design and construction, and part design.
1. Tooling Design Points
1.1 Incorrect material selection – The coefficient of thermal expansion for all steel, hardened steels, copper & aluminum, must be taken into consideration when designing and building tools. Large differences can cause damage and uneven wear to all components.
1.2 Incorrect back plates – Most molds have 2 back plates, one on the fixed side and one on the moving side. Back plates have 3 main functions:
- To hold the mold in the molding machine using clamps
- To form part of the runner system
- To support the entire mold against excessive platen deflection
If the back plates are too thin, the resultant repetitive deflection during each cycle eventually causes the following part quality problems:
- Short shots
- Weight variation
- Sink marks
- Balancing issues in the runner system
These quality problems occur because the plates do not provide enough support to stabilize the mold against cavity injection pressure and clamp tonnage.
1.3 Incorrect runner design – A runner system that is not balanced will cause filling issues and inefficiency in running the tool. Some parts may be over-packed and other under-filled; both scenarios could cause quality issues with the part.
1.4 Incorrect gate design – Selecting the incorrect gate has a similar effect with filling issues and inefficiency in running the tool. The gate is the area where the polymer transits from the runner to the part, and if this is wrong the gate could cause restrictions, incorrect polymer flow, turbulence, or other defective filling issues.
2. Part Design Points
2.1 Part designed with incorrect wall thickness – The part should be designed with the thinnest wall section the part can tolerate (taking into account strength, functionality and other critical aspects of the design). The two risks as:
- Too thin – if the wall section is too thin it may not fill correctly or it may break off in the tool.
- Too thick – wall sections that are too thick take longer to cool and solidify before ejecting and may result in sink marks, warping and even cracking.
2.2 Sharp internal corners – Having a sharp corner at an intersection will cause stress within the part and could result in part failure. Best practice is to design corners with a radius.
2.3 Thick solid sections – Designing a part with thick solid section because you need the strength or rigidity is not the answer. Having a thick solid section will only result in sink marks and defective molding due to the cooling effect of the polymer after injection. Best practice is to core out the section and add ribs to strengthen that area (this will allow the molten polymer to fill the part and for cooling to be even).
2.4 Deep pocket with parallel sides – In order for a part to be ejected off of the core without sticking, there has to be draft on the walls. A parallel-sided part will inevitably cause molding issues and high reject rates. Adding draft will overcome these problems.
3. A few real-life examples we saw in China
Some of the issues our team has experienced, and that have caused issues with the tool running, are listed below:
- Not standard parts used in the construction of the tool, springs cut to length and not ground flat resulting in uneven forces being applied to ejector plates. This caused ejector system to snag and pins to get stuck.
- Core/cavity finished to the wrong dimensions so the toolmaker ‘welded the tool’ to add metal then reworked that area to the correct dimension. This ultimately led to inconsistent parts. Remedy was to machine out that section, add an insert of the correct steel specification and rework the shape and dimensions.
- Poor workmanship with incorrect tools being used for the job, steel hammers on the tool, screwdrivers to upon the tool on the bench, tools not protected in storage resulting in rust and contamination.
What other design issues did you encounter?